Design Guide 28 - Stability Design of Steel Buildings

Floor Dead load, wFD = 2.4 kip/ft Live load, wFL = 4.0 kip/ft Estimated floor beam weight = 0.149 kip/ft Estimated floor

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Floor Dead load, wFD = 2.4 kip/ft Live load, wFL = 4.0 kip/ft Estimated floor beam weight = 0.149 kip/ft Estimated floor interior column weight = 0.065 kip/ft Estimated floor end column weight = 0.048 kip/ft Wind load, WF = 20 kips (Note: Wind load on roof surfaces as specified in ASCE/SEI 7 is not considered in this problem.) Solution: From AISC Manual Table 2-3, the material properties are as follows: Beams and columns ASTM A992 Fy = 50 ksi Fu = 65 ksi Bracing ASTM A36 Fy = 36 ksi Fu = 58 ksi The analysis was performed using a general second-order elastic analysis program including both P-Δ and P-δ effects. See Figure 2-5 for the member labels used. Refer to Section 1.7 for a more detailed discussion of the analysis computer models utilized in this Design Guide. See Section 2.2 for a step-by-step description of the ELM. Description of Framing All lateral load resistance is provided by the tension only rod bracing. The tension rods are assumed to be pin connected using a standard clevis and pin (see AISC Manual Tables 15-3 and 15-7). Beams within the braced frame are bolted into the column flanges using double angles (see AISC Manual Figure 13-2(a)). A single gusset plate connecting the tension rod is shop welded to the beam flange and field bolted to the column flange (see AISC Manual Figure 13-2(a)). All other columns outside the braced frames are leaning columns with simple beam-to-column connections. Design Approach Design is an iterative process. Preliminary sizes should be chosen based on experience or a preliminary analysis. Braced frame structures are often controlled by strength as opposed to wind or seismic drift. Thus, for this problem, the sizes are estimated from a preliminary strength check and then used in the computer analysis. The member sizes used in the following analyses are those shown in Figure 2-6. Analysis Load Combinations The member design forces are obtained by analyzing the structure for 1.6 times ASD load combinations and then dividing the results by 1.6. It should be noted that the notional loads already include, by definition, the 1.6 amplification. The load combinations from ASCE/SEI 7 Section 2.4.1 used for the second-order analysis are given in Table 2-1. If the B1-B2 approach is used to account for second-order effects, the analysis need not include the 1.6 multiplier on either the load combinations or in the determination of notional loads since this is already included in the B1-B2 calculation. Note that since the structure loading and geometry are symmetric and symmetry of the frame is enforced in the member selection, the wind load is considered in only one direction.

Fig. 2-5. Braced frame member labels.

AISC DESIGN GUIDE 28 / STABILITY DESIGN OF STEEL BUILDINGS / 17