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Reinforcing a PEMB
6

Reinforcing a PEMB

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
The proposed renovation includes adding new load to the PEMB in the form of added insulation, ceiling and sprinklers. This will likely double the existing dead loading. Because the original design loading is light (or, very tight) I’m considering treating the existing PEMB regular steel building with continuous roof beams and adding BAR material to reinforce it for the moment caused by the added loading without moments at the exterior columns (like a PEMB), without going into a complicated rigid frame design. Can anyone suggest an alternative method or any pitfalls?

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.
pipe

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.

My additional design dead load, will be approximately 10 psf.

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. ponder

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.


And no engineers that they referenced would work with us (or answer the phone). So we had to do the work ourselves (grumbling all the way), absorbing a few million in liability to make $500.

Do PEMB companies have a shredding machine instead of storage files?


RE: Reinforcing a PEMB

(OP)
Thanks Jed...

You'd think so... I'll try contacting a couple of them tomorrow to see if they have any suggestions. I suspect their market is highly competitive and they do not have the resources to do 'extra' work. Their main 'buck' is made in shipping the box out of their plant. ponder

This is one of the reasons that I don't like PEMBs. I've encountered their use, several times, for industrial buildings; these are the last type of buildings I'd use them for. Most owners are unaware of the restrictions that come with them. In addition to advising a client that any alteration may void any warranty they might have, my drawing notes have numerous additional loads that have to be incorporated that can make it easier for an industrial building to use PEMBs. These are from my Project Notes, which I edit for each project:

MBS LOADING

MBS SHALL BE DESIGNED FOR THE FOLLOWING SUPERIMPOSED LOADS IN ADDITION TO THE LOADS STIPULATED ABOVE:

ALL ROOF PURLINS:
DEAD LOAD = 5 PSF UDL
LIVE LOAD = 5 PSF UDL AND TWO POINT LOADS OF 250 LBS AT ANY LOCATION

BUILDING FRAME:
DEAD LOAD = TWO 1000 LBS VERT DOWNWARDS AT ANY LOCATION
LIVE LOAD = TWO 2000 LBS VERT DOWNWARDS AT ANY LOCATION

HAUNCH:
DEAD LOAD = 500 LBS VERT AND/OR HOR AT BOTH HAUNCHES
LIVE LOAD = 1000 LBS VERT AND/OR HOR AT BOTH HAUNCHES

MBS SHALL BE DESIGNED FOR THE FOLLOWING OPENINGS IN ADDITION TO THOSE SHOWN ON THE CONSTRUCTION DOCUMENTS:

MAN DOORS
PROVIDE FOR THE FUTURE ADDITION OF 3' HIGH x 7' WIDE MANDOOR AT ANY LOCATION AT ALL BAYS ON THE SIDEWALLS AND ENDWALLS

OVERHEAD DOORS missed one...
PROVIDE FOR THE FUTURE ADDITION OF 10' HIGH x 16' WIDE OVERHEAD DOOR AT ANY LOCATION AT ALL BAYS ON THE SIDEWALLS AND ENDWALLS

I haven't heard anything negative about my proposed manner of dealing with the added load. It seems reasonable and expedient.

I seem to attract interesting projects... not complaining, I enjoy them... it makes up for the last month of boring projects. pipe

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

-Dik

RE: Reinforcing a PEMB

I'm sure the PEMB was designed for negative moment at the beam/column connection, so you'll likely have a thicker bottom flange than top flange there. If you're going to assume a plastic hinge forms in the beam at the column face, you do need to make sure that the beam/column connection itself can actually reach that load required. PEMB manufacturers don't develop the capacity of the member for connections, they design it for the load actually present in the beam based on their design criteria. It's possible that some connection components will fail prior to the member, especially if you take into account the likely thin plates and bolt prying.

Also, keep in mind that the built-up shapes are likely only welded on one side. Just something to be aware of and you'll want to run shear flow checks for the web/flange welds.

Source: former PEMB design engineer.

Go Bucks!

RE: Reinforcing a PEMB

(OP)
Thanks for the 'heads up', Staub:
Flanges are both the same. I didn't design it plastically and that may be why the loading was light. I use plastic design all the time and have for 50 years... but only with systems I have a lot of confidence in. If properly designed the entire frame would go 'plastic' at limit load. I checked it for prying and it actually had a little spare capacity. The thin plates increase forces due to prying action to a fair extent. I was a bit surprised. Shear flow was OK. As I noted earlier, I don't like PEMBs, but 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

Quote (dik)

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 took this as utilizing a plastic hinge. I think you still need to consider the frame action though, since the connection is detailed to transmit moment across. If the connection can pass moment, it will pass it to the columns and act as a frame. If the connection can't handle the increased moment, then there's real risk of failure with the lack of redundancy. I think that treating the beam as a 3-span beam for designing the rafter reinforcing is ok, but you still need to run the frame analysis to verify that the connections and columns are ok.

Go Bucks!

RE: Reinforcing a PEMB

(OP)
Thanks again... connections would be checked, and I'll re-run the frame to check, just to make sure; I wasn't planning to, but will.

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

-Dik

RE: Reinforcing a PEMB

I don't like the sound of a 4' high brick parapet supporting the frame thrust. How does that work?

RE: Reinforcing a PEMB

(OP)
I wasn't keen about it either, Hokie, but it's been in place for over a decade. The horizontal thrust is resisted by a diagonal anble braced to the existing concrete roof. No signs of distress or cracking, and I looked real close. The load is relatively small and the brick can easily accomodate the load.

Photo Added:



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

-Dik

RE: Reinforcing a PEMB

Quote (dik)

Their main 'buck' is made in shipping the box out of their plant.

I used to think that as well until I saw the fees they are getting to actually engineer the PEMB. In my area it's becoming clearer as PEMB's are no longer allowed to be deferred submittals for many jurisdictions, instead having to submit during permitting. One particular project that comes to mind was where they designed a roughly 80k sqft straight forward PEMB and I as the EOR had to design the foundations, stages, loading docks, a connector building between their two buildings at odd angles, and their (PEMB) wall system their fee was more than 3x my fee... and I got beat up by the architect on my fee being "too high". When I saw their fee (more than a dollar a sq-ft) I was, needless to say, pissed and decided I would try to educate architects on PEMB's and the liability we take, thus far it hasn't changed a damn thing.

As for your project, I honestly wouldn't even take on such a project, it screams needing major retrofit, potentially being brought up to code (at least in the US) and seems like a total pain to deal with. I hope you got a great fee for it and wish you the best with the project.

RE: Reinforcing a PEMB

dik,

I thought you were proposing to remove the diagonal braces. But don't you have wind uplift to consider as well? The diagonal won't help much with that.

Why not just remove the whole thing, and make the roof structure a rebuild? Probably more economical. All that site welding will cost.

RE: Reinforcing a PEMB

(OP)
Yes it it Hokie. The project owner originally wanted to eliminate the diagonal struts. I went through the numbers and to 'relax' the end column to make it no longer a rigid fram using actual dead loads, I couldn't get it to work without generally reinforcing all the roof beam structure. I didn't look at the CFS roof purlins because it was too onorous to reinforce all. Lateral loading would be undertaken by discreet X-bracing. Support on the parapet was iffy, but calculations showed it was easily capable of taking the vertical loads and it was in excellent condition, being a century old. I was just checking it for existing dead and live loads.

This was the original plan. Just recently, the client will be seeking federal funding, and to do so requires the building insulation, etc. be upgraded. This requires additional insulation, a sprinkler system, and a rated ceiling. The added loading requires that I revisit the PEMB. I was just going to leave it and not modify it. Now, I have to. pipe



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)

Quote (As for your project, I honestly wouldn't even take on such a project, it screams needing major retrofit,)


I didn't realise PEMB fees were so high... I'd always thought they were part of the price. Fees are not an issue at this point... I'm on an hourly rate for this. The project originally started as a small lower floor and main floor addition to an existing six storey building to become a six storey additon. Original walls were masonry, they are now wrinkletin and steel. It's a really neat project, and I'm slowly working my way through it. Any reiforcing of PEMBs I've been involved with tend to be very expensive. That's why I consider them as 'throw away buildings'.

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

-Dik

RE: Reinforcing a PEMB

I imagine the "design fees" are just a BS way of them making it look like the building itself is cheaper. Would be interesting to hear from some of the PEMB folks on here. I mean, what is involved? Inputting some design criteria and hitting return?
Hopefully the MPC truss folks won't catch on!

RE: Reinforcing a PEMB

Quote (XR250)

I mean, what is involved? Inputting some design criteria and hitting return?

Oooooof. You realize the big fabricators staff PEs and SEs right? The whole thing is signed off by someone who went through the same process every other PE or SE has to go through to get their license.

The simplest buildings are pretty much what you suggested. Most (75%+) go through a design process that in terms of QA/QC is better than most, if not all, engineering consulting firms.

Engineers determine the loads on the frame (yes, using specified design criteria, but through spreadsheets similar to other firms), and then tweak frame geometry to come up with an economical (read, as close to 1.000 as possible) design. They'll work on layout of bracing and columns to work with the customer to give them what will be easiest to erect and work from a layout perspective.

A lot of engineers doing the intial design work have 0-4 years of experience, and are hired straight out of college. All those designs are checked, but they are the ones tasked with answering most questions, so a lot of interaction with PEMB engineers from outside is probably with someone with less than 3 years of engineering experience. You'll understand then why some "don't know what they're talking about" and aren't as familiar with concrete breakout and the intricacies of foundation design.

Go Bucks!

RE: Reinforcing a PEMB

(OP)
I like the engineering that goes into PEMB... just not the product.

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

-Dik

RE: Reinforcing a PEMB

As someone who's worked on both sides, I understand that aspect hairpull2

Go Bucks!

RE: Reinforcing a PEMB

(OP)
I had a venerated and reviled prof once tell me that if a structural engineer were to ever design an aircraft, it would never get off hte ground. pipe

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

-Dik

RE: Reinforcing a PEMB

Who do you think designs the structure of aircraft? It's not architects or electrical engineers.

RE: Reinforcing a PEMB

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.

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.

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.

Also to speak to some of the other conversations in this thread, PEMB companies design for the lowest possible loading and take members as close to a unity check of 1.0 because they have to to be in business. PEMB companies only win a job if they provide the lowest cost building (at least most of the time.) Providing the lowest cost building means using the least amount of steel. Consulting engineers would be doing the same thing if they had to determine the total cost of all the materials in their solution and provide that in their bid against other consulting engineers and the one with the lowest cost solution would be awarded the design work. I realize that's not plausible but the point is that the market forces it to be this way.

Also, our design fees aren't that high, at least not for engineering. On any job, 15% of the in-house "design" work is engineering. 85% is detailing and 3D modeling. We estimate how many hours we think it will take and that part of the cost is factored into the price with a flat hourly rate. I've never seen that cost broken out and I can't imagine it would really be as high as $1 per square foot but I'll offer my guess. On whatever job Aesur saw, my guess is that there were a lot of changes and the GC wanted the PEMB supplier to break out every item of cost so that if some item of steel was removed, the GC could get credit back for it. Then the PEMB company converted some of the steel cost to appear as design fees to give back less money when steel was removed. I'm not saying I've seen that done but there is a lot that goes into negotiating changes between a GC and a sub and to me, that would be a much more plausible answer than a PEMB company legitimately charging $1/sqft and actually winning the job.



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? ponder

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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