Progressive Collapse Requirements

7/25/2019 Progressive Collapse Requirements

http://slidepdf.com/reader/full/progressive-collapse-requirements 1/6STRUCTURE magazine August 2007

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Progressive Collapse RequirementsCold-Formed Steel Load Bearing ConstructionBy Nabil A. Rahman, Ph.D., P.E., Michael Booth and Gary Bennett

In 2001, the Department of Defense(DoD) published its design guidelinesto resist the progressive collapse of

structures resulting from an extremeloading event in the Unified FacilitiesCriteria UFC 4-023-03 Design ofBuildings to Resist Progressive Collapse .This document provides the designrequirements necessary to reduce thepotential of progressive collapse fornew and existing DoD facilities.The use of UFC 4-023-03 is controlled

by the applicability requirementsof UFC 4-10-01 document DoD

Minimum Antiterrorism Standards forBuildings . UFC 4-023-03 covers the

design strategies and requirements forbuildings made of reinforced concrete,structural steel, masonry, wood, andcold-formed steel (CFS). The criteriaincludes design examples and designdetails for all the above materialsexcept cold-formed steel. It is theobjective of this article to discuss theprogressive collapse requirements inUFC 4-010-01 and UFC 4-023-03 asit relates to mid-rise construction usingcold-formed steel load bearing walls.

CFS Load BearingConstruction

In the opinion of the authors, the

use of CFS load bearing framing intypical mid-rise construction (Figure 1)brings the value of reducing associatedconstruction cost in comparison toother structural framing systems suchas reinforced masonry, structural steeland wood. A key component of the costsavings is the lighter mass of the structurerelative to the design of the lateral forceresisting system and the foundation. Itis important to note that several typesof floor systems can be integrated withCFS load bearing walls. Among these

types are composite steel deck-concreteslabs, CFS joists or open bar joists witha concrete or wood floor diaphragm,and pre-cast hollowcore concrete planks.The structural system combining CFSload bearing walls and hollowcoreplanks is ideal for low and mid-riseconstruction of three to eight storiesin height. Because precast hollowcoreconcrete planks provide the advantagesof high stiffness-to-weight ratio andrapid field installation, this allows for a

wide range of building usage includingapartment buildings, office buildings,health care facilities, hotels, schools,and dormitories.

Progressive CollapseRequirements

UFC 4-010-01 requires that all newand existing buildings of three storiesor higher be designed to avoid progres-sive collapse. UFC 4-023-03 offers twolevels of design procedures to resist pro-gressive collapse:• The first level of progressive collapse

design employs the “Tie Forces”method, which is based on the catenary(membrane tension) response of thestructure. This design level may be usedfor buildings assigned Very Low andLow Levels of Protection (VLLOP andLLOP). Horizontal ties only are requiredfor buildings assigned VLLOP, whilehorizontal and vertical ties are requiredfor buildings assigned LLOP.• The second level of progressive

collapse design employs the “AlternatePath” method, which is based on flexuralperformance of the floor system, as thebuilding must bridge across removedvertical supporting elements. Thisdesign level must be used for buildingassigned Medium and High Levels ofProtection (MLOP and HLOP).Tie Forces requirements and additional

ductility requirements must be alsosatisfied for MLOP and HLOP. In the

case that an adequate Tie Force cannotbe developed in a vertical structuralelement, then the Alternate Pathmethod is allowed to be used to verifythat the building can bridge over thedeficient element.DoD mid-rise building projects

include, but are not limited to, housingfacilities (barracks) in addition todining, gathering, operations centers,administrative buildings, and healthcare facilities. These types of facilitiesare typically assigned VLLOP or

LLOP (UFC 4-010-01, Table B-1). As a result, the Tie Forces method canbe used by the designer to satisfy theprogressive collapse resistance part ofthe design. The Tie Forces method is asimple, indirect design approach thatenhances the continuity, ductility andstructural redundancy of the building byrequiring horizontal and vertical “ties” tomechanically keep the building togetherin the event of an extreme loading thatcauses local damage. Figure 2 (see page 10 )

Figure 1: Cold formed steel load bearing mid-rise construction.



shows the different types of ties that mustbe provided in the building design. Thereare two main groups of ties; horizontal andvertical. Horizontal ties include internalties within the floor system, ties to edgecolumns or walls, ties to corner columns,and peripheral ties around the perimeter ofthe building. While horizontal ties mustprovide a straight continuous load path fromone building edge to the other, vertical tiesmust provide a straight continuous load path

in columns and load bearing walls. Along aparticular horizontal or vertical load path,different structural elements may be used toprovide the required tie strength. Adequatemanufactured connections must be providedbetween these structural elements to ensurethe continuity of the load path.

Other additional design requirements thatapply to all levels of protection (VLLOPthrough HLOP) are:a) Multistory vertical load bearing

elements must be capable of supportingthe vertical load after the loss of lateralsupport at any floor level with an un-supported length (Lu) of:

Lu = 2 l s Eq. 1

where ls is the floor height in (ft). This

should be analyzed with the followingfactored load combination:

(0.9 or 1.2)D + (0.5L or 0.2S ) + 0.2W Eq. 2

where D is Dead Load, L is Live Load,S is Snow Load and W is Wind Load.

CorridorBeam

Stairs

Columns

Interior CFSLoad Bearing Wall

Plank Direction

C o r r i d o r

Max. Panel = 25’ - 8”

Exterior CFS LoadBearing Wall

Exterior CFSLoad Bearing Wall

330’ - 4”

Plank

Direction

26’ - 9”

59’ - 8” 6’ - 2”

9’ - 4”

26’ - 9”

N

b) In each bay (one at a time) and at alfloors and the roof, the slab/floor systemmust be able to withstand a net upwardfactored load of:

1.0D + 0.5L Eq. 3

Floor/Roof Tie ForceRequirements

Internal Ties

The requirements for floor and roof internaties vary based on the construction materiaused. As a general rule: internal ties musbe provided at each floor and roof level itwo directions; they must be effectivelcontinuous; and they must be anchored tperipheral ties at each end. Additionally, themay be spread evenly or grouped. In wallsthese ties must be within 1.6 ft. of the top obottom surface of the floor diaphragm.For reinforced concrete and CFS floo

and roof systems, internal ties must have required tensile strength (in kips/ft width

equal to the greater of:

Eq.4

DeadLoad

LiveLoad

WindLoad

Roof 20 psf 20 psf ±15 psf

Ceiling 15 psf – –

Typical Floor(other thanCorridor)

80 psf 40 psf –

Corridor 80 psf 100 psf –

Exterior Walls 48 psf – 25 psf

Interior Walls 12 psf – 5 psf

Table 1: Unfactored Design Loads

Figure 3: Typical floor plan.

Peripheral Tie(dashed lines)

Vertical Tie

Horizontal Tie toExternal Columnor Wall

Internal Ties(dotted lines)

CornerColumn Ties

Figure 2: Tie forces in a frame structure (Reference UFC 4-023-03).(1.0D +1.0L)l r

8475.21.03.3

F t or F t



where D and L are in pounds-per-square-foot(psf) and lr is in feet. Ft is the “Basic Strength”

which is the lesser of (4.5 + 0.9 no) or 13.5for reinforced concrete floor/roof systems andthe lesser of (1.62 + 0.33 no) or 4.92 forCFS floor/roof systems. The variable no is the number of stories.

Cold-Formed Steel WallsTie Force Requirements

Peripheral Ties

At each floor level and roof level,provide an effectively continuousperipheral tie capable of sustaining arequired tensile strength equal to:

1.0 F t (kips) Eq. 5

This tie must be located within 3.9ft. of building edges or within theperimeter wall.

Horizontal Ties to External Walls

Every 3.3 ft. length of external loadbearing wall must be tied horizontallyinto the structure at each floor and rooflevel with a tie having a required tensilestrength equal to the greater of:

a) Lesser of 2.0 F t or Eq. 6

or b) 3% of the total ultimate vertical loadcarried by the wall at that level where l s is in(ft). Where the peripheral tie is located withinthe wall, provide horizontal ties adequate toanchor the internal ties to the peripheral ties.

Vertical TiesTie each column and load-bearing w

continuously from the lowest to the highlevel. The tie must be capable of resista sile force equal to the maximum des

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Tie ForceBasic

StrengthReferenceEquation

Parameters Required Force

Horizontal

Internal, TFi (Floor)

Ft = lesser of

(4.5 + 0.9 no)or 13.5

no = 4

Ft = 8.1

Equation (4)

D = 80 psf

L = 40 psf lr (E-W) = 25’ 8”lr (N-S) = 26’ 9”

TFi (E-W) = 2.94 kips/ftTFi (N-S) = 3.07 kips/ft

HorizontalInternal, TFi (Corridor)

Equation (4)

D = 80 psf L = 100 psf lr (E-W) = 25’ 8”lr (N-S) = 6’ 2”

TFi (E-W) = 4.41 kips/ftTFi (N-S) = 2.45 kips/ft

HorizontalPeripheralTFp

Ft = lesser of(1.62 + 0.33no) or 4.92

no = 4

Ft = 2.94

Equation (5) – TFp = 2.94 kips

Horizontalto External

Walls TFe

Equation (6)

ls = 11’ 4”Max. 3.3-ft. wall load(E-W) = 27.2 kipsMax. 3.3-ft. wall load(N-S) = 15.4 kips

TFe (E-W) = 1.23 kips/ft

TFe (N-S) = 1.23 kips/ft

Vertical TF v

(Walls)N/A

Equation (7),one story

–TFv (Interior) = 4.3 kips/ftTFv (Exterior-Sides) = 2.7 kips/TFv (Exterior-Long) = 2.3 kips/

Vertical TF v

(Columns)Equation (7),one story

– TFv (Columns) = 20.3 kips

Table 2: Required Tie Forces

l s

8.2 F t



ultimate dead and live load received by the wall or column from any one story:

1.2D + 1.6L (kips) Equation 7

This is the 2003 IBC factored load combi-nation.

Worked Design Example

Building Description

A four-story barracks building designed withCFS load bearing walls and hollowcore precastconcrete planks for the floor slabs is considered

in this example. The barracks building isassigned LLOP and is approximately 330 feetlong and 60 feet wide, with an 11-foot, 4-inch floor to floor height and a CFS truss roofsystem. The building incorporates a normal

weight brick veneer finish on the exterior walls. The typical structural system consistsof interior and exterior load bearing walls inthe N-S direction that have variable spacingbetween the bearing walls with a maximumspan of 25 feet, 8 inches (Figure 3, see page10 ). Exterior walls in the E-W direction arealso load bearing walls, but only support

the truss roof loads in addition to their self weight and the lateral wind pressure. Theinterior corridor is supported by corridorbeams resting on individual CFS columns atthe inside edges of the load bearing walls.Hollowcore planks are selected to be 8

inches thick to accommodate the maximumspan of 25 feet, 8 inches between the N-S loadbearing walls. The planks require a seating/bearing width of 2 to 2.5 inches on top ofthe stud walls as recommended by the 1998Design Manual of the Precast/PrestressedConcrete Institute. A minimum of 6-inch

width wall framing can accommodatthe required plank seating width, but 8inch walls are typically recommended fointerior walls to allow enough gap betweeplanks to insert diaphragm connectors. Thunfactored design loads for this building arsummarized in Table 1 (page 10).

Required Tie ForcesThis barracks building is considered

composite construction since it has CF walls and columns with hollowcore concretslabs and CFS roof trusses. Hence, the floo

system is required to meet the internal tirequirements for reinforced concrete desig

while the roof system is required to meet thinternal tie requirements for CFS designThe walls and columns are required tmeet the peripheral, external and vertical tirequirements for CFS design, as applicable

Appropriate over-strength factors (W) anstrength reduction factors (F) must be usefor different materials as given in UFC 4023-03. The required horizontal and verticatie forces for the typical floor and interioand exterior walls and columns for thi

building are calculated as given in Table (see page 11) to generate tie schemes anmechanical connections.

Tie Forces Schemes

a) Internal Ties

The required internal tie forces betweenhollowcore planks can be achieved using various connection details offered by the PrecastPrestressed Concrete Institute to satisftheir structural integrity and diaphragmaction requirements. Reinforcement stee

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6” Stud Wall

6” Stud Wall

22 ” Bearing

Screw Connection

Hollowcore Plank

Stiffener Plate

Wall Top Track

(Min. 54 Mil)

Wall Bottom Track

(Min. 54 Mil)

Pour-Stop Angle

#4 Reinforcement Bars

Grouted in Plank

Figure 4: Peripheral tie and horizontal tie connection to external wall.

continued on page 1



bar ties may be run inside the core of theplanks in their longitudinal direction andin the connection joint between the planksin their perpendicular direction. Groutingis used to provide the bond between thesteel bars and the planks. Reinforcementsteel bars can also be used in the concretetopping in the longitudinal direction of theplanks, in addition to shorter anchored barsto tie adjacent planks in the perpendicular

direction. Using #4 Grade 60 rebar (60ksi yield strength) spaced 2 feet on centersatisfies all the internal tie force requirements(Maximum required strength = 4.41 kips/ft).b) Peripheral Ties The required peripheral ties around the

perimeter of the building can be achievedthrough the steel runner tracks bounding theexterior load bearing CFS stud walls. 6-inchexterior stud walls are recommended alongwith a minimum of 54 mil (0.0566-inchdesign thickness) runner tracks (Figure 4, seepage 12 ). A single 33 ksi yield strength standard

track (1.25-inch flange) provides an ultimatetensile strength of 14 kips, which exceeds therequired strength of 2.94 kips. Care must betaken at splice locations of runner tracks inorder to provide a splice connection capableof resisting the required tensile strength.Another method to provide the requiredperipheral ties is to use longitudinal steel barsembedded in the end grout. Two #4 Grade 60rebar provide an ultimate tensile strength thatexceeds the required tie strength.c) Horizontal Ties to External Walls The required horizontal ties to external walls

can be achieved by using reinforcement steelbars grouted into the hollowcore planks from

one side and anchored to the runner tracksof the CFS stud walls from the other side.Figure 4 (see page 12 ) shows these ties wherethe planks are supported by the stud wall andtransferring vertical loads (E-W direction ofthe barracks building). Similar ties should beprovided where the planks are only restingon the stud wall but not transferring anyvertical loads (N-S direction of the barracksbuilding). Anchorage of the horizontal ties

to the runner tracks ensures the continuitybetween internal ties and peripheral ties asrequired by UFC 4-023-03. Number 4 Grade60 rebar can be used at spacing 2 feet oncenter, which provides a tensile strength thatexceeds the required strength of 1.23 kips/foot. If steel bars are used as peripheral ties,the horizontal ties must extend to overlap

with the peripheral ties to provide the requiredanchorage between the two sets of ties.d) Vertical Ties The specifications for vertical ties are the

most critical requirements in this building’s

structural system due to the relatively heavy weight of the floor slab. For interior loadbearing walls, mechanical connectors are T-shape steel sections made of d-inch thickplates welded together with stiffeners andattached to both the bottom and top studs ofthe wall with ½ inch bolts (Figure 5 ). The T-shape connectors are tied together using two½-inch through bolts that run through theseparation gap between the planks. A testedT-shape connector with the same dimensionsprovides a tensile strength of about 21.8 kips.Use of this connector to tie every third stud

(every 4 feet) in interior walls would satisfythe required tensile strength of 4.3 kips/foot

(4.3 kips/ft x 4 ft = 17.2 kips < 21 kips). Thtensile capacity of two 2-inch through boltexceeds the required tensile strength. Thnumber of ½-inch bolts required to connecthe T-shape to the studs (acting in shear) idependant on the thickness of the stud. Foexample, a stud that is 54 mils thick and 50ksi yield strength requires eight ½-inch boltto develop connection shear strength of 17.kips. The stud itself must also be checked t

resist the same strength in tension. Similaor lighter mechanical connectors can bused for exterior load bearing walls in thN-S and E-W directions of the barrackbuilding since the required tensile strengthis significantly less.Corridor beams are typically designed a

structural steel W-or WT-shape resting otop of the corridor columns. The columns arbuilt-up CFS studs attached together wit

weld or screws, and capped from each en with a welded closure plate (Figure 6 ). Thhollowcore planks rest on top of the structura

steel beam. The required vertical ties for thcolumns can be achieved, as shown in Figur6 , first by welding the closure plate of thbottom CFS column to the W-shape steebeam. Then, four ½-inch through bolts (twon each side of the column) run through thhollowcore planks and are fastened at one ento the top flange of the W-shape beam and athe other end to the closure plate of the toCFS column. Stiffener plates for the W-shapsteel beam are provided to connect the beamflanges together at the vertical tie locationThe tensile capacity of four ½-inch through

bolts exceeds the required tensile strength o20.3 kips for the vertical tie.

A325 2” Bolts

22” Bearing

(2) 2” ThroughBolts

Wall Bottom Track

Wall Top Track

T-SectionConnector

HollowcorePlank

Screw Connection

Stiffener Plate

A

A

Min. 6” Stud Wall8” Recommended

Grout

Figure 5: Vertical tie connection in internal wall.

A325

2” Bolts

(2) Stiffener Plates

Stud Wall

(A-A)

T-Section Connector

WallBottom Track

14



Column Closure PlateWelded to Column

(2) Stiffener PlatesWelded to Beam

Column Closure PlateWelded to Column and Beam

3-Stud Built-Up CFSColumn (Weld Attached)

Wall Bottom Track

Wall Top Track

Grout

Wall Typical Stud

Stiffener Plate

HollowcorePlank

W-Shape Beam (4)2” Through Bolts(2 per Side of Column)

Design for Loss of Lateral Support

Loss of lateral support for load bearing wallsand columns necessitates that these loadbearing elements be designed at any floorlevel with an un-supported length (Lu) that istwice the floor height (Equation 1). For CFS

walls, the lateral direction of the wall is thestrong-axis direction of the studs. Therefore,only the strong-axis unsupported length ofthe studs should be doubled when design isperformed to satisfy this requirement.In addition, the loss of the lateral support

in exterior walls subjected to lateral windpressure requires that the connection between

wall sections must be capable of resisting theresulting bending moment; otherwise the

wall system becomes unstable under lateral wind loads. The governing factored loadcombination to design this condition is givenin Equation 2 and the factored design load

would be (0.2W). For the barracks buildingunder consideration, if this connectiondetail is used every third stud (every 4feet), the maximum factored moment to beresisted for a continuous 22 feet, 8 inchesbeam is 15.4 in-kips.

ConclusionsThis article summarizes the progressive

collapse requirements in UFC 4-010-01and UFC 4-023-03 as it relates to mid-riseconstruction using CFS load bearing walls.

A worked design example for a four-storybarracks building with CFS load bearing

walls and hollowcore precast concrete slabsassigned LLOP has been used to demonstratethat all the tie forces requirements canbe achieved through existing structuralelements of the building or added mechanicalconnections. Special requirements such as

loss of lateral support in exterior walls have

also been discussed. Design details have beenproposed for all the tie forces requirementsand special requirements. If CFS load bearing

walls are used with other floor systems (suchas composite steel deck or CFS joists), someof the presented design details will needto be adjusted accordingly; however, thesame method of achieving the tie forcesrequirements would still apply.▪

Nabil A. Rahman, Ph.D., P.E. is the Directof Engineering & Research for The SteelNetwork, Inc. Dr. Rahman currently servesas the Chairman of the Wall Stud Task Grouof the Committee on Framing Standards(COFS) of the American Iron and SteelInstitute (AISI). He is also a member of the

ASCE-SEI Committee on Cold-Formed

Steel, as well as the AISI Committee onSpecification. He can be reached via e-mail [email protected].

Michael Booth is the Director of Project Management at The Steel Network, Inc. Mr. Booth has held the titles of RegionalSales Manager and National Managerof Technical Services, in addition to 20+

years experience within the constructionindustry. He can be reached via e-mail [email protected] .

Gary Bennett is the Engineering Services Manager for the Steel Network, Inc. Mr.Bennett has extensive experience in designand analysis of cold formed steel structures.

Mr. Bennett is a member of severalsub-committees for AISI-Committee onSpecification (COS). He can be reached viae-mail at [email protected].

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Figure 6: Vertical tie connection in corridor column.

Progressive Collapse Requirements

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