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 byJames 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 PresidentComputerized Structural DesignMilwaukee, WI
3 Technical Digest No. 11The 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. 11The 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, andSpecify 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 Buildings2005 AISC Seismic Provisions for Structural Steel BuildingsASCE 7-05
6 Technical Digest No. 11Designing 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 MethodologyThe 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 MethodologyThe 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 MethodologyThe 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 ModelModel for IBC or ASCE Load CombinationsIt 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 AnalysisTrial 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 pointsP = panel point load (kips) at factored load level for LRFD, and at nominal load level for ASDL = girder length (ft.)d = girder depth (inches)
14 AnalysisThe SJI equation for the approximate moment of inertia for a joist equals26.767(WLL)(L3)(10-6) for both LRFD and ASD.where:WLL = The RED figure in the K-, LH-, and DLH-Series Load TablesL = (Span – 0.33) in feet for K-Series joistsL = (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 193991187984222.02L6 x 6 x 7/882880978174919.52L6 x 6 x 3/470569867865016.92L2.5 x 2.5 x 3/16494841341.80Goes down to 2.5 x2.5 x 3/16
16 Model for AISC-Strong Beam, Weak Column Frame ModelModel for AISC-Strong Beam, Weak Column
27 Specification of Required Forces and Moments Seismic Criteria:R = 3.5 for OMFSDS = gSD1 = 0.39gr = 1.0QE = 49 kipsImin = 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:
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 SLRSConfiguration of the connectionsConnection material specifications and sizesLocations of demand critical weldsLocations and dimensions of protected zonesWelding requirements as specified in Appendix W, Section W2.1
36 Example 1The 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 Membrane2 psf Deck2 psf Insulation3 psf Joists and Bridging2 psf Girder10 psf Total
39 Example 2 Wind Load = 120 MPH – Exposure C Seismic Load: Charleston, South CarolinaServiceability Requirement:Maximum drift = H/100 (10 year wind)
40 Examples 1 and 2 Comparison Example 1: Charleston, SCWind 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 lineExample 2: Jackson, MSSeismic Base Shear (R=3.0)14.3 kips per frame line
46 Appendix AAppendix A contains a complete design of the Joist Girders for Example 1
47 AcknowledgementThe 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.