Simply Supported Beam Design

[KCCE]

I am reviewing a design that has a beam subjected to 200k-ft max moment and a 32 ft span, the designer has specified a W21x44. When I checked the beam via ASD the compressive flange would have to be braced at least every 4.7-ft to meet the flange criteria. I think the designer is just selecting the beam by Mr method without checking the unbraced length. Am I interpreting this correctly?

 

[jec67]

Please provide some additional information. What is the beam supporting - concrete deck, metal deck, etc? Are there members framing into the beam?

 

[KCCE]

There are purlins bolted to the bottom flange of the beam, the roof deck is then attached to these purlins. The purlin spacing varies from 4' to 12'. I am also concerned about the roof deck pans to purlin connection which is via a beam clamp. Do you know of any load capacity information on this type of connection?

 

[SlideRuleEra]

Using AISC Manual of Steel Construction, Ninth Edition, ASD:
Page 2-168: W21 X 44 (A36 Steel) is good for maxiumum moment of about 163 kip-ft, no matter how short the unbraced length. The limiting factor is the allowable shear strength of the web.

However - For steel with Fy = 50 ksi, the W21 x 44 is the lightest section for the 200 kip-ft moment (page 2-202). The unbraced length would have to < or = about 6.2 ft (the intersection of the line for the W21 x 44 and 200 kip-ft moment).

 

[broekie]

Based on the information that you have given, it does not sound that the compression flange of the beam is braced. If this is the case, I think a W16x89 or so is the lightest section that will work (that's for 50 ksi steel). I don't know anything about the beam clamp connection you are talking about.

 

[KCCE]

Sorry I failed to mention the beam has an Fy=50 ksi

 

[SlideRuleEra]

Concur with broekie.