Industrial buildings in the US 6

[Tom Livingston]

Hi All,

My company has just become involved in a US- based project which requires design of a number of low-rise industrial type buildings. My query is regarding the type of steel frame construction commonly used in the US for this type of building. In the UK we normally use straight hot-rolled beam and column sections for the portal frames, and add sloping tee sections to create haunches to the undersides of the beams at the connections to the columns to get deeper bolt groups etc. I understand that in the US it is quite common to have the whole of the members (both beams and columns) sloping rather than just using locally deepened haunches at the beam supports. Can anyone advise whether both types of construction are commonly used in the US, or is there a preference for one or the other? Thanks, Tom

 

[Ron247]

I have not dealt with a PEMB in several years, but PEMB suppliers are just like all other suppliers, there are good ones and bad ones. I have worked several jobs that did not include a metal building anywhere in the project and I had contractors who did not supply what was specified in the drawings or in the specs.

When dealing with a metal building, I used a "Letter of Certification for Loading" as a requirement to bid the job. I believe the version of the letter I had was from Butler Metal Buildings. It was a 2-page letter that was to be signed by the PEMB bidder. The wording was very precise about what I was getting in my bid. It was very common to have 10 people bidding a job and 4 of them bow out rather than fill out a simple letter that certified the design loads, codes to be adhered to and some other information most engineers would think are just common sense. They would say their legal department would not let them sign mine but most of them had their own version. As an example, my letter may say " in accordance with AISC 9th Edition" and the less reputable PEMBS wanted to say "use AISC 9th edition as a guideline". My favorite substitute one of them had was "We design in strict accordance to our own standards". If they wanted to bid, and would sign the letter, I noticed their price was not lower than the reputable ones anymore.

Another piece of advice is to critique your canned PEMB specs. I have not read one in years but I found a lot of poor wording that left the door open for faulty design by low-end companies. Years ago I called the PEMB expert at one of the canned spec companies to ask him to please add the AISI Code as a reference because each time there was a new release of their spec, I had to make a lot of edits. Their representative that was the specialist in PEMBS, asked me what AISI was. Like myself, anyone who ever worked in metal buildings know what the AISI code is, it is the steel code we were not taught in college and therefore had to learn on our own. Or as I call it, the purlin, girt and sheeting code. As an example, I have gone to several jobs to look at something not related to the PEMB but since I had worked in the PEMB field for 5 years (long ago), I was familiar with most aspects. The roof purlins sometimes looked flimsy for the span. When I asked out of curiosity I was shown how they met the specifications. The specifications said all purlins and girts must be 8" deep. There were no other requirements in the canned spec. The purlins were the thinnest available 8" Z and the narrowest flange width they could get. some of them did not have stiffening lips on the flanges. My edited can spec used to say "minimum 8" deep and designed to safely support all required design loads." Again, a common sense statement but leave it out and you open the door for poor design.

 

[Settingsun]

Ron247, sometimes you may want exactly 8" depth for some reason. And it would have to be a very airy spec to be able to argue that the design loads don't need to be supported - I have trouble imagining it.

Another approach to compliance with specifications is to have a tender schedule of all departures, with no departures stated elsewhere in the tender submission being accepted, ie the schedule takes precedence. You can then enforce the requirements after signing the contract at no additional cost.

 

[dik]

Do you, by chance, have a representative copy of that (redacted as req'd)?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[Ron247]

Steve49,
I see it on cold-formed studs a lot. The Arch specs says 6" stud, nothing more. What do they supply? The thinnest 6" stud that is really a track. It has no stiffening lips, narrow flanges. I work in the south and thanks to a lot of design loads never occurring, they get away with a lot. I hate it when I go to a site with this issue because it is so obvious, you can't act like your not capable of noticing it. Even though it is not in my scope of work, I know if something bad happens, I may get blamed for not seeing the obvious even though I had nothing to do with it.

I understand I can easily argue the specs, problem is, we are on site, under construction when it gets noticed. What I am saying is that disreputable suppliers/contractors will do it if they can get away with it and where I live, they can get away with it.

I know it sounds un-imaginable, I am not saying I believe this, I am saying I actually have seen it more times than you would think. The specs may be better now, as I said, I have not edited one in years. Last year I saw one with 20' studs, 16" oc, 3-5/8" deep, 1-3/8" unreinforced flanges. Contractor showed me the Arch spec and it said "minimum 3-5/8" stud at 16" oc". I called the architect because I knew him, told him cold-formed studs require at least 4 pieces of info (depth, style, thickness and steel strength). He corrected me and told me it only needed to have the depth. For some reason, the city inspectors never question it.

 

[Ron247]

dik,
I will look but I am pretty sure it was from Butler. I have not written a PEMB spec in 10 years so it may take a little digging to find it.

 

[Ron247]

dik,
Attached is a handout from 20 years ago where I gave a presentation to some engineers and architects on PEMBs. At the end is the Letter of Certification for Loading that I used and a real substitute I was given one time. Based on the code changes since the time it was written, it surely needs to be enhanced. It is a base letter, even back then we had to add mezzanine info, crane info, and other specific criteria as needed. At first, I thought the letter was not needed until I saw how may PEMB suppliers would not sign it. Hopefully, over the 20 years since I wrote this, some companies have improved but most likely not all of them.

I forgot about this handout but when I did a wild card search on PEMB, it showed up. The entire document is somewhat germane to most engineers or architects being involved in a PEMB so I included the whole thing. It was a handout that went with a powerpoint type presentation I appear to have lost. I also have it in word format.

 

[dik]

Thanks Ron... I'm not a big fan of them, and have designed foundations for hundreds of them. I'm impressed by the engineering that goes into them. ONe of my more interesting projects, was the removal of an outside frame leg to make way for a wider door. The owner was surprised when the effected span was in extended for three bays. Sand.. has shown an impressive collection of 'fine buildings' and am very impressed. The firm I was with 50 years back did nearly all of Butler's PEMB foundations in Winnipeg. They had the odd one like Sand..,

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[Settingsun]

Ron247, the E in PEMB doesn't invoke a duty of care that covers your spec wording about supporting design loads anyway?

 

[XR250]

I forgot about this handout but when I did a wild card search on PEMB, it showed up. The entire document is somewhat germane to most engineers or architects being involved in a PEMB so I included the whole thing. It was a handout that went with a powerpoint type presentation I appear to have lost. I also have it in word format.

Geez, it seems if you to were to do a really comprehensive spec on a PEMB, it would not be any cheaper than a stick built structure!

 

[Ron247]

Steve49,
You would think what I am saying would not be needed, but what I am saying is that each time I see this, they show me the specs. You are 100% right, you should not need to do this, but the combination of loose specs, low-end PEMBS and inspection departments who just look for a stamp on something rather than actually review it leads to this. Add to that, younger engineers fresh out of college and going to work for a PEMB. If their mentor convinces them something is OK, but we would not agree, they have little to go on because after all, that is their mentor. I worked for one of the more reputable PEMBs, and even we had situations where what the company was "suggesting" was highly questionable. The people making the suggestions, were not always engineers. Difference was, we pretty much said no.

XR250,
I think PEMBs will always be more cost effective because of reuse of standard details, standardization in general and the fact they try to move steel around from where it is not needed to where it is need (i.e the tapered members). A prismatic member is highly stressed at one place and the balance is less stressed. By moving the steel from the low-stressed pinned-base to the highly-stressed fixed rafter to column area, there is a savings. Also, cold-formed secondaries are hard to beat with mill-steel. Z-purlins can be lapped to be continuous. There are a lot of reasons a PEMB will be cheaper even with equivalent design loads and design criteria

I want to be clear about one thing, I have no fear of using a PEMB for any project provided I have a good supplier and I do my work right. I let them clearly know what I want from them.

This thread was partly about mill sections (prismatic members) versus tapered ones. One thought on that. If a PEMB is 50' or less in clear span, and you are not in a heavy snow load area, straight parts may be cheaper than tapered. While tapered uses less steel, the labor is higher. The webs are generally cut on a burn table rather than sheared, that costs much more. Even a web taper of 1" requires a burn table. I rarely used tapered parts once I figured this out for smaller width buildings. Now that A992 is more common, that is probably more true than when I learned this. Also, all the flange braces are the same length for prismatic , whereas on a tapered section, each one is a different length.

 

[Settingsun]

Maybe there's something about the PEMB business I'm not grasping. Seems like a form of D&C to me where you have to be up for the fight if it's an open tender, or even if you've invited the el cheapo company to bid. If they say 'show me the spec', they get a copy of the loading code. If it isn't fixed, they get another copy as an attachment to a notice of breach of contract.

Maybe what I'm missing is that PEMBs don't have drawings issued for approval?

 

[Ron247]

Steve49,
I am not just talking about PEMBs. The cold-formed wall studs and cold-formed floor joists are not PEMB related. But it has the same issue. If the cold-formed framing spec only cites some minimum depth, never even mentions design loads, never clearly notes the eng/arch has not designed the members, and does not state the supplier is to design them, the contractor supplies that depth noted and contends he thought that the engineer or architect had already determined that any stud that deep or greater would work. I worked 3 jobs last year with an architect who said he would design the walls. He only called out a cold-formed wall stud depth and no where in his specs did he clearly note they stud supplier has to perform a design. His entire "wall design" was calling out a 6" deep cold-formed stud. My drawings always clearly stated I did not do the wall design in all 3 cases. For me, it was just easier to add the design note to my specs rather than fight with them later if I am responsible for them. Think about it like a Philadelphia lawyer rather than an engineer. Wouldn't you prefer your engineer client had at least said they had to design the stud for the building code loads rather than never mention it at all. And for the other attorney, "My client saw that PE stamp on the drawings and therefore assumed all design work was already done. Like I said, a spec that is too loose can lead to this.

Most reputable PEMBs do issue approval drawings "if requested".

 

[Brad805]

We have never had any issue getting complete PEMB specs from any of the suppliers we work with. By specs, I mean only the typical approval package that shows the general arrangement, design loads, and foundation reactions. Good luck reverse engineering anything from those drawings. If you get into a cost competitive job you will find the larger suppliers tend to have a leg up.

The cost benefit to the PEMB is the fact they can supply a complete package (frames/girts/purlins/cladding....). They save a few nickels out of each step and a proper crew can stand up these buildings very efficiently. Those we have worked with are very large companies. They are processing hundreds of jobs at anytime, and that reduces their waste greatly. They also buy their materials in bulk based on the market. Few custom shops can do that because of the number of different sections they require, and the fact nobody else in their market space does so. I do not think standardization is really all that important to the PEMB suppliers we work with. They rely heavily on automation and they have very complex systems to track all the products.

As for quality with the PEMB, you get what you pay for. Pounds of steel is king in the PEMB market. If it costs less, you undoubtedly have less pounds of steel. If you do not have a clear spec for your building envelope you will end up with a terrible building envelope. We have put torch-on roofs on many old PEMB's over the years when the owner got tired of leaks, ice or other problems. Even the addition of that little load can be a problem with some of the modern PEMB's. It is great fun when you try adding a new make up air unit to a PEMB CRU.

In the case of SandwichEngine's nicer pictures I am not sure one would see much savings unless you stick to products the building supplier rolls or stocks. If you break down your typical project to only the steel frames the potential to save pounds decreases dramatically. That said, you would still have single source responsibility for the building.

The other huge advantage of the PEMB is their marketing setup. Over the years we have been involved in the foundation design for endless clients that buy a steel building without fully understanding the overall costs. The level of automation makes it possible for their sales force to give an owner a quote in very short order. Many of the sales team can produce preliminary foundation reactions as well. In todays "want it now" culture that is brilliant. By the time the owners have figured out the total cost, the PEMB supplier has been paid and the building is in production.

Steve, delegated design in NA is a mess. It seems in many locations we are in a race to find the absolute bottom. Steel stud is an excellent example where you can see a lot of waste or added cost in a traditional design.

In the case of the OP, I am not sure how many are setup to deal with blast loads.

 

[SandwichEngine]

It sounds like some of you have had some very bad experiences with PEMB companies. Having only worked at reputable companies, it's very eye-opening for me to hear and gives me some understanding of the negative perception I often see in this forum and others.

All I can say is that all the companies that I'm aware of design to the relevant codes and any deviations from the specifications are negotiated as I've explained above.

My best advice is to make it non-negotiable that your PEMB supplier is IAS certified. IAS audits each company twice a year. They go into enormous depth. They look into tons of stuff not related to engineering like whether or not you store your bolts in a way that will ensure the manufacturer applied lubricant doesn't get dusty. The engineering audit is thorough. They look at design standards, methods of design, processes for making sure what gets designed makes it to the field, etc. IAS is a subsidiary of ICC who makes the IBC code. There would be zero tolerance for design methods that don't align with the codes. You can trust an IAS certified company has designed the building properly for whatever loading, deflection requirements, etc. that have been agreed on.

Based on the comments above, it sounds like there are maybe a lot of non IAS accredited PEMB companies out there doing the rest of us a disservice. Do whatever you can to not allow these type of companies on your projects. I'm realizing that there's a reason this type of certification exists--because people like you need to have a way to discern whether or not a company uses quality and some would say legal engineering and manufacturing methods.

Also, we do issue approval drawings on every job. At my previous company, one of their big sales pitches was that they always deliver drawings and buildings on time. That can only happen if you don't go through the approval process because that adds uncertainty so they would try not to do approvals but they would do them if they were required.

To speak to the issue of cold formed studs, we got into a lawsuit a few years ago with a job where the architect designed the studs. This was down near the Texas coast and so it had to get certified by the EOR as meeting the requirements of the Texas Department of Insurance. When the EOR inspected with that in mind, he found that the studs weren't close to working at the lengths shown in the drawings. But he was friends with the architect and got lots of work through him. Their solution? Say that the PEMB company, us, should have known that the studs couldn't span as far as their sealed drawings showed and should have gifted them a ton of additional material to make the existing stud design work. We wouldn't provide that material for free but called out beam sizes for them to source locally to get their studs to work. Then they withheld payment for the entire building as a way of attempting to get us to pay for the material anyway. We sued them and put a lien on their building. When it came time to do depositions, they cancelled theirs knowing they didn't have any chance and eventually agreed to pay the full price through arbitration. Very immature in my opinion. "Fine we'll pay for our building but you'll have to incur lawyer fees so you're out either way." Really wish they would have just engaged a PE to design the studs.

 

[Ron247]

For those curious about the Letter of Certification for Loading (LCL) I mentioned earlier, the IAS accreditation that SandwichEngine is referencing is another item that could be added to the LCL to better weed out low-end suppliers. I think IAS did not exist 25 years ago when I got my copy of the LCL.

Now that I think of it, a LCL may not be a bad idea for any supplier who is doing the design provided you know what codes they are to adhere to.

 

[Brad805]

In all my years I have not seen any PEMB suppliers try to use loads other than the code in effect. That is just begging to get sued. The problem with loads is there is no reserve capacity. I do not fault the PEMB suppliers at all for that.

Below is your classic low end pre-eng building envelope. This is snipped from a current website of an IAS certified supplier. This does not perform well in many climates, and few owners think much about in my experience. In Canada, this should not be built in most municipalities due to the energy code, but we see it frequently. That said, we see tons of architects that want all their custom steel to fit nicely in the exterior wall as well. I don't fault the PEMB suppliers at all for any of that. Buyers need to be aware and look thru the brochures with something other than cost in mind. I think the PEMB is a great solution, but you need to know the options and evaluate them carefully.

 

[Ron247]

Brad805,
When you say it does not "perform well", in what sense do you say that? It is poor structural, energy or some other desired trait?

you need to know the options and evaluate them carefully.

I couldn't agree more, but trying to teach architects about PEMB options is not easy. They want what they want, and they want it now.

 

[Brad805]

Ron, the problems we have had with the low end building envelope are all non-structural. We have worked with quite a few pre-eng suppliers and even the smaller companies have all produced a fine structure. I get involved in the renovation aspect, so that is where I see some of the headaches down the road. Some of the issues with the roof/wall envelope are:
1. Roof insulation is heavily compressed at each roof purlin. This can cause snow to melt at these points and and this can start icing problems. I have pictures of this type of roof with 24" or so of ice at the eave.
2. The exposed fasteners have a rubber grommet to form the seal. That hardens over the years, and we have found instances of leaks at these locations. I am sure the grommet quality has improved over the years. We generally steer our clients to a better roof and wall system.
3. The icing problem usually leads to a gutter failure. Many in sales have suggested ice rakes, but those typically fail as well.
4. This roof and wall envelope does not meet many current energy standards.

Keep in mind I generally work in a cold climate, so some of my notes will not apply to many.

 

[Ron247]

The items you are pointing out are typical for low or high end PEMB companies. It seems more like every PEMBs "Low-end" offering has the same problems. I am in the sunbelt so I have to turn on a TV to see snow. Problem is that some low-end suppliers do not have the higher end roof and wall panel offerings, so for a low-end supplier there are no alternatives to cure your problems.

Their solution is for you to seek out other panel and insulation options and apply those to the structurals you buy from them. There goes single-source supplier and it adds all the mating and fit-up details to your scope.

 

[Brad805]

Ron, even in the sunshine belt your owners are spending extra on energy with the poor details. They just spend the money at a different time of year. For owners using a triple net lease contract their interest rarely extends beyond building cost, but as more and more tenants become a bit savvy I think (hope) some of those details will be more important. I get why the PEMB suppliers need to offer this option. In some building sizes they are competing with the pole building (aka wooden tent), and that is just more of the race to the bottom. In modern design I am constantly baffled how few look at long term operating costs. The focus by the vast majority of our clients seems not to extend beyond the construction cost.