ECONOMIC AND DESIGN IMPACTS OF CROSS-FRAME LAYOUT OPTIONS FOR STEEL I- GIRDER BRIDGES WITH SEVERE SKEWS DUSTEN OLDS, P.E. NORMAN L. (NORM) MCDONALD, P.E. AHMAD ABU-HAWASH, P.E. TODD HORTON, P.E. BIOGRAPHY Dusten Olds is a Professional Associate and Bridge Engineer with HDR Engineering in Omaha, NE. His background includes experience in complex design and modeling as well as load rating of multiple structure types. Mr. Olds received his BS and Masters of Engineering from Washington University in St. Louis along with an MBA from University of Nebraska at Omaha. Norman L. (Norm) McDonald is the State Bridge Engineer for the Iowa Department of Transportation. He has worked for the DOT for 30 years with the last 15 years as State Bridge Engineer. Mr. McDonald is a member of the AASHTO Subcommittee on Bridges and Structures and serves as Chairman of the Technical Committee for Structural Steel Design (T-14), Vice-Chair of the Technical Committee for Structural Supports for Signs, Luminaires and Traffic Signals (T-12), and is a Region III member on the Technical Committee for Bridge Preservation (T-9). Ahmad Abu-Hawash is the Chief Structural Engineer with the Iowa Department of Transportation and has been working with the DOT in highway construction, bridge rating, and bridge design since 1983. He is responsible for overseeing the design of major bridge projects, design policy review, coordination of bridge research, and the resolution of structural fabrication issues. Ahmad received his BS degree from the University of Iowa and his MS degree in Structural Engineering from Iowa State University. Todd Horton is a vice president and a senior project manager for HDR Engineering in Omaha, NE. His background includes extensive experience in the design and analysis of tangent and horizontally-curved steel plate girder and box girder bridges using conventional and finite element methods. Mr. Horton received his BS and his MS degrees in Civil Engineering from the University of Nebraska. SUMMARY Two multi-span severely skewed steel bridges with alternate cross-frame layouts were designed to assess the economics of a staggered cross- frame layout versus a contiguous layout. For each bridge, the cross- frames and girders were designed utilizing consistent design parameters to assure a uniform comparison and produce a practical design. Factors affecting the economy of the designs and parameters influencing the various limit states for the design of the two example bridges are presented. Further, the study evaluates the recent AASHTO LRFD specification changes specific to cross-frames on the bridge designs. The paper investigates how the choice of cross-frame layout influences the design and cost of the structure, thus helping the engineer make educated decisions for future steel I-girder superstructure designs.
12
Embed
ECONOMIC AND DESIGN IMPACTS OF CROSS-FRAME … · ... and bridge design since ... steel I-girder superstructure ... For example, the 7th Edition of AASHTO LRFD Design Specification
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
ECONOMIC AND
DESIGN IMPACTS
OF CROSS-FRAME
LAYOUT OPTIONS
FOR STEEL I-
GIRDER BRIDGES
WITH SEVERE
SKEWS
DUSTEN OLDS, P.E.
NORMAN L. (NORM)
MCDONALD, P.E.
AHMAD ABU-HAWASH, P.E.
TODD HORTON, P.E.
BIOGRAPHY
Dusten Olds is a Professional
Associate and Bridge Engineer
with HDR Engineering in
Omaha, NE. His background
includes experience in complex
design and modeling as well as
load rating of multiple structure
types. Mr. Olds received his BS
and Masters of Engineering
from Washington University in
St. Louis along with an MBA
from University of Nebraska at
Omaha.
Norman L. (Norm) McDonald is
the State Bridge Engineer for
the Iowa Department of
Transportation. He has worked
for the DOT for 30 years with
the last 15 years as State Bridge
Engineer. Mr. McDonald is a
member of the AASHTO
Subcommittee on Bridges and
Structures and serves as
Chairman of the Technical
Committee for Structural Steel
Design (T-14), Vice-Chair of
the Technical Committee for
Structural Supports for Signs,
Luminaires and Traffic Signals
(T-12), and is a Region III
member on the Technical
Committee for Bridge
Preservation (T-9).
Ahmad Abu-Hawash is the
Chief Structural Engineer with
the Iowa Department of
Transportation and has been
working with the DOT in
highway construction, bridge
rating, and bridge design since
1983. He is responsible for
overseeing the design of major
bridge projects, design policy
review, coordination of bridge
research, and the resolution of
structural fabrication issues.
Ahmad received his BS degree
from the University of Iowa and
his MS degree in Structural
Engineering from Iowa State
University.
Todd Horton is a vice president
and a senior project manager for
HDR Engineering in Omaha,
NE. His background includes
extensive experience in the
design and analysis of tangent
and horizontally-curved steel
plate girder and box girder
bridges using conventional and
finite element methods. Mr.
Horton received his BS and his
MS degrees in Civil
Engineering from the University
of Nebraska.
SUMMARY
Two multi-span severely
skewed steel bridges with
alternate cross-frame layouts
were designed to assess the
economics of a staggered cross-
frame layout versus a
contiguous layout.
For each bridge, the cross-
frames and girders were
designed utilizing consistent
design parameters to assure a
uniform comparison and
produce a practical design.
Factors affecting the economy
of the designs and parameters
influencing the various limit
states for the design of the two
example bridges are presented.
Further, the study evaluates the
recent AASHTO LRFD
specification changes specific to
cross-frames on the bridge
designs. The paper investigates
how the choice of cross-frame
layout influences the design and
cost of the structure, thus
helping the engineer make
educated decisions for future
steel I-girder superstructure
designs.
Page 1 of 11
ECONOMIC AND DESIGN IMPACTS OF CROSS-FRAME
LAYOUT OPTIONS FOR STEEL I-GIRDER BRIDGES
WITH SEVERE SKEWS
Introduction
It has been well documented that steel plate
girder bridges with severe support skews can
develop large cross-frame forces due to relative
girder deflections particularly in the vicinity of
the skewed supports. In recent publications, it
has been suggested that skewed bridges with
staggered cross-frame layouts (staggered) could
be more cost efficient than contiguous cross-
frame layouts which are in-line transverse to the
girders (contiguous). However, there has been
limited data in the form of complete designs to
evaluate the economic impact of cross-frame
layout. It is understood that a staggered cross-
frame layout can result in significantly reduced
cross-frame forces for severely skewed bridges;
however, the trade-off is an increase in girder
bending moment and flange lateral bending
moments.
To evaluate the impact of the cross-frame layout
on a bridge design, two real world bridges, I-80
mainline in Council Bluffs, Iowa and US-75 in
Bellevue, Nebraska were chosen due to their
span lengths, bridge widths, skews and stiffness
to evaluate the effects on girder bending, shear
and fatigue as well as axial forces in the cross-
frame members. Three dimensional finite
element analysis was utilized to account for the
cross-frame stiffness, relative deformations and
flange lateral moments.
Additionally, the influence of recent AASHTO
LRFD specification changes specific to cross-
frames were evaluated on the two bridges noted
above. For example, the 7th Edition of
AASHTO LRFD Design Specification (2014) (§ C4.6.3.3.4)(1) allows the reduction of the axial
rigidity of the cross-frame members to 0.65AE
for single angle members and flange-connected
tee-sections to account for end eccentricity. In
2015, the Interim Revisions (§ C6.6.1.2.1)
changed the fatigue loading for cross-frames to
confine truck placement to one critical
transverse position per each longitudinal
position throughout the length of the bridge. In
the previous code provisions, the truck
positioning included two different transverse
positions and allowed a reduction factor of 0.75
to account for the reduced probability of
adjacent truck positioning over millions of
cycles. These two provision changes were
incorporated into the designs to assess their
effects on each design element and their effects
on the design economy.
Bridge Layout
The purpose of the study was to try to quantify
the design impact and, consequently, the
economic impact of altering the cross-frame
layout on severely skewed bridges. It was
important to select actual bridges that are
representative of current design practice. Studies
have proven that a staggered cross-frame layout
can reduce the cross-frame forces by reducing
the transverse bridge stiffness. The consequence
of reducing the stiffness is an increase in
primary bending moments of the girders and the
more impactful increase in the flange lateral
bending moments due to the cross-frames
staggered (non-contiguous) alignment. It has
been suggested by others that the decrease in the
cross-frame forces would outweigh the increase
in the demand on the girders producing a more
economic design. To evaluate the impact of the
cross-frame layout, the bridges needed to
possess certain physical attributes. The
following attributes were deemed important to
the evaluation of cross-frame layout.
Bridge Attributes Contributing to
High Cross-frame Forces
Skew is the primary reason cross-frames in
bridges with a tangent alignment are designed
for primary force effects. These force effects are
a result of differential deflections between
adjacent girders and are dependent on the skew
and girder spacing. As skew and girder spacing
Page 2 of 11
increases, substantial forces can be induced in
the cross-frames.
As Bridge Width increases, the impact of the
skew is magnified in the negative moment
regions where cross-frames extend from a rigid
point at the support outward to a more flexible
location along adjacent girders.
Span Arrangement: Shorter span lengths or
unbalanced span arrangements are often
susceptible to primary bending fatigue with high
ADTT. Fatigue can have a significant impact on
the cross-frames and the girder flange sizes due
to being subjected to higher fatigue stress
ranges. Bridges which have details governed by
the Fatigue Limit State may be impacted by the
additional flange lateral bending resulting from a
staggered cross-frame pattern.
L/D Ratio is a measure of girder flexibility.
More flexibility increases the demand on cross-
frame members due to the increased relative
deflections between adjacent girders. The span-
to-beam depth (L/D) ratio limit as specified by
AASHTO is merely a guide due to the fact it
does not incorporate the girder stiffness nor the
girder spacing. However, a high L/D ratio
suggests the bridge is a relatively flexible
structure which may increase the demand on the
cross-frames.
Bridge Configurations
Two bridges, each with skews of approximately
45 degrees were designed with alternate
staggered and contiguous cross-frame layouts.
The first structure is the I-80 mainline bridge in
Council Bluffs, Iowa which consists of a 1227
foot, 5 span unit with 44 degree skewed supports
and 270 foot maximum span lengths (Figure 1).
The bridge has
relatively constant
support skews of
approximately 45
degrees at the abutment
and piers, a fairly wide
bridge width of 63 feet,
L/D ratio of 32.7 which is less than the
suggested AASHTO limit (indicating a stiffer
bridge) and the dead load to live load ratio is
such that girder fatigue is not a limiting
criterion. Based on the bridge attributes
discussed above, the I-80 Bridge would be
expected to have high cross-frame forces.
The second bridge is the
US75 Bridge in
Bellevue, Nebraska
which consists of a 462
foot, 3 span bridge with
45 degree skews and 130
to 190 foot spans (Figure
2). The US75 bridge has relatively constant
support skews of approximately 45 degrees at
the abutment and piers, bridge width of 45 feet,
L/D ratio of 40 which is shallower than the
suggested AASHTO limit (indicating a flexible
bridge) and the dead load to live load ratio and
span balance are such that girder fatigue is a
limiting criteria in the positive moment regions
for girder bending. Based on the bridge
attributes discussed above, the US75 Bridge
would be expected to have high cross-frame
forces, also.
Cross-Frame Layout Options
A contiguous pattern and a staggered pattern of
cross-frame layouts were investigated for the
design of the two bridges noted above. The two
cross-frame layouts are shown in Figures 1 & 2
for each of the two bridges.
Staggered cross-frame layouts are typically used
to mitigate the “nuisance stiffness” (2) of the
structure resulting from the transverse stiffness
of the cross-frames at the supports. If the cross-
frames were totally eliminated or the transverse
stiffness significantly lowered, the girder shear
forces would be carried by the girder directly to
the bearings neglecting any slab stiffness.
Increasing the cross-frame stiffness will create
another load path to the pier supports attracting a
percentage of the girder shears. A staggered
cross-frame layout reduces the transverse
stiffness by offsetting the cross-frames rather
than aligning them transversely across the
structure. The offset allows the girder flanges to
deform transversely reducing the stiffness.
Unfortunately, rarely is anything free. The
resulting lateral deformation of the flange plate,
due to the staggered cross-frame alignment,
results in lateral bending moments in the flange
that must be accommodated in the design of the
girders.
Page 3 of 11
Contiguous cross-frame layouts can lead to
excessive cross-frame forces at the skewed
supports. Therefore, there is an advantage to
eliminate select cross-frames near the supports
to reduce the transverse stiffness at these
locations. Cross-frames were removed parallel
to the pier on each side of the pier as shown in
Figures 1 & 2. This pattern is very similar to the
staggered layout except that the stagger is only
located adjacent to the pier and not throughout
the length of the bridge. The advantage of this
layout is the nuisance stiffness near the pier is
reduced while the more effective contiguous
layout is utilized in the positive moment regions.