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Design of Lateral Load Resisting Frames
Using Steel Joists and Joist Girders
Presentation by:
James M. Fisher, Ph. D., P. E.
Vice President
Computerized Structural Design
Milwaukee, WI
Authored by
James M. Fisher, Ph.D., P.E.
Perry S. Green, Ph.D.
Joseph J. Pote, P.E.
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Technical Digest 11
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Technical Digest No. 11
The purpose of TD No. 11 is to present
information to the EOR, and the joist
manufacturer, for the design of single
story moment resisting joist and Joist
Girder frames.
Design considerations for both wind and
seismic lateral loads are presented.
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Technical Digest No. 11
The digest has been limited to single storyframes, not because of wind requirements,but because of current requirements for
seismic design; in particular, the use ofstrong beam, weak column systems whichare typically necessary when using trussconstruction in lieu of beams and girders.
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Technical Digest No. 11
The Digest illustrates procedures to:
Analyze,
Design, and
Specify joist and Joist Girder moment frames to resist
wind and seismic lateral loads.
The reader is assumed to be familiar with:
2005 AISC Specification for Structural Steel Buildings 2005 AISC Seismic Provisions for Structural Steel
Buildings
ASCE 7-05
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Technical Digest No. 11
Designing joist and Joist Girder structures as rigid
frames is no more difficult than designing rigid
frames with wide flange beams and columns.
To obtain a cost effective design the engineer must
be aware of the inter-relationships between framing
elements, i.e. joists, Joist Girders, columns, bracing
members and connections.
In general, the most economical design is one
which minimizes manufacturing and erection costs,
and one which reduces the special requirements
(seat stiffeners, chord reinforcing, etc.) for the
joists, Joist Girders and columns.
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Design Methodology
The first consideration relative to the design of
the structure is to determine if rigid frame action
is required.
For single story structures the optimum framingsystem generally consists of braced frames in
both directions, and the use of a roof diaphragm
system to transfer wind and seismic loads to the
vertical bracing elements.
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Design Methodology
The specifying professional and the joistmanufacturer must communicate design data andinformation to each other.
The specifying professional must specify thenecessary loading and stiffness data to the joistmanufacturer.
The specifying professional must indicate thetype of joist to column connections so that the
joist manufacturer can provide the joists with thegeometry that meets the design intent.
Dialog must occur between all involved partiesprior to final pricing and design.
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Design Methodology
The joist manufacturer must design
the joists in conformance with the SJI
Specifications and other contract
requirements specified by the
specifying professional.
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Analysis Requirements
Forces and moments in single story joist rigid
frames are determined in a manner similar to
other Ordinary Moment Frames (OMF).
The first step is to perform a preliminary analysis.
In general, it is suggested that the OMF be
considered as a pinned based frame to eliminate
moment resisting foundations; however, for drift
control partially restrained or fixed bases can beconsidered.
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Analysis Requirements
After selecting trial member sizes for the columnsand joists, a computer analysis is performed todetermine forces, moments, and deflections (both
first-order and second-order) for the loadcombinations prescribed by the ApplicableBuilding Code.
Because a second-order analysis is a non-linearproblem, the analysis must be performed for each
required load combination.
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Frame Model
Model for IBC or ASCE Load Combinations
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Analysis
Trial joist stiffness can be obtained from the SJIequations for the approximate moment of inertiafor a joist or a Joist Girder. The SJI equation for a
Joist Girder equals 0.018NPLd (LRFD),and 0.027NPLd (ASD)
where:
N = number of panel points
P = panel point load (kips) at factored load levelfor LRFD, and at nominal load level for ASD
L = girder length (ft.)
d = girder depth (inches)
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Analysis
The SJI equation for the approximate
moment of inertia for a joist equals
26.767(WLL)(L3
)(10-6
) for both LRFD and ASD.where:
WLL= The RED figure in the K-, LH-, and
DLH-Series Load Tables
L = (Span
0.33) in feet for K-Series joists
L = (Clear span + 0.67) in feet for LH- and
DLH-Series joists
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Analysis
Angle Size Unbraced Leng th
feet
Area
in.2
L = 4 L = 5 L = 6 L = 7
2L6 x 6 x 1 939 911 879 842 22.0
2L6 x 6 x 7/8 828 809 781 749 19.5
2L6 x 6 x 3/4 705 698 678 650 16.9
2L2.5 x 2.5 x 3/16 49 48 41 34 1.80
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Frame Model
Model for AISC-Strong Beam, Weak Column
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OMF Analysis
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Pseudo Columns
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Typical Connections
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Basic Connection
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Eccentricity Effect
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Added Reinforcing
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End Plate Type Connection
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Plate Connection-Sidewall
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Plate Connection-Interior
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Specification of RequiredForces and Moments
IBC LRFD load combinations are used.
Nominal loads:
D = 15 psf L = 20 psf (reducible)
S = 5 psf
W (uplift gross) = 27.25 psf (windward roof)
= 17.3 psf (leeward roof)
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Specification of RequiredForces and Moments
Seismic Criteria:
R = 3.5 for OMF
SDS= 0.9297g
SD1= 0.39g r= 1.0
QE= 49 kips
Imin
= 6790 in.4for the exterior girders and 4570 in.4for the interior girder (analysis requirements).
Minimum width of top chord = 7.0 in. (weldrequirements).
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Minimum thickness of bottom chord = 3/8 in.(weld requirements).
All top chord axial loads and end moments aretransmitted directly into the columns via the tieplates. No horizontal forces are transferredthrough the girder seats.
Chord splices must conform to the requirementsof the 2005 AISC Seismic Provisions, Section7.3a.
Controlling IBC Load Combinations are givenbelow for Joist Girder Mark Numbers G1 and G2,respectively:
Specification of RequiredForces and Moments
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Controlling IBC LoadCombinations
Mark G1
LRFD
Load Combination:
Panel
Load
(kips)
Left End
Moment
(kip-ft.)
Right End
Moment
(kip-ft.)
TC
Force
(kips)
BC
Force
(kips)
Remarks
1.4D + 1.4C
1.2D + 1.2C + 1.6(Lror S)
1.2D + 1.2C + 1.6W +
0.5(Lror S)
1.2D + 1.2C + 1.0E +0.2S
(1.2 + 0.2SDS) (D+C) +
QE + 0.2S
0.9D + 1.6W
+
+
+
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Main Wind Force ResistingPressure Table
2005
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2005 AISC Seism ic Prov is ionsSection 5.1
Designation of the seism ic load resis t ing system(SLRS)
Designation of the members and connections that
are a part of the SLRS Configuration of the connections
Connection material specifications and sizes
Locations of demand cr i t ical welds
Locations and dimensionsofpro tected zones Welding requirements as specified in Appendix W,
Section W2.1
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Bracing
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Examples 1 and 2
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Examples 1 and 2
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Example 1
The building is located in Charleston,
South Carolina. The building code to be
used is 2006 International Building Code
(IBC 2006).
The precast concrete shear walls at the
north and south ends of the building are
non-load bearing shear walls, and areused to resist the forces between the first
interior rigid frame and the end wall.
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Example 1
Loading requirements are specified as:
Roof Loads:
Dead Load:1 psf Membrane
2 psf Deck
2 psf Insulation
3 psf Joists and Bridging
2 psf Girder
10 psf Total
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Example 1
Collateral Load:
3 psf Sprinkler
2 psf Mechanical & Lighting
5 psf TotalLive Load:
20 psf Reducible per Code
(12 psf on Joist Girders)
Ground Snow Load = 5 psf
Roof Snow Load = 5 psf (ASCE 7, Section 7.3,low slope roof criteria)
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Example 2
Wind Load = 120 MPH Exposure C
Seismic Load: Charleston, South Carolina
Serviceability Requirement: Maximum drift = H/100 (10 year wind)
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Examples 1 and 2 Comparison
Example 1: Charleston, SC
Wind Base Shear (120 mph) 22.9 kips per frame line (Factored by 1.6)
Seismic Base Shear (R=3.5) 49.0 kips per frame line
Example 2: Jackson, MS
Wind Base Shear (120 mph) 22.9 kips per frame line (Factored by 1.6)
Seismic Base Shear (R=3.0) 14.3 kips per frame line
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Example 1: Exterior Columns
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Example 1: Interior Columns
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Example 2: Exterior Columns
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Example 2: Interior Columns
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Example 1 (120 mph, SDC D) Columns: Exterior W18x86, Interior W18x97
Total Column Weight = 12,200 lbs
Girder Weight = 6,300 lbs Total Weight = 18,500 lbs per bay
Example 2 (120 mph, SDC B) Columns: Exterior W21x111, Interior HSS 8x8x3/16
Total Column Weight = 8700 lbs Girder Weight = 3200 lbs
Total Weight = 11,900 lbs per bay
Examples 1 and 2 Comparison
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Appendix A
Appendix A contains a complete design of
the Joist Girders for Example 1
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Acknowledgement
The authors of Technical Digest 11 wouldlike to thank: The Engineering Practice Committee and the
Research Committee of the Steel Joist Institute fortheir review and contributions to the writing of thisdocument.
John A. Rolfes, S.E., P.E. Vice President ofComputerized Structural Design for his assistance
in the preparation of the digest, and James O.Malley, S.E. Senior Principal,
DegenkolbEngineers, for his insightful review of the digest.
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Thank you