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
Thanks in advance






RE: Analyzing Existing Metal Building
RE: Analyzing Existing Metal Building
RE: Analyzing Existing Metal Building
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
RE: Analyzing Existing Metal Building
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
the PEMB and MBMA have their own standards.
RE: Analyzing Existing Metal Building
RE: Analyzing Existing Metal Building
I hate these buildings as much as tilt-ups.
RE: Analyzing Existing Metal Building
RE: Analyzing Existing Metal Building
RE: Analyzing Existing Metal Building
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
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
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
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
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
RE: Analyzing Existing Metal Building
RE: Analyzing Existing Metal Building
RE: Analyzing Existing Metal Building
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
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
RE: Analyzing Existing Metal Building
RE: Analyzing Existing Metal Building
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
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
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.