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Authored by James M. Fisher, Ph.D., P.E. Perry S. Green, Ph.D.

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Presentation on theme: "Authored by James M. Fisher, Ph.D., P.E. Perry S. Green, Ph.D."— Presentation transcript:

1 Design of Lateral Load Resisting Frames Using Steel Joists and Joist Girders
Authored by James M. Fisher, Ph.D., P.E. Perry S. Green, Ph.D. Joseph J. Pote, P.E. Presentation by: James M. Fisher, Ph. D., P. E. Vice President Computerized Structural Design Milwaukee, WI

2 Technical Digest 11

3 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.

4 Technical Digest No. 11 The digest has been limited to single story frames, not because of wind requirements, but because of current requirements for seismic design; in particular, the use of strong beam, weak column systems which are typically necessary when using truss construction in lieu of beams and girders. This is because the flexural strength and stiffness of the trusses is usually significantly larger than most reasonable column sections. JP-Conventional trusses with slender elements have a reputation as being notoriously poor performers under inelastic cyclic loading.  They look great and absorb lots of energy for the first two or three cycles.  Then the cyclic reversals of compression buckling followed by tensile yielding leads to member cracking, followed by rapid deterioration of strength and stiffness.  In order to prevent this very undesirable behavior, in high seismic design categories, we are strongly recommending the use of the Strong-Beam / Weak-Column design approach, with not only the connections, but also the entire joist being designed based on the column plastic moment.  This ensures that the joist remains elastic throughout, and that all inelastic behavior stays safely in the compact column section where it does not lead to rapid deterioration of the slender truss elements under inelastic stress reversals.

5 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

6 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.

7 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 framing system 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. This system should always be evaluated by the Engineer of Record (EOR) as a first option. Only if the building foot print, or other bracing restrictions prevent the use of this system, should moment frames be considered. As a “rule of thumb” if the length to width ratio of the building exceeds 4 to 1 roof diaphragm forces become large and use of roof diaphragms to span lateral loads to perimeter bracing may not be practicable. In addition, strut forces become potentially excessively large as well as the vertical bracing forces and foundation uplifts.

8 Design Methodology The specifying professional and the joist manufacturer must communicate design data and information to each other. The specifying professional must specify the necessary loading and stiffness data to the joist manufacturer. The specifying professional must indicate the type of joist to column connections so that the joist manufacturer can provide the joists with the geometry that meets the design intent. Dialog must occur between all involved parties prior to final pricing and design.

9 Design Methodology The joist manufacturer must design the joists in conformance with the SJI Specifications and other contract requirements specified by the specifying professional.

10 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 be considered. The specifying professional is encouraged to consider serviceability criteria and drift control at the preliminary design phase of the project.

11 Analysis Requirements
After selecting trial member sizes for the columns and joists, a computer analysis is performed to determine forces, moments, and deflections (both first-order and second-order) for the load combinations prescribed by the Applicable Building Code. Because a second-order analysis is a non-linear problem, the analysis must be performed for each required load combination. Individual load cases cannot be used and then summed to obtain a correct result. The 2nd order analysis must be performed using the cumulative, factored loads associated with each load combination.

12 Model for IBC or ASCE Load Combinations
Frame Model Model for IBC or ASCE Load Combinations It is suggested to use a simplified model for the joist frame by modeling the joist as an equivalent beam section with an approximate moment of inertia. The node at the interface of the column and joist should be located at the mid-height of the joist to more closely approximate the relative stiffness of these two elements and to predict lateral drift in the frame. This model is referred to as Model 1

13 Analysis Trial joist stiffness can be obtained from the SJI equations for the approximate moment of inertia for 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 level for LRFD, and at nominal load level for ASD L = girder length (ft.) d = girder depth (inches)

14 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 ) in feet for LH- and DLH-Series joists

15 Analysis Angle Size Unbraced Length feet Area in.2 L = 4 L = 5 L = 6
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 Goes down to 2.5 x2.5 x 3/16

16 Model for AISC-Strong Beam, Weak Column
Frame Model Model for AISC-Strong Beam, Weak Column

17 OMF Analysis

18 Pseudo Columns

19 Typical Connections

20 Basic Connection

21 Eccentricity Effect

22 Added Reinforcing

23 End Plate Type Connection

24 Plate Connection-Sidewall

25 Plate Connection-Interior

26 Specification of Required Forces and Moments
IBC LRFD load combinations are used. Nominal loads: D = 15 psf L = 20 psf (reducible) S = 5 psf W (uplift gross) = psf (windward roof) = 17.3 psf (leeward roof)

27 Specification of Required Forces and Moments
Seismic Criteria: R = 3.5 for OMF SDS = g SD1 = 0.39g r = 1.0 QE = 49 kips Imin = 6790 in.4 for the exterior girders and 4570 in.4 for the interior girder (analysis requirements). Minimum width of top chord = 7.0 in. (weld requirements).

28 Specification of Required Forces and Moments
Minimum thickness of bottom chord = 3/8 in. (weld requirements). All top chord axial loads and end moments are transmitted directly into the columns via the tie plates. No horizontal forces are transferred through the girder seats. Chord splices must conform to the requirements of the 2005 AISC Seismic Provisions, Section 7.3a. Controlling IBC Load Combinations are given below for Joist Girder Mark Numbers G1 and G2, respectively:

29 Controlling IBC Load Combinations
Mark G1 LRFD Load Combination: Panel Load (kips) Left End Moment (kip-ft.) Right End TC Force BC Remarks 1.4D + 1.4C 1.2D + 1.2C + 1.6(Lr or S) 1.2D + 1.2C + 1.6W + 0.5(Lr or S) 1.2D + 1.2C + 1.0E +0.2S ( SDS) (D+C) + QE + 0.2S 0.9D + 1.6W +

30 Main Wind Force Resisting Pressure Table

31 2005 AISC Seismic Provisions Section 5.1
Designation of the seismic load resisting 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 critical welds Locations and dimensions of protected zones Welding requirements as specified in Appendix W, Section W2.1

32 Controlling IBC Load Combinations
Mark G1 LRFD Load Combination: Panel Load (kips) Left End Moment (kip-ft.) Right End TC Force BC Remarks ( SDS) (D+C) + QE + 0.2S (L-R) ( SDS) (D+C) + QE + 0.2S (R-L) (0.9 – 0.2SDS)D + QE (L-R) (0.9 – 0.2SDS)D + QE (R-L) Column Mp, (AISC OMF Criteria)

33 Bracing

34 Examples 1 and 2

35 Examples 1 and 2

36 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 are used to resist the forces between the first interior rigid frame and the end wall.

37 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

38 Example 1 Collateral Load: 3 psf Sprinkler 2 psf Mechanical & Lighting
5 psf Total Live 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)

39 Example 2 Wind Load = 120 MPH – Exposure C
Seismic Load: Charleston, South Carolina Serviceability Requirement: Maximum drift = H/100 (10 year wind)

40 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 Seismic Base Shear (R=3.0) 14.3 kips per frame line

41 Example 1: Exterior Columns

42 Example 1: Interior Columns

43 Example 2: Exterior Columns

44 Example 2: Interior Columns

45 Examples 1 and 2 Comparison
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

46 Appendix A Appendix A contains a complete design of the Joist Girders for Example 1

47 Acknowledgement The authors of Technical Digest 11 would like to thank: The Engineering Practice Committee and the Research Committee of the Steel Joist Institute for their review and contributions to the writing of this document. John A. Rolfes, S.E., P.E. Vice President of Computerized Structural Design for his assistance in the preparation of the digest, and James O. Malley, S.E. Senior Principal, Degenkolb Engineers, for his insightful review of the digest.

48 Thank you


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