Metal Buildings - Independent Analysis 3

[Martino8]

Looking for guidance regarding structural design for pre-fabricated metal buildings.
We're talking the classic structural steel moment frame "bents" with light gauge purlins & girts with metal deck/siding.
Seems like the standard in the US is for these structures to be designed by the supplier (usually with in house privately developed software).

But let's say an independent engineer wanted to separately analyze this type of building.
What sort of software package would be best suited for the commonly used tapered built-up members etc?

 

[DoubleStud]

You are right about that. I keep getting printouts of the reactions from various different PEMB companies. They all have very similar printouts. They are horrible and difficult to understand! When I have questions, it takes forever for them to answer the questions. I am interested to know and also wonder why there aren't other software with better outputs. I want to know the software as well.

 

[human909]

Using you favourite structural frame software you can analyse tapered built up members by chopping the members up into discreet non tapered members. If you pick suitable segment size you will get pretty accurate deflections and stresses.

For LTB you can check segment by segment using an approach suitable for the code you are using. For AS codes that would mean considering the section properties, the bending effective length and the moment profile.

I understand US codes approach LTB in a somewhat different manner so I can't directly comment on US treatment. But for AS codes it is relatively easy.

*** For extreme tapers without suitable lateral/twist staggered along the length I'd be careful with assumptions used. But if you have suitable LTB restraints then you can get suitable results.

The software I use even has a tool for this:

https://www.spacegass.com/manual/Input_and_Editing_Tools/Generate_taper.htm

 

[TLHS]

Most output is the same because it's often this software:

https://mbsweb.com/

Actually a neat bit of kit. It's not for an occassional designer though. It's not cheap and you have to setup parts databases and things.

However, you can do analysis in any old structural software. The systems themselves aren't compicated usually. They're just using a bunch of custom sections and potentually making aggressive assumptions on certain checks.

 

[dold]

You can define tapered WF members directly in RISA 3D for analysis and design.

I was looking at some calcs for a metal building the other day. It appears that DCR < 1.00 is merely a suggestion for these PEMB guys. 1.01, 1.02, 1.03, 1.04.... looks good! smh

 

[Retrograde]

You can refer to AISC Design Guide 25.

 

[JAE]

You typically don't know the grade of steel used. The size of the members are most often NOT standard WF shapes, etc. but built up shapes out of plates.

The thin members typically require much more attention to buckling (local and global), thus the numerous braces for column interior flanges, etc.

The welding used to connect the "parts" also is an unknown to some extent. Older PEMB members I've surveyed in the past sometimes have variable weld sizes down the lengths.

Overall, we do everything in our power to avoid such re-analyses.

To me, these structures are:
like tin-foil members built-up like a house of cards,
designed to the gnat's ass,
guaranteed to be the first to fail in a storm, and

 

[Once20036]

Ram-Elements can also analyze tapered members.

 

[ThomasH]

But let's say an independent engineer wanted to separately analyze this type of building.
What sort of software package would be best suited for the commonly used tapered built-up members etc?

I would use a software that allows me to model the beams with plate elements. Then the tapered beams can be modelled correct and stiffeners and other "plates" can be included.

 

[JLNJ]

The one-sided stitch welds from the column webs to the column flanges give me pause.

 

[Martino8]

Appreciate the feedback. Thanks for all the resources.
As far as the detailing that falls into the "not how I would have done it" camp...
Do we think insisting on detailing that make sense to non-PEMB structural designers would make suppliers non-competitive?

 

[dauwerda]

Do we think insisting on detailing that make sense to non-PEMB structural designers would make suppliers non-competitive?

My first guess is that it would just make PEMB suppliers no-bid your project.

 

[XR250]

Listen to JAE

 

[bones206]

The level of effort required to do a full reverse-engineered analysis is huge unless you have the original shop drawings and calcs. Like JAE said, gathering as-built information from the field is a daunting task in itself.

 

[Aesur]

@Martino8 What exactly are you trying to do here? Retrofit an existing PEMB or design a new PEMB without going through a supplier?

If you are trying to retrofit an existing PEMB, depending on what you are doing, you will need to as-built the structure and most likely heavily reinforce areas of the beams, columns, bracing etc.. unless it's a simple addition of small mechanical units, maybe the girders work and you just need new purlins.

If you are trying to design a new building to be as cheap as a PEMB, good luck. There is a high chance you will not get a PEMB supplier to even bid on the project and will therefore end up at a normal steel fabricator which will be much more expensive being non-typical for them. I'm not saying it can't be done, I'm just going off what I have seen.

We recently had a project where an owner went online and bought a kit PEMB that had no collateral capacity, and many other issues (fire walls, deflection for the required finishes, etc.), they tried to get it permitted and it was rejected so we were brought in, in the end we had to add collateral capacity to the structure and the parts were already sitting on site. We went in at a fair, but much higher fee than if we engineered a completely new conventional building. There was some complaining but they still signed it. We spent every last dollar of that fee. Then the repair was so expensive that it would have been cheaper, if time allowed, to purchase a whole new PEMB and sell the one they had for scrap. Our repair consisted of adding many new columns, foundations, doubler plates, new wall system, new bracing system and more. This was simply because we went from 0 collateral dead load to 8 to 10 psf, which for a PEMB is 3 to 4x what it's designed for with self weight.