Tributary Width!! 3

[ssdpe]

When dealing with a simple span beam subjected to uniform distributed load, one can consider the tributary LENGTH to obtain the reactions at the support to be L/2. However, when continuous beams are used this is not correct.

If you refer to the AISC Table for Continuous Beams,(2-312), you will find that for a two-bay system the reactions at the center support are 25% greater than if continuity were not considered, (5+5)/8=1.25L. Recall that my argument is for equal spans, equally loaded. If you check the remaining cases, you will realize that the first interior support greatly affected by the continuity.

For instance, if I am going to design a 150ft clear span rigid frame and let's say that there are 5 bays at 25ft, I believe that my tributary width for the design of the first interior frame should be (23+20)/38*25ft=28.29ft.

I had an argument with my boss about this issue. He insists that everybody ignores the increase in loading for the first interior frame and that he would design the frame for 25ft. I think he is wrong. What are your comments or suggestions?

 

[LPPE]

When I design interior supports for continuous beams I always design for the additional reaction.

 

[haynewp]

The reactions for a 2 equal span continuous beam will depend on the section being considered, 25% additional is conservative for gravity loads.
There are some situations, (ex. calculating resistance to uplift loads from cross braces) where this would not be conservative.

 

[JAE]

Arguments with the "boss" are always a joy!

He is right that generally, for most shear conditions the difference ends up being quite small...especially for steel members where shear rarely controls in the beam. And for column/footing design, the difference can be important if you are designing right on the edge.

In many cases, you end up with a design that is somewhat conservative and the difference doesn't really affect the final design.

But it may be important for some cases - we always simply analyze it to account for the continuity and the difference -

 

[trainguy]

You're right, he's wrong, period. His slide rule approach probably makes it a challenge to perform the arithmetic. As JAE points out, it probably doesn't affect the final design.

What I do, is to introduce reasonable amounts of conservatism wherever I can in the design. So, use the "increased" reaction approach when designing for downward loads, and use your boss' approach when checking uplift (as hawnewp suggests). That may be a decent, defendable compromise.

tg

 

[ssdpe]

Pylko, Haynewp, JAE, and Trainguy:

Thank you for your responses on this matter. I think that when designing structures you not only need to be concerned about exceeding a budget but about safety and compliance. Like JAE said, "Arguments with the boss are always a joy". Have a good day!