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Re: Maximum Moments in Beams

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You don't say what the beam span, tributary load width, dead load, beam material type and occupancy are or what size the reactor is so it is difficult to assess your values. You also haven't said anything about where the beam is braced (e.g., at the ends, at intermediate points, continuously by a slab). Without this information, it is difficult to provide meaningful comments.

You probably need to go to a basic steel design textbook and understand the various failure modes for a steel beam before proceeding further.

Have you considered that the moment may not be the control in the design. Depending on the span, tributary width, bracing and type of loading, deflection, buckling or shear might control. The beam may have been designed to prevent vibrations and as such has the need for a higher moment of inertia than might have been required by strength alone.

You say the only load is a 13 kip reactor. What is the building code LL for the structure. It may exceed the load from the reactor. A light storage load on an area under UBC would be 125 psf, a heavy storage load 250 psf plus applicable dead load in both cases for allowable stress design. A factory may have had even higher design loads depending on its intended usage.  Also, has the building changed occupancies in it's life. It might have been designed for loads based on a different occupancy.

Bill Cain, SE
Berkeley CA

-----Original Message-----
From: Mauricio Castro <rico355(--nospam--at)>
To: seaint(--nospam--at)
Sent: Tue, 17 Jul 2007 7:27 am
Subject: Maximum Moments in Beams

I am currently revising the structure of the floor of a building to make sure that the old beams will manage to carry an increase in dead load of about 7/5 of the current load. I calculated the new maximum moments and compared them to the maximum moments allowed by the beam (Using my 13th edition AISC steel construction manual).
I am getting maximum moments of about 8 kip-ft and according to the manual these beams (12 B 16.5 which are supposed to be W12x16) should resist moments up to 75.4 kip-ft. This is a noticeable difference which makes me thing that either the beam highly safe or the calculations wrong. I haven't considered safety factors yet, but even then, the safety factor would be too high. The floor is the top floor and practically only carries the weight of a reactor (around 13kips) and unfortunately the only plans are the layouts of the beams.
Does this difference in maximum moments sound right?
Mauricio Castro
Civil Engineering Student

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