I am currently trying to use STAAD to model a building where my floors primarily consist of concrete on metal deck over steel beams. I have some real irregular framing due to architectural features, so I wanted to model the floors in STAAD to get accurate moment and deflection values.
My question is simple: is there a way to manually define the PNA location when using the composite beam design feature in STAAD? Or, perhaps a way to define a range of % Composite Behavior?
I read the reference manual and how it goes thru the composite beam design theory, and I'm pretty disappointed that the program goes thru a series of steps to determine the location of the PNA instead of just asking the user. Using the STAAD theory, just about every one of my beams will fall under Case 1 (PNA in Slab) from a geometric standpoint, because I am using beams that are lightweight with respect to their depth (e.g. W18x35) and I have a pretty substantial concrete deck (6" total thickness on 1.5B deck). While this is well and good, we all know that getting 100% composite action is neither practical, nor useful in many situations. If the program is assuming the PNA is in the concrete slab without my ability to tell it otherwise, then it is assuming we are always going to achieve 100% composite action and, consequently, have an absolute ton of studs required.
In my case, the majority of my members are governed by loading that occurs in the construction phase. I already realize that I will need (2) separate models for my project: one to determine moments and deflections in the construction phase and one to determine moments and deflections in the final stage. I'm going to account for creep in another fashion. Because of the members mostly being governed in the construction phase, they don't require a great deal of composite action in the final stage.
My current workaround is that I am adding members to the section database that exactly mimic the steel member in the composite section, but have altered Ix values to account for the composite action that I want. For instance, I added a member "W18x35PNA7" which has all of the same properties of a W18x35, except it's Ix value represents a value from Table 3-20 (AISC 13th edition) with the PNA located at Location 7 and a Y2 value that I have conservatively pre-determined. Obviously, if my slab depth changes, these numbers become obsolete in addition to the fact that I have to be conservative with my Y2 value. These are not ideal fixes.
Anyone have any better ideas or workarounds? Normally, I use excel to do composite beam designs because of the multiple stages, but in this case, the framing is so complex that it doesn't really lend itself to hand calculations.
Thanks - Anthony
