×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Old beam...fMn values

Old beam...fMn values

Old beam...fMn values

(OP)
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.

RE: Old beam...fMn values

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.

RE: Old beam...fMn values

Is it cast iron or steel?

BA

RE: Old beam...fMn values

That's a good point, BA.. That never even crossed my mind.

RE: Old beam...fMn values

(OP)
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.

RE: Old beam...fMn values

(OP)
Is the above method a correct way to size a beam?

RE: Old beam...fMn values

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.

www.PeirceEngineering.com

RE: Old beam...fMn values

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.

RE: Old beam...fMn values

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.

RE: Old beam...fMn values

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.

RE: Old beam...fMn values

Don't forget to check the beam capacity based on the existing connection. That may control your design.

RE: Old beam...fMn values

(OP)
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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources