How to designate a beam as fully restrained or braced in STAAD ? 1

[anonymous119]

I have to design a steel I-beam which is placed on a concrete slab upon which (I-Beam) load of skylight has to come. I want to take advantage of bottom concrete surface on which beam is placed directly. So, where can I give the command of fully restrained member to Staad? Second should I design the member like that or there should be different procedure?

 

[Ron]

anonymous119...this is an international forum, so unless it is obvious from your question, it would be good to state where you are located and under which codes you are working.

I'm not sure I understand completely your issue. Is the steel beam spanning anything? A skylight is usually a fairly light load with only wind load and/or snow load really affecting its attachment and supports. A sketch would be helpful.

 

[SteelPE]

Are you trying to fix the ends of the beams for moment or are you trying to fix the beam against lateral torsional buckling? If against lateral torsional buckling are you trying to fully restrain the top flange or the bottom flange?

 

[anonymous119]

The structure is in Jeddah, Saudi Arabia & AISC code is being considered. The beam span is 5.5 meters. The skylight is inclined which is supported by hollow steel tubes with all four sides supported by concrete slab. Actually the contractor hasn't constructed the concrete up stand at the lower part, for placing the skylight base over, so that's why they are saying to place an I-Beam there instead.

I have attached the autocad file. Pardon me with the drawing I know its only a demonstrating one. Hope you would find it helpful.

 

[anonymous119]

Steel PE, I am trying to restrain it against lateral torsional buckling with the bottom flange fully restrained.

 

[SteelPE]

The program will not care about whether or not your beam is restrained until it comes down to the code check process of the computer program. I usually enter my LTB information by hand using the following commands under the parameters section (note this is for AISC 360-05):

UNT Member Length Unsupported length of the top* flange for calculating flexural strength. Will be used only if compression is in the top flange.
UNB Member Length Unsupported length of the bottom* flange for calculating flexural strength. Will be used only if compression is in the bottom flange.

I believe these commands can be found under the "design"-"steel"-"define parameters" tabs. using the GUI. These parameters are entered into the program using a length variable for example:

UNT 5 MEMB 11 TO 18

This command will give a unbraced length of 5 length units (feet or inches) to top flange of members 11 to 18.

 

[anonymous119]

Thanks SteelPE, what should I do to fully restrain the beam against moment?

 

[SteelPE]

If you restrain the beam against moment you don't have a beam. All you can really do is restrain the ends. You usually take care of this when you define your supports.

 

[anonymous119]

"If you restrain the beam against moment you don't have a beam" . Do you mean that beam is a member which has to carry moment and its the design code which limits it up to the permissible limits, otherwise it won't remain a beam it may become a tension or compression member.

SteelPE Please elaborate.

 

[SteelPE]

What I meant to say was that if you fully restrain a beam against moment along its entire length you will no longer have a beam as there will be no bending. You have to remember that providing restraint against LTB has nothing to do with restraining a beam against bending.