Yield Strength & Steel Design 2

[tkday]

I am a little rusty on steel design. I have some old plans which state the building was designed using ASTM A36. I've been using LFRD but realized that the manual uses 50ksi for wide flange only. What resources are there for checking moment resistence for A36 of this shape?

 

[csd72]

The method is the same as for 50ksi. If you look at the older manuals there will be charts for this grade (albeit to the older code).

Better still, write yourself a spreadsheet.

 

[frv]

The charts (in the 13th Ed) are set up for 50ksi; However, the specification is set up for any yield strength- just go through the specification.

 

[tkday]

Thanks much for the info. So in supporting my calculations, what specifications should I reference. I've been told that I should be adjusting my allowable bending for A36 vs. grade 50. So correct me if I'm wrong, what you are saying is that my numbers are good for A36 if using the grade 50 tables and I need not make any adjustments. Am I correct?

 

[Lion06]

NO, NO, NO!!!
The spec is set up for any yield strength means that you can use the formulas in the spec to calc the allowable bending strength based on any yield strength. That does not mean that you can use a 50 ksi table for a 36 ksi beam!!
Is there an engineer in your office you can talk to about this?

 

[tkday]

O.k. I think I got it: Plastic Moment=Fy*Z, so I use 36 instead of 50 and reduce by 0.9 factor? Sorry Head Engineer is out. Been awhile but it's coming back slowly.

 

[PMR06]

You're factoring your loads too, right??

 

[Lion06]

The platic moment is only applicable for adequately braced beams (Lb is small enough to negate LTB). Is that your situation? It most likely is, especially for 36 ksi, but verify that.

 

[dgkhan]

tkday,
I do not think you are rusty, rather you never were polished on steel deisgn the way you are approaching. Better have someone review your work.You cannot use this equation Mp=Fy*Z unless section is compact and laterally braced etc.

 

[tkday]

Thanks for all inputs. Very helpful. Beam is 27x94 and fastened to concrete deck and 7.5' o.c. joist bracing so assuming continuous? Lateral-torsional buckling doesn't apply for this case, yielding only based on specs. Also checked shear based on specs. Beam does fail with proposed loading now based on these calcs. Loading calculation spreadsheet has been verified by higher ups and greatest load combination applies based on IBC 2006.
If not continuous Lp is border line (7.49'). I will do the math to be sure (F1.2) and being compact, based on B5.1.

 

[dik]

Whether a beam is compact or non-compact is related to the b/t ratios for the flange and web and not related to unsupported length. For A36, your beam is compact and Z can be used. There are likely other bracing issues, if you are using it for plastic design.

Dik