Question on design assumptions and structural modeling 4

[STpipe]

Hi everyone,

I've run into a scenario that has me puzzled with one of the projects I'm working on. I'm more interested in the theoretical aspect of the problem.

It's a retrofit, where I'm adding some beams and columns to provide additional support to a roof. The roof itself is made of material with a much lower stiffness than steel. I came up with the preliminary member sizes using hand computations using classical methods, and then subsequently modeled the structure (including the roof) in a commercial program to perform the detailed analysis.

The issue I'm having trouble resolving is that due to the nature of the analysis, the roof is providing significant lateral support to the beams, and is inducing significant axial loads into them. When analyzing the original structure, or in past projects where we have to interface a steel support structure with this same type of roof, the other engineers did not include any sort of lateral support that the it might provide in their analysis and calculations. However, based on what I'm seeing, not doing so may lead to the analysis missing a large axial load in the beams.

So my questions:

1) Are those axial forces in my beam "real", or simply a function of the modeling techniques used where I may need to revisit my assumptions.
2) Before the advent of these advanced 3D structural analysis programs, how would one go about making the correct assumptions for the structural behaviour when dealing with these scenarios which are not typical. The classical techniques which we learned typically would only consider the steel framing.

Thanks.

 

[STpipe]

racookpe1978,

If I'm understanding your post correctly, are you saying that I shouldn't follow the general consensus (which is to ignore the composite action between the beam and the roof and to simply design the beam on its own), but rather it should be modeled, albeit accurately taking into account the accurate properties of the roof and the actual stiffness of the connection?

 

[BridgeSmith]

My use of the cover plate analogy was simply as a way to explain the effects of composite action, where it exists. If the roof was added as a structural component attached to the beam, it would increase the capacity of the section, and if the actual roof did not have the strength capacity or stiffness that it did in the model, the system would not have the capacity predicted by the model. That is why I advised conservatively ignoring the roof except for lateral restraint and the loading it adds.