The Pros and Cons of Using Physical Members

Most of the Structural programs from Bentley, including STAAD.pro, RAM Structural System, and RAM Elements, consider the notion of a physical member vs. a segmented finite element model, and there are certain pros and cons to that approach.

Here’s a simple structural framing plan to illustrate what I mean, the girder (beam #1) supports unequally spaced beams (8’; 8’; 8’ and 6’).

Simple Framing Plan with Girder supporting unequally spaced beams.

The RAM Structural System probably has the smartest treatment of physical members since that has been an essential part of the application since its inception. In RAM SS the design considers the forces, unbraced lengths, and Cb factors for each segment individually in the code check. It even goes so far as to consider independent segments for positive and negative bending, since single flange might be braced (e.g. by an open web steel joist or the decking). The program reports the only most critical case to keep the output concise.  There is an override available so that the user can specify a larger or smaller unbraced length, but the program simply uses that value in lieu of the calculated segment length in the design calculations. The design moments for each segment are not affected the override.

With the upcoming version of RAM Elements (version 11.0), the specification of member unbraced length has been expanded to allow the user to enter multiple unbraced length segments for a single member. The advantage of this is that the user can get closer to the sophisticated design approach used in the RAM Structural System, but there are some disadvantages too.

First, it is now required to define unbraced length segments that sum to the member length. In other words, you can no longer specify that a 30’ long beam is braced every 8’. Rather you need to enter 8: 8: 8; 6 or some other combination of segment lengths that equals 30’.  This means that converted models from previous versions will generate an error unless the member length is a multiple of the user specified unbraced length. 

Next there is the issue of Cb. I think most engineers would agree that Cb should be evaluated for each unbraced segment since the moment demand values change dramatically along the span, but if the program calculates Cb, it does so only considering the physical member end moments.  *** Correction, RAM Elements v11 does consider various Cb factors for the various unbraced segments of a physical member and then reports the most critical segment overall ***

So, this begs the question, “Why not segment the model and use the default segment length for the unbraced length?” The segmented model will not behave any differently from the model with physical members from an analytical point of view, but the user gains independent control over the unbraced length and Cb values used for each. If the defaults values are left in place, the results are more likely to be correct.

There are a few drawbacks to using a segmented model. With a segmented model you have more members to report on, and if you are printing a hard copy of the designs, then that might be a legitimate argument. Furthermore, if you are optimizing the design you need to make sure that the segments all use a common “Description” variable.

There are exceptions to the rule, and here are a few cases where using physical members is really a better approach:

1.       When designing Cap Plate supporting a continuous beam, or a Chevron Brace Connection with RAM Connection it’s necessary for the segments of the beam on either side of the joint to be defined as a single physical member.

2.       When integrating the model with other applications, like Revit, using the ISM interface, it’s probably best to define single physical members throughout.

I welcome your comments; have you found other good reasons to use physical members or segments in your work?

 

 

 

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