SPAN & Physical Member commands in SACS

Hello all,

There are some ambiguities for me about the use of SPAN and PHYSICAL MEMBER commands in SACS. Consider the following simple senario:

Imagine we have a main deck beam (say HEB600), on top of which stringer beams( say IPE200) are running in perpendicular direction. IPE200 stringers will provide LTB support for HEB600.Also assume we want to model all members in SACS, so the HEB600 should be broken along its length, at intersection with the top IPE200s.

for the design of HEB600 beam , as per AISC or EUROCODE, each segment of the broken beam should be designed separately, based on the its end moments. If SPAN command is not used, SACS does exactly as codes instruct and treat each beam segment separately and uses the end moment ( at intersection with IPE200s) to calculate Mcr and LTB resistance.

However, if we want to check the deflection of the HEB600, we have to use SPAN command which basically instructs the program to join all beam segments together and treat it as one. in this case, the end moments used for the LTB resistance of the beam are not the ones at the location of lateral supports, but those at the two ends of the beam itself ( even if we identify the Lcr equal to beam segment, still end moment are chosen wrongly) .This will result in incorrect design results.

What is the solution? How can we check deflection using SPAN without causing mistake in design of the beam?

Also, I was not able to find good reference for the PHYSICAL MEMBER ( say PM) command.  what is the use of this command?. how does it compare to SPAN command? can it be the solution for the problem above?

Any comment is appreciated.

Thank you

Parents
  • I guess you need to do two separate analysis, one for beam design without SPAN definition and one for span deflection check. And what design code are you using?  

    Normally physical member is generated by SACS program for reference(commented line at bottom of model) but it has no effect to member check. For example for jacket all leg members along one pile will be combine to one physical member.

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  • I guess you need to do two separate analysis, one for beam design without SPAN definition and one for span deflection check. And what design code are you using?  

    Normally physical member is generated by SACS program for reference(commented line at bottom of model) but it has no effect to member check. For example for jacket all leg members along one pile will be combine to one physical member.

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