Plate Girders and Plate Channels

[Sam78053]

Hello Fellow Engineers,

I just started working at a curtain wall manufacturing company and I am lost. I would like to design the steel reinforcing we use in these mullions. Mostly are channels but they are not the type in the steel manual. The are either 10GA or 3/16" thick plates that are bent to form a channel. The depth of the channels vary from 4" to 10".

1) What issues should I worry about with respect to these steel reinforcing.
a) web buckling
b) lateral torsional buckling
c) unbraced length
d) is there any other?

2) Are the AISC equations valid for this type of configuration? Is the bent place channel as same as the channels that are in the Steel manual? To me they are not! But I may be wrong.

3) Because W sections are too large to fit in the mullion systems, we use plate girders, which we take three plates and weld them together to create a W beam. Alsom can I use the AISC equations for the failures I have listed above?

4) If I like to design the girder plates and calculate the length and the spacing of stitch welding I like to use, how am I suppose to approach this design? what parameters should I consider?

I greatly appreciate all your help. I hope I do not come out as a stupid engineer but I am not experienced in this field. I have only designed simple easy stuff and no one is an engineer in this office.

Thanks again.
Sam

 

[ishvaaag]

At your thicknesses and sizes quite likely proper use of the AISC specs shouldn't cause any particular problem; I however would better look unto some provisions for cold formed design (such in IBC codes) since this is the kind of member you are working on.

Respect particulars I would look unto apart from those you quote I would examine if the current practices in the firm warrant that no member can be overloaded by unforeseen compression (i.e., the mullions show that they can effectively slip with differential movement at the ends) and the quality of the fixity against rotation at the floors levels, since assuming full fixity when there is not could cause problems. At your proportions following the code it seems then would be enough.

Built-up double tees of no slender proportions use to be designed just like rolled except when the code so provides with different equatios for built-up members. For compact double tees the additional checks may reduce to designing the stitch welds for the shearing forces therein on a tributary basis for the separation of the welds at the location. Better don't adjust the weld strength too tight since if there is some gap between flange and web you could incurr in non considered concomitant unaccounted stresses.

The use of provisions for cold formed members will address the subject of the particular shapes your firm uses.

 

[Sam78053]

Yes, you are absolutely correct, I am using the Aluminum Design Manual for the mullion members. This is when we add steel reinforcing to obtain greater heights for the mullion. I am assuming that the mullion will take all the stress before steel reinforcement will help the mullion and they work together to resist the load.

Where do I find examples of designing plate girders and splice plate design?

Thank you again.

 

[ishvaaag]

I did a pair of worksheets for plate girder design to the specs of AISC LRFD 1992; obviously they won't prove by themselves compliance with the standing regulation but can give a start. I attach both as in a zip file.

The worksheets were geared to big plate girders with some uniform live and dead loads and a number of point live and dead loads. (More than anything a transfer beam setup in a building).