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Simply supported beam assumption

Simply supported beam assumption

(OP)
I am a mechanical engineer who is trying to initially size members in a steel one storey building, and I have very little experience with structural steel.

My first question is how to approach a flexure member that is not necessarily able to be considered simply supported. In general I know that the max moment of an indeterminate beam with a point or constant distributed load is going to be less than that of a simply supported one, albeit it will happen at the end points rather than it’s center. So, I’m wondering if because the magnitude is less can I just assume that I’m being more conservative by using the tables and formulas which specify simple supports? Given that the calculation of C.b is based on the internal moments it seems plausible that I could.

On a related note do shear vs moment connections have different considerations when it comes to bracing? It is my assumption that an end plate for instance would be adequate at bracing against beam torsion and lateral displacement (assuming the column did not bend/curve) but I was not sure if that would be true for a shear plate? In the case I am analyzing it appears that the roof purlins will not adequately brace my beam and I need to size as if it’s completely unbraced, but I know I have to ensure the ends are secure and am essentially trying to confirm what it takes to assume that the endpoints are braced, either via the connection itself or additional joists between columns perpendicular to my main beam.

RE: Simply supported beam assumption

First of all, these are pretty easy questions that someone in your company should be helping you out on. If someone asked me to preliminary design a pump lift station, I would have to ask questions.
Your design should match your analysis and your analysis should follow your design. But most of the times, simple ended members are assumed. It's a pain to design fixed connections, the moments must be carried into the structure and it's just not worth it.
Both moment and simple (not shear) connections provide lateral bracing. However, moment connection affect other parameters (Cb) that can increase your beam's capacity. But why get too refined? If this is a preliminary design, assume simple connections. I would rely on the tables and charts in the AISC manual.
And don't forget about deflection. This normally controls, at least the beam depths. You don't want a W8 spanning 35 ft.

RE: Simply supported beam assumption

Usually when you use fixed end conditions on a beam, you reverse the compression flange to the bottom of the beam - this can affect the unbraced length of the beam and affect moment capacity.
In your case you suggest that even the top flange connected purlins aren't adequate so if you are using the full beam length as the unbraced length then this issue doesn't come into play.

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RE: Simply supported beam assumption

(OP)
In my analysis I considered the unbraced length to be the full length of the beam span. My concern was how to determine the necessary strength of bracing at the endpoints given that there would be no nodal braces against lateral torsional buckling. I have analyzed deflection as well as the other modes of failure, and now my concern is the end point conditions and what level of strength and or bracing needed there. From your responses I have come to understand that the connection should be adequate due to the stiffness of the column that the beam is attached to and that additional lateral bracing would be unnecessary at the endpoints. Is my understanding correct?

RE: Simply supported beam assumption

That may or may not be correct. If you are going to do anything more than cursory sizing for a very rough cost estimate, it is best that you sit down with an experienced structural engineer. Without knowing the loads or configuration of what you're doing, it is difficult to answer your question.

Will you be doing the final design as well?


-5^2 = -25 winky smile

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RE: Simply supported beam assumption

Helpmeout,

I agree with swearingen regarding the difficulty in providing assistance, particularly since your question is fairly broad. I am going to preface my suggestions with the assumption that you are only providing very preliminary sizes to be reviewed by a competent structural engineer. So... 2% of the plastic moment force couple is a conservative approach to beam bracing force. People have using that since the beginning of steel design. Usually, *usually*, simple shear connections can easily resist this force to prevent the beam section from twisting at supporting members, but not always. That is really all that I can say not knowing the details.

Have you read the AISC manual thoroughly, or do you not have access to it? Also AISC publishes technical papers and guides - you will certainly find a primer on beam bracing if you look there. The steel industry and thus AISC pumps more money into free technical guides and notes than any other industry in structural engineering. The internet is busting at the seams with all of this information.

Good luck.

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