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To recipients of the Construction Handbook for Bridge Temporary
Works, First Edition (1995): Instructions
Interim revisions have been made to the Construction Handbook
for Bridge Temporary Works, First Edition (1995). They have been
designed to replace the corresponding pages in the book and are
numbered accordingly.
Underlined copy indicates revisions that were approved in 2007
by the AASHTO Highways Subcommittee on
Bridges and Structures. A listing of newly changed and deleted
articles is included with these interim revisions as an addendum to
the preface of the book.
All revised pages also display a box in the lower outside corner
indicating the interim publication year. Any
non-technical changes in page appearance will be indicated by
this revision box alone to differentiate such changes from those
which have been approved by the AASHTO Highways Subcommittee on
Bridges and Structures.
To keep your Specifications correct and up-to-date, please
replace the appropriate pages in the book with the
pages in this packet.
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ISBN: 978-1-56051-397-1 Publication Code: CHBTW-1-I1
American Association of State Highway and Transportation
Officials
444 North Capitol Street, NW Suite 249 Washington, DC 20001
202-624-5800 phone/202-624-5806 fax www.transportation.org
2008 by the American Association of State Highway and
Transportation Officials. All rights reserved. Duplication is a
violation of applicable law.
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Transportation Officials Provided by IHS under license with AASHTO
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1
CHAPTER 1. INTRODUCTION
SCOPE
This construction handbook has been developed for use by
contractors and construction engineers
involved in bridge construction on Federal-aid highway projects.
This document may also be of interest to
falsework design engineers, and supplements information found in
the Guide Design Specification for Bridge
Temporary Works.(1) The content is construction-oriented,
focusing primarily on standards of material quality
and means and methods. This handbook contains chapters on
falsework, formwork, and temporary retaining
structures. For more in-depth discussion on a particular topic,
related literature and references are identified.
Chapter Two. Falsework identifies material standards, the
assessment and protection of foundations,
construction-related topics, loading considerations, and
inspection guidelines. Methods for in situ testing of
foundations are identified. General guidelines regarding timber
construction, proprietary shoring systems, cable
bracing, bridge deck falsework, and traffic openings are also
discussed.
Chapter Three. Formwork identifies and describes the various
components and formwork types
commonly used in bridge construction. Information on load
considerations and design nomographs are
provided. General guidelines relating to formwork construction
and form maintenance are also discussed.
Chapter Four. Temporary Retaining Structures focuses primarily
on cofferdams and their
application to bridge construction. As indicated by the chapter
title, however, general topics relating to a wide
range of temporary retaining structures are also addressed.
Specific topics include classification of construction
types, relative costs, sealing and buoyancy control, seepage
control, and protection. The construction of timber
sheet pile cofferdams, soldier pile and wood lagging cofferdams,
and steel sheet pile cofferdams is reviewed.
Methods of internal bracing and soil and rock anchorage are also
discussed.
Section properties of standard dressed and rough lumber, bridge
deck falsework design examples,
recommended thicknesses for wood lagging, and steel sheet pile
data are included as appendixes. Definitions
and related publications are identified below.
DEFINITIONS
For the purpose of this manual, the following definitions apply.
These definitions are not intended to
be exclusive, but are generally consistent with the common usage
of these terms.
Falsework Temporary construction work used to support the
permanent structure until it becomes
self-supporting. Falsework would include steel or timber beams,
girders, columns piles and foundations, and
any proprietary equipment, including modular shoring frames,
post shores and adjustable horizontal shoring.
Shoring A component of falsework such as horizontal, vertical,
or inclined support members. For the
purpose of this document, this term is used interchangeably with
falsework.
Formwork A temporary structure or mold used to retain the
plastic or fluid concrete in its
designated shape until it hardens. Formwork must have enough
strength to resist the fluid pressure exerted by
plastic concrete and any additional fluid pressure effects
generated by vibration.
2008 by the American Association of State Highway and
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violation of applicable law.
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2 Interim 2008
Cofferdam A temporary watertight enclosure that allows
construction of the permanent structure
under dry conditions.
RELATED PUBLICATIONS
California Falsework Manual, California Department of
Transportation, Sacramento, CA.
Certification Program for Bridge Temporary Works
(FHWA-RD-93-033), Federal Highway Administration, Washington,
DC.
Formwork for Concrete (SP-4), Seventh Edition, American Concrete
Institute, Detroit, MI.
Foundation Construction, A. Brinton Carson, McGraw-Hill, New
York, NY.
Guide Design Specifications for Bridge Temporary Works
(FHWA-RD-93-032), Federal Highway Administration, Washington, DC.
See also AASHTO GSBTW-1 (1995) and GSBTW-1-I1 (2008).
Guide Standard Specification for Bridge Temporary Works
(FHWA-93-031), Federal Highway Administration, Washington, DC.
Handbook of Temporary Structures in Construction, R.T. Ratay,
Ed., Second Edition, McGraw-Hill Book Company, New York.
Lateral Support Systems and Underpinning, Vols. I, II, III
(FHWA-RD-75-128, 129, 130), Federal Highway Administration,
Washington, DC.
Soil Mechanics, Foundations, and Earth Structures (NAVFAC DM-7),
Department of the Navy, Alexandria, VA.
Standard Specifications for Highway Bridges, 17th Edition
(HB-17), American Association of State Highway and Transportation
Officials, Washington, DC.
Synthesis of Falsework, Formwork, and Scaffolding for Highway
Bridge Structures (FHWA-RD-91-062), Federal Highway Administration,
Washington, DC.
Temporary Works, J.R. Illingworth, Thomas Telford, London,
England.
2008 by the American Association of State Highway and
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violation of applicable law.
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3 Interim2008
CHAPTER 2. FALSEWORK
MATERIALS AND MANUFACTURED COMPONENTS
Structural Steel Quality of Steel Steel grades greater than ASTM
A 36/A 36M are generally not recommended for
falsework construction unless certified or test samples are
taken. The Guide Design Specification for Bridge
Temporary Works permits the use of higher working stresses for
other grades of steel, provided the grade of
steel can be identified. Identification is the contractors
responsibility. If steel properties are unknown and test
samples are not taken, steel can generally be assumed to be ASTM
A 36/A 36M. For reference, some of the
more common steel designations predating ASTM A 36/A 36M are
provided in table 1.
Table 1. Early ASTM steel specifications.(2)
ASTM requirement
Date Specification Remark Tensile strength, lbf/in2 Minimum
yield point, lbf/in2
1924-1931 ASTM A 7 (withdrawn 1967) Structural steel 55,000 to
65,000 T.S. or not less than 30,000
Rivet steel 46,000 to 56,000 T.S. or not less than 25,000
ASTM A 9 (withdrawn 1940) Structural steel 55,000 to 65,000 T.S.
or not less than 30,000
Rivet steel 46,000 to 56,000 T.S. or not less than 25,000
1939-1948 ASTM A 7-A 9 Structural steel 60,000 to 72,000 T.S. or
not less than 33,000
1939-1949 ASTM A 141-39 (withdrawn 1967) Rivet steel 52,000 to
62,000 T.S. or not less than 28,000
Conversion: 1,000 lbf/in2
Dimensional Tolerances Rolling structural shapes and plates
involves such factors as roll wear,
subsequent roll dressing, temperature variations, etc., which
cause the finished product to vary from published
profiles. Mill dimensional tolerances are identified in AASHTO M
160M/M 160 (ASTM A 6/A 6M), Standard
Specification for General Requirements for Rolled Steel Plates,
Shapes, Sheet Piling, and Bars for Structural
Use.(3) This information is provided in tables 2 and 3 for
general reference.
Conditioning of Salvaged Steel AASHTO M 160M/M 160 (ASTM A 6/A
6M) also provides
guidelines for the condition of plates, structural shapes, and
steel sheet piling, as follows:
Plate Conditioning Plates may be conditioned by the manufacturer
or processor for the removal of
imperfections or depressions on the top and bottom surfaces by
grinding, provided the area ground is
well faired without abrupt changes in contour and the grinding
does not reduce the thickness of the
plate by: (1) more than 7 percent under the normal thickness for
plates ordered to weight per square ft,
but in no case more than in (3.2 mm); or (2) below the
permissible minimum thickness for plates
ordered to thickness in inches or millimeters.
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4
Table 2. Permissible variations in cross section for W and HP
shapes.(3)
A, depth, in B, flange width, in Section nominal size, in
Over theoretical
Under theoretical
Over theoretical
Under theoretical
T + T', flanges, out of square, max., in
Ea, web off center,
max., in
C, max., depth at any cross section over theoretical
depth, in
To 12, incl. 1/8 1/8 1/4 3/16 1/4 3/16 1/4
Over 12 1/8 1/8 1/4 3/16 5/16 3/16 1/4
Notes: (a) Variation of 5/16-in max. for sections over 426
lb/ft. (b) Conversion: 1 in = 25.4 mm; 1 lb/ft = 1.49 kg/m.
Table 3. Permissible variations in camber and sweep.(3)
Permissible variation, in
Sizes Length Camber Sweep
Sizes with flange width equal to or greater than 6 in All 1/8 in
x
(total length, ft) 10
Sizes with flange width less than 6 in All
1/8 in x (total length, ft) 10
1/8 in x (total length, ft) 5
4 1/8 in x (total length, ft) with 3/8 in max. 10
Certain sections with a flange width approx. equal to depth and
specified on order as columnsa Over 45 ft 3/8 in + 1/8 in x (total
length, ft - 45)
10
Notes: (a) Applies only to: W 8 x 31 and heavier, W 10 x 49 and
heavier, W 12 x 65 and heavier, W 14 x 90 and heavier. If other
sections are specified on the order as columns, the tolerance will
be subject to negotiation with the manufacturer. (b) Conversion: 1
in = 25.4 mm; 1 ft = 0.305 m.
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5 Interim2008
Imperfections on the top and bottom surfaces of plates may be
removed by chipping, grinding, or arc-
air gouging and then by depositing weld metal subject to the
following limiting conditions:
The chipped, ground, or gouged area shall not exceed 2 percent
of the area of the surface being conditioned.
After removal of any imperfections in preparation for welding,
the thickness of the plate at any location must not be reduced by
more than 30 percent of the nominal thickness of the plate.
(AASHTO
M 160M/M 160 (ASTM A 6/A 6M) restricts the reduction in
thickness to a 20 percent maximum.)
The edges of plates may be conditioned by the manufacturer or
processor to remove injurious imperfections by grinding, chipping,
or arc-air gouging and welding. Prior to welding, the depth of
depression, measured from the plate edge inward, shall be
limited to the thickness of the plate,
with a maximum depth of 1 in (25.4 mm).
Structural Shapes and Steel Sheet Piling Conditioning These
products may be conditioned by the
manufacturer for the removal of injurious imperfections or
surface depressions by grinding, or chipping
and grinding, provided the area ground is well faired without
abrupt changes in contour and the
depression does not extend below the rolled surface by more
than: (1) 1/32 in (0.8 mm) for material less
than 3/8 in (9.5 mm) in thickness; (2) 1/16 in (1.6 mm) for
material 3/8 to 2 in (9.5 to 50.8 mm) inclusive
in thickness; or (3) 1/8 in (3.2 mm) for material over 2 in
(50.8 mm) in thickness.
Imperfections that are greater in depth than the limits
previously listed may be removed and then weld
metal deposited subject to the following limiting
conditions:
The total area of the chipped or ground surface of any piece
prior to welding shall not exceed 2 percent of the total surface
area of that piece.
The reduction in thickness of material resulting from removal of
imperfections prior to welding shall not exceed 30 percent of the
nominal thickness at the location of the imperfection, nor
shall
the depth of depression prior to welding exceed 1 in (32 mm) in
any case except as follows:
The toes of angles, beams, channels, and zees and the stems and
toes of tees may be conditioned by
grinding, chipping, or arc-air gouging and welding. Prior to
welding, the depth of depression,
measured from the toe inward, shall be limited to the thickness
of the material at the base of the
depression, with a maximum depth limit of 2 percent of the total
surface area.
Welding Most of the ASTM-specification construction steels can
be welded without special
precautions or procedures. The weld electrode should have
properties matching those of the base metal. When
properties are comparable, the deposited weld metal is referred
to as matching weld metal. See AWS
D1.1/D1.1M(4) for requirements. Table 4 provides matching weld
metal for many of the common ASTM-
designated structural steels. In general, welding of
unidentified structural steel is not recommended unless
weldability is determined.
Most of the readily available structural steels are suitable for
welding. Welding procedures can be
based on specified steel chemistry because most mill lots are
usually below the maximum specified limits.
Table 5 shows the ideal chemistry for carbon steels.
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6 Interim 2008
Table 4. Matching filler metal requirements.(4) Welding
Processa,b
Group Base metal steel specificationc Submerged metal arc
welding (SMAW) Submerged arc welding (SAW)
Gas metal arc welding (GMAW)
Flux cored arc welding (FCAW)
I ASTM A 36, A 53 Grade B, A 500, A 501, A 529, A 570 Grades 40,
45, and 50 A 709 Grade 36
AWS A5.1 or A5.5 E60XX or E70XX
AWS A5.17 or A5.23 F6X or F7X-EXXX
AWS A5.18 ER70S-X
AWS A5.20 E5XT-X and E7XT-X (except -2, -3, -10, -GS)
II ASTM A 242,d A 572 Grades 42 and 50 A 588 A 709 Grades 50 and
50W
AWS A5.1 OR A5.5 E70XXe
AWS A5.17 or A5.23 F7X-EXXX
AWS A5.18 ER70S-X
AWS A5.20 E7XT-X (except -2, -3, -10, -GS)
III ASTM A 572, Grades 60 and 65
AWS A5.5 E80XXe
AWS A5.23 F8X-EXXXf
AWS A5.28 ER80Sf
AWS A5.29 E8XTf
IV ASTM A 514 (over 2 in thick), A 709 Grades 100 and 100W (2 in
and under)
AWS A5.5 E100XXe
AWS A5.23 F10X-EXXXf
AWS A5.28 ER100Sf
AWS A5.29 E10XTf
V ASTM A 514 (2 in and under), A 709 Grades 100 and l00W (2 in
and under)
AWS A5.5 E100XXe
AWS A5.23 F11X-EXXXf
AWS A5.28 ER110Sf
AWS A5.29 E11XTf
Notes: (a) When welds are to be stress relieved, the deposited
weld metal shall not exceed 0.05 percent vanadium. (b) See AWS
D1.1/D1.1M(4), Sec. 4.20 for electroslag and electrogas weld metal
requirements. (c) In joints involving base metals of two different
groups, low-hydrogen filler metal electrodes applicable to the
lower strength group metal may be used. The low-hydrogen processes
shall be subject to the technique requirements applicable to the
higher strength group. (d) Special welding materials and procedures
may be required to match the notch toughness of base metal or for
atmospheric corrosion and weathering characteristics. (e) Low
hydrogen classifications only. (f) Deposited weld metal shall have
a minimum impact strength of 20 ft-lbf (27 J) at 0 F (-18 C) when
Charpy V-notch specimens are used. This requirement is applicable
only to bridges. (g) Conversion: 1 in = 25.4 mm
Table 5. Preferred analysis of carbon steel for good
weldability.(5)
Element Normal Range (%) Carbon 0.06 - 0.25 Manganese 0.35 -
0.80 Silicon 0.10 max Sulfur 0.035 max Phosphorus 0.030 max
Guidance with respect to workmanship, qualification, and
inspection of weldable steel can be obtained
from Structural Welding Code, AWS D1.1/D1.1M.(4) Acceptable and
unacceptable weld profiles prescribed by
AWS are illustrated in figure 1.
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7 Interim2008
Figure 1. Acceptable and unacceptable weld profiles.(4)
Timber Timber Quality The design values for new lumber are
obtained from grading rules published by
several agencies, including: National Lumber Grades Authority (a
Canadian agency), Northeastern Lumber
Manufacturers Association, Northern Softwood Lumber Bureau,
Southern Pine Inspection Bureau, West Coast
Lumber Inspection Bureau, and Western Wood Products Association.
Design Values for most species and
grades of visually graded dimension lumber are based on the
provisions of ASTM D 1990, Establishing
Allowable Properties for Visually Graded Dimension Lumber from
In-Grade Tests of Full-Size Specimens.
Design values for visually graded timbers, decking, and some
species and grades of dimension lumber are based
on the provisions of ASTM D 245, Establishing Structural Grades
and Related Allowable Properties for
Visually Graded Lumber.
The methods in ASTM D 245 involve adjusting the strength
properties of small clear specimens of
wood, as given in ASTM D 2555, Establishing Clear Wood Strength
Values, for the effects of knots, slope of
grain, splits, checks, size, duration of load, moisture content,
and other influencing factors, to obtain design
values applicable to normal conditions of service. ASTM D 245
describes the procedures for rating lumber on
the basis of strength ratio. Strength ratio of a structural
timber is the ratio of its strength to that which it would
have if no weakening characteristics were present.
Used Lumber Where the origin and grading of the material is no
longer known, it should be
regraded by a qualified agency or individual. Timber should be
discarded if it has been painted such that it
prevents assessment, if there is any sign of rot (fungal or
chemical), if there is mechanical damage, or if there is
any undue distortion of shape. Timber should never be reused
without careful inspection.
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8
Timber Characteristics Because wood is an organic material, it
is subject to variations in structure
or properties or both. Some important anatomical characteristics
of wood and their effects on the strength of
wood members are as follows:
Knots A knot is a portion of a branch or limb, which has been
surrounded by subsequent growth of
the wood of the trunk. Knots reduce the strength of wood because
they interrupt the continuity and
direction of wood fibers. They also cause local stress
concentrations where grain patterns are abruptly
altered. The influence of a knot depends on its size, location,
shape, soundness, and the type of stress
considered. In general, knots have a greater effect in tension
than in compression, whether stresses are
applied axially or as a result of bending. Shapes of knots in
various structural members and methods of
measurement are illustrated in figure 2.
Slope of Grain Slope of grain or cross grain are terms used to
describe the deviation in wood fiber
orientation from a line parallel to the edge of the specimen. It
is expressed as a ratio such as 1 in 6 or 1
in 14, and is measured over sufficient distance along the piece
to be representative of the general slope
of the wood fibers. Slope of grain has a significant effect on
wood mechanical properties. Strength, for
example, decreases as the grain deviation increases. Specimens
with severe cross grain are also more
susceptible to warp and other dimensional deformations due to
changes in moisture content. The
technique to measure slope of grain is illustrated in figure
3.
Checks and Splits Checks and splits are separations of the wood
across or through the rings of
annual growth, usually as a result of drying shrinkage during
seasoning. Checks are partial depth
fractures, while splits extend through the full cross section.
If members are subject only to tension or
compression, checks and splits do not greatly affect strength,
unless they occur in zones of severe grain
slope.
Moisture Content Design values prescribed by the National Design
Specification for Wood
Construction (NDS) are for normal load duration under dry
conditions of service.(8) Dry lumber is defined as
lumber that has been seasoned to a moisture content of 19
percent or less by weight. Green lumber is defined as
lumber having a moisture content in excess of 19 percent.
Because the strength of wood varies with the
conditions under which it is used, these design values should
only be applied in conjunction with appropriate
design and service recommendations from the National Design
Specification.
Member Size Timber members should be generally assumed to be
standard dressed (S4S) sawn
lumber unless otherwise shown on the falsework drawings. Section
properties of S4S lumber are furnished in
appendix A. While these sizes are generally available on a
commercial basis, it is good practice to consult the
local lumber dealer(s) to determine availability.
Typically, the dimensions of rough-cut lumber will vary
appreciably from nominal, particularly in the
larger sizes commonly used in falsework construction. If the use
of rough-cut material is required by the
falsework design, the actual member size should be verified
prior to use.
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13 Interim2008
soils, field verification of soil strength and compressibility
is more difficult. Simple tests include a dynamic
cone penetration test where the number of blows to advance a rod
with a cone at the tip is recorded. The number
of blows is a rough indicator of relative density. Field density
tests (AASHTO T 191) can also be performed to
determine the unit weight of the soil from which an estimate of
relative density can also be obtained. The
measured unit weight can be compared with published information
on maximum or minimum unit weights for
various soil types. Alternatively, the maximum and minimum unit
weights can be determined by performing
laboratory tests AASHTO T 180 for the maximum unit weight and
ASTM D 4254 for minimum unit weight and
calculation of relative density.
A better and more sophisticated procedure for determining the
suitability of granular and mixed soil
deposits to support the footings is to perform pressuremeter
testing (ASTM D 4719) or dilatometer testing in
shallow hand auger holes extended below bearing level.
Test Pits Test pits can be dug throughout the area to
investigate the various soil or rock formations.
Test pits should be used to supplement other field monitoring
wherever erratic or discontinuous subsurface
conditions are present. Determining the thickness and character
of these deposits from a large excavation is
more accurate than from examination of small diameter samples
from borings. Block samples can also be
obtained for laboratory testing.
Proof-Rolling Proof-rolling is a field observation test that can
be used to indicate if and when
problem soils are located at shallow distances below grade. The
procedure consists of making multiple passes
over the area with a fully loaded dump truck having a minimum
weight of 20 tons (18,000 kg). As the dump
truck traverses the area, the amount of ground deflection under
loading shall be observed. Deflections of 2 in
(50.8 mm) or less are indications of reasonably good support
conditions. Large deflections and severe rutting
are indicative of very poor support conditions. The depth of
influence of proof-rolling is likely to be on the
order of 2 to 5 ft (0.6 to 1.5 m). Any weak soil below this
depth will remain undetected.
Load Testing The procedures for performing plate bearing tests
are described in AASHTO T 235.
The plate load test consists of a loading plate with a minimum
12-in (305-mm) diameter with a jack to provide a
force, and with a truck or other heavy object used as a
reaction. Deflections are measured with either survey
instruments or dial gauges. As the jack loads are applied,
deflection readings should be taken at the design load
and at twice the design load. The test results are analyzed in
accordance with figure 7. The depth of influence of
a plate load test is only about 1.5 times the diameter of the
plate. Thus, larger foundations that stress the soil to
greater depths may perform differently than the plate load test
would indicate.
Deep Foundations If piles are driven to support the falsework,
the driving resistance of each pile should be recorded and
compared to the required driving resistance that has been
developed for the project using either a wave equation
analysis or acceptable driving formula. Plumbness, length of
pile installed, type of hammer and cushion, surface
alignment of the driven pile, and any other observations that
could affect pile performance should also be
recorded. This data should be given to the designer for review.
If a load test is required, it should be performed
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14
in accordance with ASTM D 1143. The procedure to calculate the
failure load from a load test is identified in
figure 8.
If drilled piers are used to support the falsework, the strength
of the soil at bearing level should be
determined in a manner discussed in the previous section for
shallow foundations. The length and diameter of
the drilled shaft and bell (if used) should be recorded along
with the plumbness and surface alignment. Any
other pertinent field observations that could affect
performance, such as the presence of squeezing or caving
soils, water inflow, or accumulation of debris at the base of
the drilled pier should be brought to the attention of
the drilled pier contractor and reported to the design
engineer.
Protection of the Foundation Area Falsework foundations, in
general, are set at a very shallow depth compared with those of
permanent
structures. This places them within the zone affected by
seasonal moisture content changes, frost action, scour,
and so forth. The area covered by the foundations under the
falsework should be considered in relation to the
general topography of the surrounding ground and the likelihood
of outside influences affecting it. Steps should
be taken to safeguard it, and avoid undermining conditions such
as shown in figure 9. The stability of the
ground under and around the falsework foundations will depend on
the ground remaining unaffected by the
following: local influences of water from water courses, extreme
rainfall, melting snow, or burst water mains;
severe frosts or excessively dry and hot weather; movements of
surrounding ground subjected to excavation,
filling, or other changes; and all pressures applied by adjacent
construction operations.
Falsework in Streams Where supports (usually consisting of piles
or piers) are installed in rivers or
streams, they should be designed to withstand the horizontal
loads arising from flood conditions, applied to an
area of resistance substantially greater than that offered by
the supports alone. This increase should account for
the accumulation of river debris. To minimize this accumulation
and avoid the impact of larger pieces, measures
should be specified and installed upstream to divert such debris
from the supports or to retain it independently.
The measures adopted will depend on the circumstances. The use
of fenders, floating booms, and cutwaters
should be considered for this purpose.
Scour is likely to occur in areas of increased stream velocity.
It is likely to affect the bed of the
waterway around and under the falsework and any banks, channels,
or other existing features of the waterway.
Protection should be provided where such scouring forces are
likely to occur.
Foundations on Sloping Ground The stability of foundations on
sloping ground should be
examined by a qualified engineer specialized in soil mechanics.
For rock slopes, special attention should be
given to the geometry of bedding, cleavage planes, or joint
planes that might provide a sliding surface for block
failure. In many sandstone, siltstone, and mudstone formations,
it is not possible to predict the shear strength at
bedding planes. Here, it is necessary to ensure that the bedding
does not intersect the slope in a manner that
would permit blocks to move out of the face.
Where the requirements are such that foundation members need to
be set other than level,
appropriately shaped packs should be used at the base of the
vertical member. The foundation member should
be effectively prevented from moving down the slope as shown in
figure 10.
2008 by the American Association of State Highway and
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17 Interim2008
Figure 9. Washout under sill support.
(Courtesy of Scaffolding, Shoring, and Forming
Institute)(10)
Figure 10. Sole plate and bracing details for falsework
supported on a sloped surface.
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18
Fill Material Where falsework is to be carried on fill of
unknown origin or quality, the fill should be
investigated. Fill may have abrupt variations in composition,
compaction, and strength. Where falsework is
supported on a compacted fill whose properties have been
determined, it is important to ensure that both the fill
and the underlying ground are protected, so that no disturbance
or loss of material results from the movement of
water or environmental changes. In cases where the fill material
is variable in consistency, and unable to receive
and transmit loads uniformly, a minimum depth of 18 in (457 mm)
of the fill should be removed and replaced
by well-compacted and stabilized granular material of known
bearing capacity.
Heavy Vibrations Deposits or layers of granular materials, if
not fully compacted, are susceptible to
consolidation and settlement if subjected to vibrations either
from the falsework above, from adjacent operations
(for example, piling), or the passage of heavy traffic. This
condition is not accounted for by modification factors
applied to the presumed bearing pressures. Either the granular
materials should be compacted, or the sources of
vibration stopped during critical stages of construction. Some
uniformly graded sands and silts may also be
adversely affected by vibration from the compaction of concrete
above the falsework.
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19 Interim2008
CONSTRUCTION
General In falsework construction, overall stability is a
function of both internal (local) and external (global)
conditions. Internally, falsework can be subject to a wide
variety of local horizontal forces produced by out-of-
plumb members, superelevation, differential settlement, and so
forth. Therefore, it is necessary for the
falsework assembly to be adequately connected to resist these
forces. Although friction often provides means of
load transfer, so-called positive connections eliminate, or at
least reduce, the probability of underestimating
the necessary restraint. The need for positive load transfer is
particularly apparent when superelevation exists or
the soffit is inclined.
Timber cross-bracing between adjacent steel beams, shown in
figure 11, is commonly used for flange
support in falsework construction. In this method, timber struts
are set diagonally in pairs between the top
flanges of the adjacent beams, and securely wedged into place.
However, timber cross-bracing alone will not
prevent flange buckling because the timber struts resist only
compression forces. A more effective flange
support method uses steel tension ties welded, clamped, or
otherwise secured across the top and bottom of
adjacent beams in combination with timber cross-bracing between
the beams.
Uplift can occur when falsework beams are continuous over a long
span, coupled with a relatively
short adjacent span. Two common examples of this condition are
longitudinal beams with short end spans and a
transverse beam with a relatively long overhang. In the
longitudinal example, uplift can occur at the end
support. For the latter case, shown in figure 12, uplift can
occur at the first interior post (support). Both of these
conditions can contribute to instability and, therefore, should
be avoided. If uplift cannot be prevented by
loading the short span first, the end of the beam must be tied
down or the span lengths changed.
In order to ensure longitudinal stability, it is necessary to
provide a system of restraint to prevent the
falsework bents from overturning when the horizontal design load
is applied in the longitudinal direction. This
type of restraint can be furnished by diagonal bracing between
pairs of adjacent bents, or by direction transfer of
horizontal load into the permanent piers.
Timber Construction Lateral Support of Wood Beams Deep, narrow
beams may fail by buckling before the allowable
bending stress is reached if they are not laterally restrained.
The amount of restraint needed to ensure beam
stability is a function of the depth-to-width ratio. Blocking of
soffit joists for haunches is also required.
Section 4.41 of the National Design Specification for Wood
Construction provides approximate
guidelines regarding the lateral restraint of rectangular sawn
lumber beams.(8) These guidelines, modified to
reflect the temporary nature of falsework construction, are as
follows:
If the nominal depth-to-width ratio of a timber beam is 3:1 or
less, no lateral support is needed. If the nominal depth-to-width
ratio exceeds 3:1, but is not more than 4:1, the ends of the
beam
should be braced at the top and bottom
2008 by the American Association of State Highway and
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20 Interim 2008
Figure 11. Timber cross-bracing between longitudinal
stringers.
Figure 12. Cantilevered ledger beam at temporary pile bent.
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23 Interim2008
In general, plan bracing should be provided at the top lifts
(tiers) and at least every third intermediate
lift. The plan bracing at the top tier may be omitted when the
grillage used to support the permanent structure is
capable of acting as a diaphragm. When shoring a sloped surface,
the tube bracing illustrated in figure 14 is
recommended.
Figure 14. Bracing detail for screw leg supporting a sloped
soffit.
Screw-leg Extensions Leg load capacity for modular frames
generally decreases as the screw-leg
extensions increase. Eccentric loads on screw (extension) heads
should also be avoided. Variations between
various proprietary systems preclude generalizations regarding
the extent of load reduction for screw-leg
extension. However, extensions at the top and bottom of a frame
totaling 12 in (305 mm) generally do not
significantly affect the allowable leg capacity.
2008 by the American Association of State Highway and
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violation of applicable law.
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24 Interim 2008
Cable Bracing Bracing systems consisting of securely anchored
cable guys are widely used to resist overturning of
falsework. In particular, cable systems provide an effective
means of ensuring the stability of heavy-duty
shoring and are relatively inexpensive when compared to other
bracing methods. Cable is also used extensively
as temporary bracing to stabilize falsework bents being erected
or removed adjacent to traffic. However, the
effect of preloading the tower legs should be carefully analyzed
before implementing this bracing technique.
The cable bracing should also be applied symmetrically to a
shoring assembly to avoid unbalanced loading or
overturning.
Cables, with their fastening devices and anchorages, are
manufactured assemblies as defined in the
Guide Design Specification for Bridge Temporary Works.
Accordingly, and in addition to information that may
be shown on the falsework drawings, the contractor should be
requested to furnish a manufacturers catalog or
brochure showing technical data pertaining to the type of cable
to be used. Technical data should include the
cable diameter, the number of strands and the number of wires
per strand, the ultimate breaking strength or
recommended safe working strength, and such other information as
may be needed to identify the cable in the
field.
Prior to installation, cable should be inspected to verify that
the type and size of the cable and its
condition (new or used) is consistent with design assumptions.
Used cable should be inspected for strength-
reducing flaws, such as obviously worn, frayed, kinked, or
corroded cable, which should not be permitted in
construction.
U-bolt clips must be placed on the rope with the u-bolts bearing
on the short or dead end of the rope,
and the saddle bearing on the long or live end of the rope.
Improperly installed clips will reduce the save
working load by as much as 90 percent. Also, the omission of the
thimble in a loop connection will reduce the
safe working load by approximately 50 percent. After
installation, clips should be inspected periodically and
tightened as necessary to ensure their effectiveness. General
guidelines regarding the number of wire rope clips
and their spacing are shown in figure 15. However, efficiency
factors and prescribed clip spacings can vary, and
the manufacturers literature should be consulted for a given
application.
Extensive further review of cable bracing may be found in the
California Falsework Manual(13),
Chapter 4, Stress Analysis, Section 4-5, Cable Bracing
Systems.
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violation of applicable law.
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25 Interim2008
Figure 15. Typical installation of wire rope clip.(16)
Bridge Deck Falsework Multiple girder bridges rarely have
ground-supported deck formwork. Deck casting is usually
performed using hanger beams attached to the interior girders,
and cantilever brackets affixed to the exterior or
fascia girders. Figure 16 illustrates this forming method, with
examples for both steel and concrete girders.
Design examples of bridge deck falsework are provided in
appendix B.
The deck forms between interior stringers are generally set on
joists hung from the top flange or
supported from the bottom flange. Proprietary hangers include
removable brackets or coil-bolt assemblies that
remain permanently embedded in the deck slab. The embedded
hangers are generally hung over the top of the
stringer, or welded to stirrups or shear studs projecting from
the top surface. Welding the hangers creates a
positive connection that will prevent movement during casting.
However, several States prohibit welding these
devices to the permanent structure.
In order to form the cantilevered portion of the deck slab, a
needle beam arrangement or overhang
bracket can be used. As shown in figure 16, a needle beam works
well for shallow steel girders where bottom
flange tension hangers can be easily attached. This support
arrangement is temporarily attached to the steel
members, with no embedment anchors required in the slab.
A more common method of forming the overhang consists of an
overhang bracket tied to the
fascia girder with a hanger support. Gravity loads from the
formwork, concrete deck, and screed machine act
downward on the bracket. These loads create a force couple on
the bracket, where tension is resisted by a
hanger support rod and compression is applied horizontally to
the girder web. This compressive force is resisted
by bending in the beam web. For steel stringers, the web could
buckle inward due to this out-of-plane force if
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26
(a) Bridge deck forming methods with steel stringers.
(b) Bridge deck forming methods with precast AASHTO girders.
Figure 16. Bridge deck falsework.
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27 Interim2008
the magnitude of compressive force resistance, and interior
diagonal struts may be required to prevent bottom
flange rotation.
Some general guidelines regarding the use of overhang brackets
are as follows:
The diagonal leg brace should bear on the web within 6 in (152
mm) of the bottom flange. The exterior stringer should have its top
flange tied at regular intervals to prevent outward
rotation. Recommended maximum spacing intervals are 2 ft (0.6
m), when finishing machine rails
are located on the bracket-supported formwork, and 4 ft (1.2 m)
when finishing machine rails are
on the top flange of the stringer.
Precast, prestressed concrete I-girders should have ties at 8-ft
(2.4-m) maximum spacing. Steel girder diaphragm cross frames are
not to be considered as ties if they do not have a top
horizontal strut.
Hardwood blocking [4 in by 4 in (102 mm by 102 mm) minimum] or
the equivalent should be wedged between webs of the exterior and
interior stringer within 6 in (152 mm) of the bottom
flange, located below the top ties.
Decks for cast-in-place box girders may use other hardware for
supporting decks. Deck forms
supported on ledgers are typically used. The ledgers may be
supported on bars/brackets cast into the girder
stems or may be nailed (shot) into, using a power-actuated nail
gun.
Traffic Openings The width of a traffic opening is generally
defined as the distance between the temporary railings and,
as illustrated in figure 17, the clear distance between
falsework posts will be considerably greater than the
prescribed width. For a vehicular opening, no portion of the
falsework should encroach into the clearance zone
established by: a vertical plane located 3 in (76 mm) behind the
back edge of the temporary barrier at its base
and extending upward to a horizontal plane at the top of the
rail; and a second vertical plane located 9 in (230
mm) behind the first plane and extending from the horizontal
plane, at the top of the rail upward to the
falsework stringer.
Temporary construction clearances often govern layout of spans.
A typical example is the required
vertical clearance over freeways in California, shown in table
6. The usual requirement is a clearance of 16 ft-6
in (5.0 m) over the traveled way, but the temporary construction
clearance may be as low as 14 ft-6 in (4.4 m).
However, for a structure constructed on ground-supported
falsework where a 40-ft (12-m) wide opening for
traffic is needed, an adequate depth of falsework may be 2 ft-6
in to 3 ft (0.8 m to 0.9 m). This results in a final
clearance of 17 ft-0 in to 17 ft-6 in (5.2 m to 5.3 m).
2008 by the American Association of State Highway and
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28 Interim 2008
Conversion: 1 in = 25.4 mm; 1 ft = 0.305 m
(a) Minimum clearance diagram.
(b) Set-back distance between traffic barrier and vertical
shoring.
Figure 17. Traffic openings.
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35
CHAPTER 3. FORMWORK
INTRODUCTION
Formwork is a temporary structure that retains plastic or fluid
concrete until it gains sufficient strength
to support itself. The formwork system includes the sheathing
that is in direct contact with the concrete, the
supporting members, hardware, and bracing.
The cost of formwork is significant relative to the cost of the
in-place concrete. Therefore, the selection
and design of formwork can significantly affect the overall cost
of the structure. Formwork selection is
influenced by many factors, including concrete pressures,
uniformity of the structure shape, accessibility to the
structure, crane capacity, materials availability and cost,
anticipated number of reuses, and crew experience.
This chapter presents an overview of formwork components and
corresponding information for design.
Formwork for Concrete, published by the American Concrete
Institute, provides extensive data for design.(19)
Allowable stresses for formwork materials are those used in
standard structural design, except when test data
give different values for proprietary products. Precautions to
be taken in the erection, maintenance, and removal
of forms are also discussed in this chapter.
FORM COMPONENTS Vertical forms are constructed from five basic
components: (1) sheathing, (2) studs to support the
sheathing, (3) walers to support the studs and align the forms,
(4) braces to prevent shifting of the forms under
construction and wind loading, and (5) form ties and spreaders
to hold the forms at the correct spacing under the
pressure exerted by the fresh concrete. The formwork structural
components and accessories should be
integrated to provide sufficient capacity in addition to easy
assembly and disassembly. Typical vertical form
components are illustrated in figure 19.
Figure 19. Formwork components.(19)
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36 Interim 2008
Sheathing Sheathing is the supporting component of the formwork
closest to the concrete. Sheathing materials
consist of wood, plywood, metal, or other products capable of
transferring the load of the concrete to supporting
members such as joists or studs. The following factors should be
considered when selecting a type of sheathing:
strength; stiffness; ease of removal; initial cost; reuse
potential; surface characteristics; resistance to damage
during concrete placement; workability in cutting, drilling, and
attaching fasteners; weight; and ease of
handling. The design information provided here relates to
plywood sheathing because it is the most common
sheathing material. Figure 20 shows horizontal plywood sheathing
for a concrete bridge deck to be supported on
steel girders.
Figure 20. Plywood sheathing for horizontal formwork.
Plywood is widely used for both job-built forms and
prefabricated form modules. Virtually any
exterior type of American Plywood Association (APA) plywood is
appropriate for forming concrete since this
plywood is manufactured with waterproof glue. However, the
plywood industry produces a product called
Plyform that is intended specifically for concrete forming.
Plyform differs from conventional exterior plywood
grades in that Plyform is constructed only from certain wood
species and veneers, and its exterior face panels
have thicker face plies for greater stiffness and are sanded
smooth. Typical Plyform trademarks, which indicate
class, veneer grade, and conformance with applicable standards,
are given in table 7.
Plyform is available in Class I and Class II, with Class I being
the stronger and stiffer panel.
Structural I Plyform is stronger and stiffer than either Class I
or II, and is often recommended for higher
concrete pressures. High Density Overlaid (HDO) Plyform is
available in any of the three classes. The face
plies of HDO Plyform are bonded with a resin-impregnated fiber
overlay, forming a hard, smooth surface that
eases removal and improves moisture resistance.
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37 Interim2008
Table 7. Grade-use guide for Plyform sheathing.(20)
Veneer grade Use these terms when specifying plywooda
Description Typical trademarks Faces Inner piles Backs
APA B-B PLYFORM Class I & IIa
Specifically manufactured for concrete forms. Many reuses.
Smooth, solid surfaces. Mill-oiled unless otherwise specified.
B C B
APA High Density Overlaid PLYFORM Class I & IIb
Hard, semi-opaque resin-fiber overlay, heat-fused to panel
faces. Smooth surface resists abrasion. Up to 200 reuses. Light
oiling recommended between pours.
B C-Plugged B
APA STRUCTURAL I PLYFORMb
Especially designed for engineered applications. All Group 1
species. Stronger and stiffer than PLYFORM Class I and II.
Recommended for high pressures where face grain is parallel to
supports. Also available with High Density Overlay faces.
B C or C-Plugged B
Special Overlays, proprietary panels, and Medium Density
Overlaid plywood specifically designed for concrete forming.b
Produces a smooth uniform concrete surface. Generally
mill-treated with form release agent.
No standard grading; for details of proprietary versions, see
manufacturers specifications.
APA B-C EXT
Sanded panel often used for concrete forming where only on
smooth, solid side is required.
B C C
Notes: (a) Commonly available in 19/32-in (15.1-mm), 5/8-in
(15.9-mm), 23/32-in (18.3-mm), and -in (19.1-mm) panel
thickness [4-ft by 8-ft (1.2-m by 2.4-m) size]. (b) Check dealer
for availability in your area.
Plywood manufactured in the United States is built up of an odd
number of layers, with the grain of
adjacent layers perpendicular. Alternating the grain direction
of adjoining layers minimizes shrinkage and
warping. In determining the flexural strength, shear strength,
and stiffness of a panel, only those layers having
their grain perpendicular to the supporting stud are assumed to
be stressed. The safe span of plywood is
therefore dependent not only on the type of plywood, but also on
whether it is used in the weak direction
(the face grain runs parallel to the supports) or in the strong
direction (the face grain runs perpendicular to the
supports).
2008 by the American Association of State Highway and
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Licensee=Praxair Inc/5903738101, User=Pitanga, Augusto
Not for Resale, 06/01/2008 15:27:08 MDTNo reproduction or
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38 Interim 2008
Formulas for calculating the maximum allowable pressures for
plywood members based on stress and
deflection are given in table 8. Table 9 summarizes section
properties for Plyform Class I and Class II, and
Structural I Plyform. Design stresses and moduli of elasticity
for these plywood classes are given in table 10.
Due to the nature of plywood, the moment of inertia cannot
simply be divided by half of the plywood thickness
to get the section modulus. Therefore, the moment of inertia, I,
is to be used to calculate deflection and the
section modulus, KS, to calculate bending stress.
The design stresses in table 10 are given for Plyform used in
wet conditions such as concrete forming.
Bending stress and rolling stress may each be increased by 25
percent under loads of short duration, though this
applies only when the number of reuses is limited. Since the
limit on the number of reuses is not well defined,
the designer must decide whether to use this factor. Also, the
design stresses may be higher if special conditions
exist, such as if the Plyform is well sealed against moisture so
that the moisture content always remains below
16 percent. In addition to plywood strength, the designer must
consider the effect of reuse on the permanent set
or deflection of the plywood.
Table 8. Formulas for stress and deflection calculations for
plywood.(20)
2 spans 3 spans
Maximum allowable pressure, wb (lbf/ft2) based on bending
stress
b
b 2
l
96F KSw
l=
b
b 2
1
120F KSw
l=
Maximum allowable pressure, ws (lbf/ft2) based shear stress
( )2
ss
19.2F lb / Qw
l= ( )
2
ss
20F lb / Qw
l=
Bending deflection, b (in) 4
3
b
wl
2220EI =
4
3
b
wl
1743EI =
Shear deflection, s (in) 2
2
2
se
Cwt l
127E I =
To calculate the maximum allowable pressure based on maximum
allowable deflection, all., calculate b and s with w = 1.0 lbf/ft2.
Then the maximum allowable pressure based on deflection, w (in
lbf/ft2) is calculated as follows:
all .
s b
w =
+
Conversion: 1 lbf/ft2 = 47.9 N/m2; 1,000 lbf/in2 = 6.89 N/mm2; 1
in = 25.4 mm; 1 ft = 0.305 m. W = uniform load, lbf/ft2 Fb =
bending stress, lbf/in2 Fs = rolling shear stress, lbf/in2 lb/Q =
rolling shear constant, in2/ft KS = effective section modulus,
in3/ft I = moment of inertia, in4/ft E = modulus of elasticity,
adjusted, lb/in2 Ee = modulus of elasticity, unadjusted, lb/in2
11 = span, center-to-center of supports, in 12 = clear span,
(in) 13 = clear span + in for 2-in framing, or clear span + 5/8 in
for 4-in framing, in = deflection, in C = constant = 120 parallel,
60 perpendicular t = plywood thickness, in
2008 by the American Association of State Highway and
Transportation Officials.All rights reserved. Duplication is a
violation of applicable law.
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Transportation Officials Provided by IHS under license with AASHTO
Licensee=Praxair Inc/5903738101, User=Pitanga, Augusto
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39 Interim2008
Table 9. Section properties for Plyform Class I and Class II,
and Structural I Plyform.(20) Properties for stress applied
parallel with face grain Properties for stress applied
perpendicular with grain
Thickness (in)
Approximate weight (lbf/ft2)
Moment of inertia I (in4/ft)
Effective section modulus KS
(in3/ft)
Rolling shear constant lb/Q
(in2/ft) Moment of
inertia I (in4/ft)
Effective section modulus KS
(in3/ft)
Rolling shear constant lb/Q
(in2/ft)
Class I
15/32 1.4 0.066 0.244 4.743 0.018 0.107 2.419
1/2 1.5 0.077 0.268 5.153 0.024 0.130 2.739
19/32 1.7 0.115 0.335 5.438 0.029 0.146 2.834
5/8 1.8 0.130 0.358 5.717 0.038 0.175 3.094
23/32 2.1 0.180 0.430 7.009 0.072 0.247 3.798
3/4 2.2 0.199 0.455 7.187 0.092 0.306 4.063
7/8 2.6 0.296 0.584 8.555 0.151 0.422 6.028
1 3.0 0.427 0.737 9.374 0.270 0.634 7.014
1 1/8 3.3 0.554 0.849 10.430 0.398 0.799 8.419
Class II
15/32 1.4 0.063 0.243 4.499 0.015 0.138 2.434
1/2 1.5 0.075 0.267 4.891 0.020 0.167 2.727
19/32 1.7 0.115 0.334 5.326 0.025 0.188 2.812
5/8 1.8 0.130 0.357 5.593 0.032 0.225 3.074
23/32 2.1 0.180 0.430 6.504 0.060 0.317 3.781
3/4 2.2 0.198 0.454 6.631 0.075 0.392 4.049
7/8 2.6 0.300 0.591 7.990 0.123 0.542 5.997
1 3.0 0.421 0.754 8.614 0.220 0.812 6.987
1 1/8 3.3 0.566 0.869 9.571 0.323 1.023 8.388
Structural I
15/32 1.4 0.067 0.246 4.503 0.021 0.147 2.405
1/2 1.5 0.078 0.271 4.908 0.029 0.178 2.725
19/32 1.7 0.116 0.338 5.018 0.034 0.199 2.811
5/8 1.8 0.131 0.361 5.258 0.045 0.238 3.073
13/32 2.1 0.183 0.439 6.109 0.085 0.338 3.780
3/4 2.2 0.202 0.464 6.189 0.108 0.418 4.047
7/8 2.6 0.317 0.626 7.539 0.179 0.579 5.991
1 3.0 0.479 0.827 7.978 0.321 0.870 6.981
1 1/8 3.3 0.623 0.955 8.841 0.474 1.098 8.377
Notes: (a) All properties adjusted to account for reduced
effectiveness of plies with grain perpendicular to applied stress.
(b) Conversion: 1 in = 25.4 mm; 1 ft = 0.305 ft; 1 lbf/ft2 = 47.9
N/m2.
Table 10. Design stresses for Plyform.(20) Plyform Class I
Plyform Class II Structural I Plyform
Modulus of elasticity E (lbf/in2, adjusted, use for bending
deflection calculation)
1,650,000 1,430,000 1,650,000
Modulus of elasticity Ee (lbf/in2, unadjusted, use for shear
deflection calculation)
1,500,000 1,300,000 1,500,000
Bending stress Fb (lbf/in2) 1,930 1,330 1,930
Rolling shear stress Fs (lbf/in2) 72 72 102
Table 10 has been increased by 25% for short duration loads.
Conversion: 1,000 lbf/in2 = 6.89 N/mm26
2008 by the American Association of State Highway and
Transportation Officials.All rights reserved. Duplication is a
violation of applicable law.
Copyright American Association of State Highway and
Transportation Officials Provided by IHS under license with AASHTO
Licensee=Praxair Inc/5903738101, User=Pitanga, Augusto
Not for Resale, 06/01/2008 15:27:08 MDTNo reproduction or
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40
In addition to plywood, reconstituted wood materials are
available for use as sheathing or as form
liners. Only those materials manufactured for forming
applications, with edge sealing and surface treatment, can
be expected to endure as well as treated plywoods. Forms that
are built similarly to steel plate girders, described
later in this chapter, are composed of webs, flanges, and
stiffeners, with the webs in direct contact with the
concrete. Steel has high strength, stiffness, and durability,
but is heavier and therefore more cumbersome to
work with. For pier caps and other applications where conduit
and plumbing penetrations are limited, however,
steel formwork is often utilized if enough reuses to justify the
cost of steel forms are anticipated. Fiberglass
reinforced plastic forms are strong, lightweight, can be readily
fabricated to non standard shapes, and can be
extensively reused. These forms are common in the construction
of round columns, as are spiral wound waxed
paper tubes and all-steel, two-piece column forms.
Structural Supports For vertical wall forms, the form ties and
sheathing transfer the lateral loads from fluid concrete to
studs and walers. As with sheathing, important considerations in
the selection of structural support members
include strength, stiffness, dimensional accuracy and resistance
to permanent deflection, workability, weight,
cost, and durability. In proprietary modular forms, these
structural supports and aligners may be made of steel,
aluminum, magnesium, or lumber. Design information for
proprietary systems is available from the
manufacturer.
Almost all formwork jobs, regardless of the types of primary
materials selected, usually require some
lumber. Lumber that is straight and free from defects may be
used for formwork. Softwoods are generally most
economical for all types of formwork. Partially seasoned stock
is usually preferred for concrete forming,
because dried lumber can swell excessively when wet and green
lumber tends to dry out and warp during hot
weather, thus causing problems in form alignment. Information on
the design of structural lumber is presented
in this chapter. Since lumber species, grades, sizes, and
lengths vary geographically, local supplies will be the
primary source of advice for the specific materials and sizes
that are available.
Lumber may be finished on all four sides and is then referred to
as standard dressed or S4S lumber.
When it is used directly as it comes from the sawmill, the
lumber is designated as rough. Properties of standard
lumber sizes common in formwork construction are identified in
appendix B.
Guidelines discussed in Chapter 2. Falsework to ensure correct
timber quality and size of material are
also applicable to formwork. Expressions commonly used to
determine support spacing are provided in table 11
and general beam formulas are provided in table 12. Allowable
stresses and strength factors are specified in the
NDS Supplement Design Values for Wood Construction.(21)
In addition to designing structural lumber to withstand bending
and shear stresses, consideration must
also be given to bearing stresses. Allowable bearing stresses
for loads applied parallel to the grain and loads
perpendicular to the grain are also given in the NDS
Supplement.
2008 by the American Association of State Highway and
Transportation Officials.All rights reserved. Duplication is a
violation of applicable law.
Copyright American Association of State Highway and
Transportation Officials Provided by IHS under license with AASHTO
Licensee=Praxair Inc/5903738101, User=Pitanga, Augusto
Not for Resale, 06/01/2008 15:27:08 MDTNo reproduction or
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-
45
(a) Flat tie.
(e) Paper tie.
(b) Snap tie.
(f) Threaded bar tie.
(c) Wire panel tie.
(g) She-bolt.
(d) Pull-out tie.
(h) Coil tie.
Figure 21. Form ties.
2008 by the American Association of State Highway and
Transportation Officials.All rights reserved. Duplication is a
violation of applicable law.
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Transportation Officials Provided by IHS under license with AASHTO
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46 Interim 2008
Construction of forms with coil tie systems begins with the
erection of one side of the form and
installation of the coil tie system as shown in figure 22. The
reinforcing steel is then positioned, the closure
forms erected, and the remaining tie hardware installed. With
this installation technique, the reinforcing steel is
not positioned in front of tie holes and therefore does not
interfere with the tie installation. However, the coil tie
system does not provide the option of being fed through the
forms. The external hardware has a high initial cost,
but can be reused.
Figure 22. Coil tie system.
Form Hangers The proprietary form hangers used with bridge deck
formwork are generally the
same for cast-in-place decks supported on steel girders and on
precast girders. A variety of formwork hangers
are available for the construction of bridge decks. Examples of
an exterior hanger and of an interior hanger are
illustrated in figure 23.
Exterior hangers are designed to support the overhanging portion
of a bridge deck on the fascia beam
of the bridge. Exterior hangers generally consist of a vertical
support on the interior side of the fascia beam and
an exterior angled support typically used to support an overhang
bracket on the exterior face of the beam. An
interior hanger, as shown in figure 23, may be equipped with a
fixed length or adjustable coil bolt assembly.
Form hanger capacities generally range from 2,000 lbf (8,800 N)
to 6,000 lbf (26,400 N).
2008 by the American Association of State Highway and
Transportation Officials.All rights reserved. Duplication is a
violation of applicable law.
Copyright American Association of State Highway and
Transportation Officials Provided by IHS under license with AASHTO
Licensee=Praxair Inc/5903738101, User=Pitanga, Augusto
Not for Resale, 06/01/2008 15:27:08 MDTNo reproduction or
networking permitted without license from IHS
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49
Conversion: 1 lbf/ft2 = 47.9 N/m2; 1 ft = 0.305 m; (F 32)/1.8 =
C
Figure 25. Lateral pressure of concrete on formwork.
FORMWORK TYPES Bridge formwork can be divided into two
categories: vertical and horizontal formwork. Vertical
formwork can be constructed using job-built systems or
prefabricated systems. Horizontal formwork can be
constructed utilizing job-built, prefabricated, or permanent
stay-in-place systems. These systems are defined as:
Job-Built Formwork a formwork system designed and built for a
specific application, most commonly using plywood and lumber.
Prefabricated or Modular Formwork a modular system that has the
durability for multiple reuses and normally is built with plywood
with a metal framing. Prefabricated formwork can be built for
custom uses on special projects.
2008 by the American Association of State Highway and
Transportation Officials.All rights reserved. Duplication is a
violation of applicable law.
Copyright American Association of State Highway and
Transportation Officials Provided by IHS under license with AASHTO
Licensee=Praxair Inc/5903738101, User=Pitanga, Augusto
Not for Resale, 06/01/2008 15:27:08 MDTNo reproduction or
networking permitted without license from IHS
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50 Interim 2008
Stay-in-Place Formwork a formwork system designed such that the
formwork is not removed after construction. This system most
commonly consists of stay-in-place metal decks or precast
concrete planks for forming concrete deck systems.
Job-Built Formwork Job-built wood forms have a low initial
material cost, but generally require much labor and can only be
used 10 to 15 times. The labor cost to repair and erect
job-built wood forms is high compared to that for
prefabricated modular forms that have much greater reuse
potential. An example of a job-built form in bridge
construction is given in figure 26.
Figure 26. Job-built formwork.
Modular Formwork The term modular form refers to all-metal forms
or metal-supported-plywood systems, whose
integrated design of tie and connecting hardware is engineered
to assure dimensional control, speed of erection,
and ease of stripping as well as structural integrity. Care must
be taken when assembling modular forms to
ensure tight and well-aligned joints with no offsets. Also,
these forms must be inspected for permanent set or
deflection that may occur after many reuses.
The most common modular forms consist of steel frames with
replaceable plywood faces. This
combination provides the job-site workability of plywood and the
large tie spacing and form durability of steel.
Overlaid plywood further extends the form-face wear, and yet can
be nailed or cut. The most successful of these
systems utilize high-carbon steel to minimize weight. The steel
portion of the form is generally designed to
protect the edges of the plywood and absorb tie loads and
stripping, wracking, and lifting stresses. Since ties fit
between panel joints (instead of through the plywood), the steel
frame absorbs the tie loads and the wear. All-
steel forms are practical for piers and columns since they
provide great rigidity and strength and can be rapidly
2008 by the American Association of State Highway and
Transportation Officials.All rights reserved. Duplication is a
violation of applicable law.
Copyright American Association of State Highway and
Transportation Officials Provided by IHS under license with AASHTO
Licensee=Praxair Inc/5903738101, User=Pitanga, Augusto
Not for Resale, 06/01/2008 15:27:08 MDTNo reproduction or
networking permitted without license from IHS
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-
51
erected, disassembled, moved, and re-erected. A sufficient
number of reuses must be expected to justify the
high initial cost. Also, special precautions must be taken when
placing concrete in cold weather since the all-
steel forms provide little or no insulation protection to the
concrete.
Lightweight modular forms are also made of aluminum and
magnesium, but are susceptible to
deterioration from contact with fresh concrete. They should,
therefore, only be used if suitably coated or as a
structural support with a separate sheathing material. Aluminum
extrusions can provide bolt slots, nailer
pockets, and other special features. Aluminum beams and
double-channel walers provide large gang-wall forms
that are exceptionally lightweight and straight due to the
nature of the extrusions.
Stay-in-Place Formwork In areas where form removal is expensive
or hazardous, the use of stay-in-place (SIP) forms may be
desirable. SIP forms help facilitate the construction of bridge
decks over high-traffic areas. The additional dead
weight of the deck slab, appearance, and corrosiveness of the
environment are some of the factors that should be
considered when deciding if metal or precast concrete SIP forms
should be used. Ribbed metal deck and precast
concrete elements may act solely as formwork for cast-in-place
concrete, or may act compositely with the
concrete and become part of the load-bearing structure. Welding
to flanges in tension zones or to structural
elements fabricated from nonweldable grades of steel is
generally prohibited.
Gang Forms Gang forms consist of prefabricated formwork panels
that include sheathing, studs, and walers, joined
into larger units for ease in erecting, removal, and reuse.
These systems are quickly assembled and permit
repetitive uses without rebuilding for efficient wall
construction. Modular units are fastened to each other and to
lift brackets, lift beams, tag lines, and possibly a work
platform while still on the ground. Vertical angles may
also be provided along the edges in order to attach individual
gang forms with bolts or special steel clamps.
Although gang forms may be used as hand-set units, they are more
commonly lifted into place by
cranes and are therefore limited in size only by the crane
capacity. The use of large gang forms helps to offset
the high cost of labor, through large forms do not easily
accommodate odd shapes or field adjustments.
Integration of relatively small modular panels with large gang
forms maximizes the benefits of both systems.
After the concrete becomes self-supporting, the forms can be
removed as large units and efficiently reused.
Lift brackets are attached to a lift beam or directly to gang
form structural elements that must have
sufficient strength to withstand the inclined loads from the
slings during lifting. Gang forms used in multi-lift
applications must be supported by specially designed inserts,
anchors, and brackets because these are in turn
supported by freshly cured concrete.
A gang form, equipped with a working platform, is shown in
figure 27. The entire unit is lifted into
place and then removed as a unit when the concrete has gained
sufficient strength. Gang forms are well suited to
the construction of walls as shown in figure 28.
2008 by the American Association of State Highway and
Transportation Officials.All rights reserved. Duplication is a
violation of applicable law.
Copyright American Association of State Highway and
Transportation Officials Provided by IHS under license with AASHTO
Licensee=Praxair Inc/5903738101, User=Pitanga, Augusto
Not for Resale, 06/01/2008 15:27:08 MDTNo reproduction or
networking permitted without license from IHS
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52 Interim 2008
Figure 27. Assembled gang form.
Figure 28. Gang form for wall construction.
2008 by the American Association of State Highway and
Transportation Officials.All rights reserved. Duplication is a
violation of applicable law.
Copyright American Association of State Highway and
Transportation Officials Provided by IHS under license with AASHTO
Licensee=Praxair Inc/5903738101, User=Pitanga, Augusto
Not for Resale, 06/01/2008 15:27:08 MDTNo reproduction or
networking permitted without license from IHS
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53
Plate Girder Forms Plate girder forms, such as the one shown in
figure 29, are well suited for the construction of bridge
pier caps. These systems are capable of forming concrete while
structurally spanning between supports with no
intermediate shoring. In many applications, these panels also do
not require external walers. The large tie
spacing and high pressure capacity provide form tie cost
advantages in spite of the high form cost and weight.
Larger plate girder modules create fewer joints to seal, align,
and finish. The most significant cost-savings result
is from the self-spanning capabilities of this system, which
makes bridge pier construction possible while
minimizing the amount of falsework.
In plate girder form systems, the web of the steel girder
doubles as a form face. The steel ribs of the
girder serve as web stiffeners to support the weight of the form
and concrete. They also act as beams to transfer
the horizontal pressures of the liquid concrete from the form
web to the form top and bottom flanges. The plate
girder forms come in modules that are bolted together, as
needed, for specific project. Proprietary bolting
hardware allows the transfer of flange forces between individual
modules, thereby allowing the formwork
system to span between supports without intermediate shoring.
Examples of plate girder forms are given in
figures 29 and 30.
Conversion: 1 ft = 0.305 m; 1 in = 25.4 mm
(Courtesy of Economy Forms Corporation)
Figure 29. Plate girder form spanning between two supports.
2008 by the American Association of State Highway and
Transportation Officials.All rights reserved. Duplication is a
violation of applicable law.
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Transportation Officials Provided by IHS under license with AASHTO
Licensee=Praxair Inc/5903738101, User=Pitanga, Augusto
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networking permitted without license from IHS
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54 Interim 2008
(Courtesy of Economy Forms Corporation)
Figure 30. Plate girder forms used to form a bridge pier.
CONSTRUCTION
It is essential that formwork is erected as designed. The
assumptions made in the design of the
formwork, such as rate of concrete placement, should be
designated on the shop drawings and confirmed during
construction. Guidelines that apply to the safe construction of
formwork are as follows:
In addition to inspection prior to concrete placement,
inspection should continue during the pour to ensure early
recognition of possible form displacement or failure. A supply of
extra bracing
materials necessary in an emergency should be readily
available.
Construction materials, including concrete, must not be dropped
or pile