×
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

Analyzing Existing Metal Building

Analyzing Existing Metal Building

Analyzing Existing Metal Building

(OP)
Anyone know of a software that easily models a preengineered metal building frame?  I have to analyze an existing frame for additional loads and I can't find a tapered girder or column option in ETABS or Enercalc.

Thanks in advance

RE: Analyzing Existing Metal Building

I beleive RISA allows you to add custom tapered members.

RE: Analyzing Existing Metal Building

Yes it does.

RE: Analyzing Existing Metal Building

Most PMBs use some very trying and almost exotic methods to show their buildings are good.  Not sure I agree with many of them.

That said - I have also found there is usually little or no fat in these buildings.  So adding ANY loads often blows them out.

Good Luck!!

RE: Analyzing Existing Metal Building

STAAD allows for this as well

RE: Analyzing Existing Metal Building

I entirely agree with Mike.
The PEMB designs go as far as to taper the members in a way that all of the sections just meet allowable. It's usually difficult to show that they work without adding any load to them.
Anytime we add any load greater than to hang a shirt on to an existing PEMB, we add an independent frame to carry the new load.  If you're worried about the cost, think of the money that was saved initially by the design.

RE: Analyzing Existing Metal Building

MiketheEngineer is right.
the PEMB and MBMA have their own standards.   

RE: Analyzing Existing Metal Building

I have a great method to analyzing metal buildings.  I refer all metal building questions to another engineer who I have made acquaintances with.  He was the chief engineer at a major company metal building company some years ago.  This way I don't have to figure out their wacky methods.

RE: Analyzing Existing Metal Building

(OP)
I was afraid of that.  There are Z-purlins as well, I'm analyzing the addition of a fall protection anchor and none of the members are typical sections I'm used to dealing with in normal building design.

I hate these buildings as much as tilt-ups.

RE: Analyzing Existing Metal Building

(OP)
SteelPE, do you have the contact info for your acquaintance who knows metal buildings?

RE: Analyzing Existing Metal Building

We have had success in going back to the original (or another local) metal building designer and having them give us both help and good information to work with.

RE: Analyzing Existing Metal Building

Good luck w/ the fall protection anchor.  If you have an "engineered" system -- shock absorbing lanyard, tested D-ring, etc., etc, you need to design for at least a 2,000 lb load.  At least that is the best I have found.  Our system  tests at just less than a 1,000 lb load.  OSHA says you can design for twice the anticipated load. Don't forget - that is for one guy ONLY.  It is additive!!

Without that, OSHA states that you must design for a 5,000 lb load.  Good Luck!!

Just tell them they can only fall when there is no snow or wind or rain or earthquakes around.

RE: Analyzing Existing Metal Building

PEMB are a pain.  I know that it is common (but typically not published) to set there unity stress checks at a minimum of 1.03 and even stretch that to 1.05 or 1.10 in certain circumstances.  By a strict analysis, it would be likely that you could not justify the loads without adding anything.

On another note, I have seen PEMB were the collateral uniform dead load added is 0.  They only use the self weight of the building and anythning that they supply such as insulation.

They have done a heck of a job in convincing people that their buildings are cost effective, but you get what you pay for.

RE: Analyzing Existing Metal Building

StructTaco

I do have his contact information however; he only works in the New England area so depending on where you are he may not be of any help to you (he usually requires some sort of site visit).  Most of my work is in New England so it is very convenient.
 

RE: Analyzing Existing Metal Building

Careful when you drill a hole in one of the members <G>...

I didn't catch the age of the building, but modification of it will void any warrantee you may have had.  Some are so 'tightly' designed that modification may have an impact a couple of bays away... (Removal of an exterior 'leg'.)

Dik

RE: Analyzing Existing Metal Building

"Most PMBs use some very trying and almost exotic methods to show their buildings are good.  Not sure I agree with many of them."

MBMA companies use the AISI and AISC Specifications as required by the building code.  What exotic method from these specifications are you referring to?  Or, are you claiming that they do not design per these specifications?

RE: Analyzing Existing Metal Building

(OP)
The client gave me the original drawings and calculations for the building...after I spent 8 hours yesterday pulling the rest of my hair out...

RE: Analyzing Existing Metal Building

I agree that they are really "efficiently" designed, but has anybody observed any major structural problems in PEMBs?

RE: Analyzing Existing Metal Building

(OP)
no I've never heard of a major structural problem with them.  They just aren't user friendly for any modifications, and clients/owners always want to change things a bit...

RE: Analyzing Existing Metal Building

There have been some collapses in heavy snow.  However, in every case I've seen, the snow exceeded the code prescribed load, so is that the fault of the MB manufacturer?

To be constructive and actually answer the OP's question: Several programs have tapered member capabilities.  SAP2000 and RISA-3D are a couple of examples.  These members are so easy to program that surely others have them also.  However, beware that the programs most likely do NOT correctly do the strength calculations for tapered members.  They also almost certainly do not correctly handle members with slender compression elements requiring Pn to be computed using E4 and E7 of the Spec. and Mn to be computed using F4 or F5 of the Spec.  They also probably do not do Kx calculations correctly nor do they correctly do the Direct Analysis Method for such a building.  Finally, there are other buckling modes such as constrained axis torsional buckling that are common in MBs and relatively uncommon (not as much as most people think, though) in conventional buildings and this mode is certainly not in typical commercial programs.  Use the program to do the analysis and use manual calcs or spreadsheets to do the strength calcs.

Let me guess what's next.  3 or 4 guys will blame the programs' inadequacies on MBMA?  Sounds like a bunch of gossiping women getting together to bash someone who's out of the room.  Conduct that's beneath a group of licensed professioonals, IMO.

RE: Analyzing Existing Metal Building

(OP)
Thanks 271828, that is very useful information.  I can imagine torsional buckling would be a problem on these members since they are usually deep with not very wide flanges.  PEMB's serve their purpose, they just aren't something I design here in California.  I get clients that want to modify tilt-ups and add to PEMB's, which is difficult at best without adding structure or strengthening existing structure.  I needed a quick way to evaluate my specific problem and was wondering if I even had the tools to get through it.

Nonprismatic plate girders are not easy to design, it usually saved for advanced steel classes in graduate degrees.

Thanks again for all the help everyone, much appreciated.

RE: Analyzing Existing Metal Building

Understood.  Unfortunately, I don't think there is a quick solution that is "correct."  All an engineer can do is build into his fee a more extensive analysis than he's used to or the money to hire someone else who specializes in MBs.  What gets me is when people (not you) don't want to do this logical course of action and then want to bash the original designer who did exactly what he was hired to do.  

RE: Analyzing Existing Metal Building

As an engineer with a PEMB company, I would reccomedn that you go back to the original manufacturer and have them do the analysis.  Are they going to charge for the analysis of the strucutre and the specification of the re-enforcment?  You better believe it, just like every SE on here that has been bashing PEMB.  Do we build excessive load into our buildings?  No, show me where in the Code it requires that we design for future potential loads.  It seems to me that we have some SE's who think a system is substandard becasue they do not take the time to learn it.

RE: Analyzing Existing Metal Building

Not sure I see a whole lot of "bashing" on here - maybe an inference or two that PEMB's are very tightly designed.  

Yes, you are only required to design to the code minimum loads.  I have no problem with that.

But it is curious that after this winter, and after several windstorms over the years, the collapsed buildings in our areas are almost always PEMB.

I agree with audeuce02 that the original building manufacturer should generally be the first choice in terms of consulting for additional loads.

 

RE: Analyzing Existing Metal Building

Some comments on PEMB's.  
1) There is not a current AISC specification that covers the design of tapered members. That section of the code was specifically removed with the 2005 version of the AISC spec.

2) Even when there was a valid specification, there were a number of restrictions on the use of those provisions.  Many metal building violated those restrictions.

I'm not saying that the metal buildings were improperly designed.  Just that the criteria upon which the design was based did not correspond to the AISC specifications.  

The MBMA did not have any true design specifications either. Therefore, companies designing these types of tapered members were often developing their own criteria based on a 1970's or early 80's text book and their interpretation of "first principles" of beam behavior. I'm not sure how consistent the various companies were in their interpretation.  

3) MBMA and AISC will soon be releasing a joint publication inteded to be used as a "design guide". I believe this publication will be a more complete treatment of tapered wide flange beams than anything we have seen before.  

Once this publication is finally release, there will likely be a new version of RISA which incorporates these design provisions into the program.  The publication of this design guide seems to be stalled for some reason.  Maybe that will give us a chance to incorporate a "final draft" version of the design criteria into the program even before it is officially published.  We'll see.   

RE: Analyzing Existing Metal Building

271828,

Here are some of my issues with pre engineered buildings:

I agree with you with your comment "However, in every case I've seen, the snow exceeded the code prescribed load". However the "prescribed" load is a minimum. To not include any additional load other than what is exactly on the structure (which appears to be the standard practice) is a bit scary to me. I doubt as a designer you know exactly where a HVAC guy is going to put his duct. To design to the absolute minimum (while within guidelines) assumes that structural construction is perfect, that you know exactly where all loads are, and that the owner will not hang any additional load. I don't agree that you can assume all of those at the same time.

It also appears (IMO) that many of these designs are outsourced to other countries (in most cases India). A few years ago i had a project (i had to design the foundation) where after I sized the foundations i realized that one of my foundations was considerably smaller than i expected for the bay spacing. After several hours of review, the designer had designed the middle column supporting a continuous beam to support 5/8 of the total load(the end beam reaction). After several conversations with the actual designer (who couldn't understand the concept of "trib width") i called the person who signed the drawings and he immediately apologized and thanked me for noticing (the column would have failed under the normal snow load). The designer didn't realize that the 5/8 in the steel manual was the reaction at each beam, not the total reaction at the middle.

I believe that PEB IF designed properly will only meet the absolute minimum. I wonder how many of your clients understand what exactly is meant when you say minimum.  

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