Analyzing Existing Metal Building 1

[StructTaco]

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

 

[StructTaco]

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.

 

[271828]

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.

 

[audeuce02]

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.

 

[JAE]

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.

 

[JoshPlumSE]

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.

 

[rharting]

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.