Simple Beam Check with ASD vs LRFD 2

[gb12345]

Hi,

I was checking a beam for a "maximum" point load in the middle of the beam (30 ft beam supported at 28ft span). This is a header beam on a gantry system with a shackle and sling directly in middle of beam. When checked with a point load in the middle i can get around 230 kip at .95 code check for AISC 14TH: ASD. On the other hand, I can get around 340kip at .95 code check for LRFD. Does this mean the beam can actually take 340kip? Just seems like a very big difference. The attached images show the beam in RISA. I am little confused on this matter. Any help greatly appreciated.
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1565188324/tips/RISA-3D_Detail_Report1-ex-----_b80dbg.pdf[/url]

 

[Dowels93]

To determine what the beam can "actually take", you'd have to calculate the beam strength without any resistance/safety factors (i.e. phi=omega=1), which should give the same result regardless of method. ASD and LRFD use different loads as well; ASD are not factored, LRFD are factored. In the end, they should remain relatively close, depending on ratio of load types.

For example, assume:
DL = 70kip
LL = 160kip

Load for ASD = DL + LL = 230kip
Load for LRFD = 1.2DL + 1.6LL = 340kip

Obviously this depends on the ratio of dead to live load, but that's intentional (since LRFD is intended to account for uncertainty in loading better).

 

[JoshPlumSE]

The key is that ASD and LRFD do not really make an attempt to say what the actual capacity of the beam is. Rather, they are methods for determining is the beam has an adequate safety factor for a given loading per those particular provisions.

Dowels93's post is great. It points out how a given sets of loads (which is also code prescribed and has it's own safety factors in it) results in a ASD load of 230k and an LRFD load of 340k..... Which produce similar "code checks".

 

[gb12345]

Okay makes sense. Thank you

 

[bhiggins]

A couple of side notes: it looks like your beam material was set to A36 steel, not sure if that is intentional. It also looks like your beam is braced at several locations, but your unbraced lengths are 30 ft in your report. You have to manually set your unbraced lengths and material in RISA.