Universal Beams

by Helmut Schmidhofer

This spreadsheet designs freely supported simple span universal beams grade 300 with full lateral restraint.

The spreadsheet expects values for the span L in m, and live load Q and dead load G in kN/m.

The program calculates two design loads: (i) for the strength limit state (WL) and (ii) for the serviceability limit state (WS).

WL = LFQ*Q + LFG*G where LFQ and LFG are load factors for live and dead loads respectively. These are preset to 1.5 and 1.25 respectively, and may be overwritten.

WS = PSIS*Q + G where PSIS is the short-term load factor for deflections, preset to 0.7, which also may be overwritten.

The program calculates the bending moment M = WL*L^2/8 and the required moment of inertia RIX = 5*WS*L^4/(200*384*L/ratio) where ratio is the required span/deflection ratio preset to 250, which may be overwritten.

It then looks up Table 5.3-5 of the AISC design capacity tables for structural steel to find the beam that meets the M, and Table 3.1-3(A) to find the beam that meets the RIX.

You should do at least one calculation manually, for two reasons:
1. to check the spreadsheet so you can have confidence in its results; and
2. to appreciate how much time the spreadsheet saves.

At the very least, you should download unibeams.xls (see below) so you can satisfy yourself that the program is using the equations correctly.

Universal beams grade 300
(freely supported, with full lateral restraint)
Given:
Span, L				m
Live load, Q				kN/m
Dead load incl. self weight, G				kN/m
Load factor for live load, LFQ
Load factor for dead load, LFG
Short term deflection factor, PSIS
Span/deflection ratio
Then:
Limit state design load, WL				kN/m
Resultant moment, M				kNm
Serviceability load, WS				kN/m
Required moment of intertia, RIX				10^6 mm4
Required beam based on M
capacity PHIM				kNm
provides IX				10^6 mm4
Required beam based on RIX
capacity PHIM				kNm
provides IX				10^6 mm4