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Old beam...fMn values 1

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jimtheengineer10

Civil/Environmental
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Apr 28, 2012
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I have an old beam (1900 or so) and I have to determine the uniform distributed load it can handle. I found an AISC document on slideruleeras page that gives the dimensions of the beam but I am looking for the φMn values. Does anyone know where I can find these values or do I have to calculate them by hand? Thanks.
 
Phi Mn is the flexural capacity (with a reduction factor), not the allowable uniform distributed load. Although, "allowable" is not the proper term for it, as you are using LRFD, you can back out the latter from the former (assuming deflection doesn't control), given the span and the load factors. However, Phi Mn depends on bracing and slenderness. IF your beam is fully braced against LTB and is compact, then phi Mn is simply Phi*Zx*Fy, using the 13th Ed. If any of what I've written sounds new to you, hire a structural engineer. This really is about as basic as it gets in Structural.
 
That's a good point, BA.. That never even crossed my mind.
 
Ok, I don't typically get into beam design but my old boss used to size a beam the following way.

Determine your moment from M=wl^2/8 and then use the LRFD manual to find a beam with a Phi b Mr values that exceeds the moment. Then you check for deflection with 5wl^4/384EI.
 
Is the above method a correct way to size a beam?
 
The deflection should be checked for a service ( w, unfactored) load or else the calculated deflection will also be factored higher than would be expected.

 
I'm not familiar with any manuals prior to the 13th edition, but what you describe is fine, provided your beam is braced and the Table you are using is 3-2. Table 3-2 accounts for non-compactness, but you have to figure out where it is braced and which value in the table you may use.

Don't disregard what BA brought up.. Cast Iron is much more brittle, and AISC would NOT apply.
 
Jim - Using ONLY modern calculations to evaluate the capacity of obsolete material could lead to grief. IMHO, at least take a look at what beam's rated loading was calculated to using late 19th century / early 20th century criteria. Then you can make an informed decision if modern methods give a reasonable answer.

BAretired's suggestion to determine if the beam is iron or steel is a good one, either is possible and it will affect your answer. A minor correction of a detail... 19th century beams were made from WROUGHT iron, not cast iron. Cast iron was used for compression members, like columns, but even 120+ years ago metallurgists knew that cast iron was way to brittle for the tensile stress in beams. Manufacture of wrought iron beams was phased out by the mid-1890's; the largest size ever made (to the best of my knowledge) was 20". Anything larger had to be steel.
 
You can use the basic principles of beam design from the Steel Manual but you certainly cannot use any of the tables for a beam from the early 1900's.

Design Guide 15 has a lot of information on older steels.

The method you are using to design the beam only makes sense if it is actually a uniformly loaded beam.
 
Don't forget to check the beam capacity based on the existing connection. That may control your design.
 
Thanks for all the replies. I let my client know that I would not be able to confidently give him the uniform load the beam can handle as I have limited experience with this type of thing and there are too many unknowns/variables. This is a great site. Thanks again.
 
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