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# Reinforcing a PEMB6

## Reinforcing a PEMB

(OP)
Background
One of my projects is the addition to an existing six storey building. The original building was used for storage, and it’s being re-purposed to Residential. It is a flat slab construction with round columns and capitals and drops. The only sign of distress observed is on the 3rd floor with a narrow, but long, flexural crack at mid span. The crack is nearly half the length of the building. The floor with the crack had almost wall to wall 10’x10’ masonry partitions to create storage compartments. I suspect that this is the cause of the flexural crack observed. The original building is over 100 years old.

A PEMB was added to the top floor (to make it a 6 storey building) several years back. The exterior columns are supported on the original building brick parapet which is about 4’ high and 24” wide. The horizontal thrust is accommodated by diagonal angle struts fastened to the existing roof and the base of the exterior columns. I looked at replacing the struts by reinforcing the structure to behave like a continuous simple steel structure and found the framing members to be a little ‘light’. I calculated the capacity using a design dead load equal to 13psf; the original supplier may have used 10psf; I don’t know, but I expect that sort of design for PEMBs.

Now the Crux

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

### RE: Reinforcing a PEMB

dik, you know what to do. But pitfalls I'd consider are:
• Measuring the sections. There's no guarantee they're of the same thickness, floor to ceiling, much less figure the tapers of the beams/columns. Webs are particularly hard to measure. If you measure a dozen purlin thicknesses, how do you know the 13th is the same? Some sections are proprietary. Some measurements are going to have to be made via a lift or ladder.
• How do you tell the owner that not only the building needs to be reinforced for the new loads, but it doesn't work for the current loads and maybe didn't work for the loads in effect when it was designed? So doing nothing is not an (legal) option.

### RE: Reinforcing a PEMB

(OP)
Thanks Jed...
I've already obtained the member sizes... roof beams are all built up and uniform X-section and columns are HSS 3x3 and the end columns are tapered to the haunch. When I first looked at it, it was to see if I could make it non-PEMB and elimnate the diagonal struts from the parapet to the original concrete roof. Based on my original rigid frame analysis it was only marginal (as I've often found PEMBs to be). To eliminate the diagonal struts, would have required reinforcing all the roof beams, so, I didn't bother to check the purlins. That was it. They are planning to add more insulation, sprinklers and a ceiling... doubling the dead load on the structure.

I figured the easiest way to accommodate this was to just look at the added dead load and determine the new moments generated and the BAR reinforcing required to provide that capacity and just beef up the PEMB with the new reinforcing without going into a detailed rigid frame analysis, and neglecting the effect of the rigid connection between the beam and the column on the outside.

I'm not concerned about the main building; the new design loads are likely significantly less than the storage building loads. The city will be approached on this and it may require load testing... I dunno.

I've already advised the owner that the original design is 'marginal'. I don't know what magic goes into PEMB design, but with their fabrication skills and highly developed software they are a tad ahead... I really dislike PEMBs, and think of them as 'throw away' buildings at the end of their life... but, I admire the engineering that goes into them.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

### RE: Reinforcing a PEMB

Interesting situation.

Your proposed method as I read it is to assume the rigid eaves plastically hinge at the original design load, which caps the horizontal thrust that can get transferred to the base (and therefore avoids increased stress on the parapet/kicker). The additional 3psf or so then is carried by simple-span action between the plastic hinges at the eaves. If you truly have to check the rigid frame, I wouldn't go this route. I'd treat a rigid frame as a rigid frame.

Alternatives:
-Can you or owner obtain the original PEMB load/reaction package (only few years ago, right)? Drawings? I'd expect at least some collateral load in there which ought to cover your assumed 3psf add, or whatever it is. Unless you found it in writing that there is none.
-If the frame is double-pitched, can you run a horizontal tie member like HSS or rod between eave connections. Add verticals if span is too far. Splice it around/through any interior columns. Depending on your actual frame geometry, this may or may not apply. If it does, now you've got a "truss" that can behaves much closer to what you want (and presumably the headroom is fine, given that you're adding ceiling). You do not seem wary of altering (and therefore owning) the struc adequacy of the PEMB rigid frame, although I might have some caution here. This tie would also reduce the horiz thrust from live load, leaving that suspect parapet/brace with much less demand from thrust, a major plus (as you already seem to have identified).
-Is parapet good for wind uplift reaction? Unreinf masonry and all.

Let us know how it goes.

### RE: Reinforcing a PEMB

(OP)
Calvin... the original manufacturer of the PEMB is no longer in business and the EOR for it has passed on to wherever engineers go. I'm really wary of altering PEMBs. I'm taking ownership of anything I do. The shop drawings are almost unreadable and there are no design loads stipulated. No existing drawings are available.

The existing concrete and masonry (brick) building goes back 100 years. Concrete unknown, but unless highly reinforced or shear issues in normally not a problem. Reinforcing size, type, spacing, and grade is unknown but the change in use likely lessens the loads originally applied.

The moments at the ends caused by rigid frame action reduce the positive moment in the end spans. I would approach this as eliminating the end moment and designing the rigid frame as a 3 span continuous beam (there are two intermediate columns) and forget about frame action. I would just apply the added dead load moment to all beams by means of BAR stock welded to the existing, flat on the bottom, and two rounded bars at the top.

Is there a company that specialises in designing PEMBs, with the exception of the suppliers?

I haven't looked at the parapet for uplift, but the loads are pretty small compared to the masonry mass of the parapet. The brickwork is in excellent condition.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

### RE: Reinforcing a PEMB

There is an engineer in BC we hire from time to time for Pre-eng design checks, but I am not sure how common they are. I suspect most like Behlen or some of the other Canadian companies have consultants they hire for things that they do not want to do. That is how we found the guy in BC.

A 3D scan of the PEMB will pick up a lot of information very quickly. You would still need to measure some parts with calipers. The old school pre-eng was not nearly as refined at those we see today since they did not have the automation we have now. It might not be that difficult to figure out.

### RE: Reinforcing a PEMB

(OP)
Thanks Brad... do you have the name of the consultant in case I need to contact him? It's my experience that usually PEMB manufacturers have their own juju team that can squeeze the last ounce from their system. Most engineers I've encountered have the same experience. I suspect they use loads accurate to 0.1 lb, rather than round them up to 1 lb like I do.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

### RE: Reinforcing a PEMB

(OP)
Brad... I forgot... I'll check with a couple of PEMB manufactures to see if they have a consultant they use.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

### RE: Reinforcing a PEMB

dik, at least your PEMB company is out of business, so you have an excuse. And it sounds like you're going in with your eyes open.
We had a case a few years ago where we needed to move a brace in a PEMB. We told the contractor to contract a structural engineer specializing in PEMBs, preferably the original firm. We didn't want our fingerprints on it in any way. And we're not a structural engineering frim, but we do most of our own structural engineering.
Their response:

And this is one of the big PEMB players. This is after they put this note on their drawings.

### RE: Reinforcing a PEMB

Dik,

There's a company named StruktureOne in Austin, Texas that does PEMB consulting. I also saw someone on a previous post saying that they were analyzing a PEMB structure using RISA because RISA has incorporated Design Guide 25 for analyzing tapered members like PEMBs use. I don't have RISA anymore so I can't speak to how well that works but I know design guide 25 was written by the guy who created the proprietary software for one of the larger PEMB companies.

### RE: Reinforcing a PEMB

(OP)
maybe aeronautical engineers?

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

### RE: Reinforcing a PEMB

(OP)
I'm a PEMB guy. The first thing I'd do is really drill down on what the actual loads are. DL for most PEMB frames calculates out to 2 psf or less. Purlins, panel and insulation typically calc out to 2-3 psf. So if that's all that was supposed to be in the original building, that's what it would have been designed for.

Using actual loadings, the estimated DL on the PEMB was just a little over 12psf. I'd normally use 20, but used 13 for checking it.

It may make sense on a normal job to assume heavier loading to leave a building with extra capacity when you design it but if I'm the owner I would want to know how the building works for true realistic loading and then maybe get options on what it would take to bring it up to a higher allowable capacity in the event that it's actually lower than what you've calculated.

Concur... and with industrial useage I stipulate added loads to be applied. This is something I get the client to agree with based on earlier problems I've encountered with PEMBs. There is a price to pay for this reserve capacity. There's n free lunch.

I've often been on calls with the EOR and GC about what loads go onto the PEMB. On the one hand, it would be nice to know that your building is rated to take additional loading down the road if something changes. On the other hand, how much are you willing to pay for that now considering that may never happen or you may not be the owner if that's desired one day. Most owners aren't interested in paying for future loading.

On one project a couple of decades back... there was a horizontal C section providing restraint for the top of a 4' high 8" CMU wall... deflection they used was L/90... This was not acceptable based on code requirements at the time, and was fixed, but the client paid addtional.

Also to speak to some of the other conversations in this thread, PEMB companies design for the lowest possible loading...

Concur... that's their job, and many of them are very good at that.

Also, our design fees aren't that high...

I have no idea what part of the price is design fee. Due to the high competition I suspect fees are pretty close, pretty much like the wood truss industry. Most PEMB manufacturers have highly specialised software that does the work. That's the part of PEMBs that I like.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

### RE: Reinforcing a PEMB

@SandwichEngine - great information. I wanted to give a bit more information in regards to the project I mentioned. There definitely was some custom aspects to it, ie 12 to 15' tall parapets that we designed but supported on the PEMB roof (not sure why architects love such tall parapets - they absolutely suck), lots of coordination between us and them, but for the most part their structure was very simple and repetitive. We had to design the walls because the jurisdictions here have started pushing for stucco finishes on PEMB's, not panels which changes building deflection criteria and the PEMB guys apparently won't design for anything but metal panels (so we are told). I believe the biggest thing may have been that they can no longer do preliminary reactions (many jurisdictions in the state are now requiring this) for foundation designs and must give final, but this is done before they even have a contract to provide the building, meaning they have to recoup their engineering cost up front. I wasn't privy to the number of hours, etc.. but was shocked at their fee. I also suspect the local fabricator/installer was adding a cut for themselves as the building itself was designed by one of the largest companies in the industry.

It was nice when I finally got to coordinate with an experienced PEMB engineer instead of the new grads who promised a lot and never delivered.

I understand how the building is made to be the cheapest possible, but in the long run, in my experience all that is happening is pushing the cost into the foundations which are very expensive, especially with shortages in concrete. There was another project where we were asked to provide a conventional steel building design because the PEMB plus crazy foundation was so expensive, in the end we provided a better building with cheaper foundation and overall less cost. I agree with dik that PEMB's are throw-away buildings, especially for anything that isn't a rectangle (which is more and more common).

### RE: Reinforcing a PEMB

(OP)
Do PEMB manufacturers have guidelines on the design of CFS roof purlins?

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

### RE: Reinforcing a PEMB

(OP)
SandwichEngine...

#### Quote (There's a company named StruktureOne in Austin,)

Do you have a link for that... there are several, but none seem to be PEMBs. dikcoatesatgmaildotcom. Just want to add it to my address book right now in case...

Just went for a quick tour through DG25... is that ever a 'book'.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

### RE: Reinforcing a PEMB

Dik,

The website to the company I know about is teamskdotcom. I know that they design PEMB but do not fabricate themselves which is what makes me think they do PEMB consulting work. I've never actually spoken to anyone there so that could be untrue.

PEMB companies do have guidelines to the design of the CFS coldform purlins--the AISI code. Just like everyone else. That is, unless they have some proprietary purlin product where they've derived the allowable loading through testing. Normal Cees and Zees though? Same as everyone else.

Aesur,

#### Quote (Aesur)

PEMB guys apparently won't design for anything but metal panels (so we are told)

I've never heard of a company that wouldn't be able to design to different deflection limits but I don't know how everyone works. It's hard for me to understand how a business model that limited could be profitable.

### RE: Reinforcing a PEMB

I wonder why the North American market has not made cold formed purlins and girts into a commodity similar to open web steel joists. In Australia, where we don't have commodity OWSJ, we just use load tables for purlins and girts. They are provided by the manufacturers, and are based on a combination of calculation and full scale load testing.

### RE: Reinforcing a PEMB

@SandwichEngine - I suspected as much. They said they could design the building frame to limit deflection, but couldn't design the girts for anything other than metal panels, so we ended up with CFS stud wall system spanning vertically. This was a local installer that uses one of the largest companies for the PEMB engineering and fabrication. If this was one project that it happened on I would chalk it up to something unique for them, but this has happened on many projects (all the same local installer however). I learned after the first proposal to exclude that piece (and have add services) or have a separate line item for the wall designs to stay competitive with the other local engineers who also exclude that piece. I will try pushing back harder next time to test the waters.

### RE: Reinforcing a PEMB

(OP)
Sandwich... thanks; I'll look them up. I use CUFSM for looking into CFS shapes...

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

### RE: Reinforcing a PEMB

(OP)
Hokie... we used to have tables for OWSJ where they had specific models for load combinations and spans, just like CFS and grating tables now...

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

### RE: Reinforcing a PEMB

Some thoughts:

1) If you let the frame joints go plastic under gravity, that might alter their availability to resist lateral loads. In the extreme, you might might be down to only one of your two frame ends resisting lateral loads. Load casing stuff would probably prevent things from ever getting quite that bad. You likely don't have the kind of roof diaphragm that would allow you to switch your VLFRS to braced frames only.

2) If there is space and fiscal appetite to do so, you might externally post tension / truss your rafters to offset the added dead loads. This could get analytically complex in a hurry too so I suspect that a fair bit of judgement would be required to make a go of it.

### RE: Reinforcing a PEMB

(OP)
Thanks Koot... I'm thinking of relaxing the exterior support haunch moments and treating it as a 3 span continuous, with X-bracing, this is because the end supports are sitting on a parapet already and I don't want to attract any added moment, and resulting horizontal load. I briefly looked at seeing how the post-tensioning of the system, but it got way too hairy, too quickly.

I have a meeting with them next week, in the interim, I'll take a gander at the roof purlins and see the best way to address reinforcing of these elements. I was just starting to do an SMath program to deal with purlins with cantilevered ends, using altenating loads of 1/2 (DL and LL) on the cantilevers... to mimic cantilevered condition that's not connected, but with doubled members at the support. I'm almost coming to the conclusion it's time to 'throw away' this PEMB, and that may be my recommendation.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

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