In general, yes. Pre-fab buildings are engineered once and produced at volume. This eliminates the design time and reduces the fabrication time required to complete the building.
A strong advantage of standard construction is that a building can be easily customized to fit the need of the consumer.
Nathaniel, I respectfully disagree. Metal building manufacturers design each building individually to meet the customer's needs. The main pre-engineering is in the design aids, standard parts, and design programs. They spend unbelievable sums of money on development of these.
The disadvantage of a MB system is simply that some types of structures are too heavy duty to be a metal building. For example, nobody would try to make a 5 story office building or hospital out of one. Wouldn't do that with wood either.
There are other aspects to cost than the initial construction prices. For certain type of buildings, like a rectangular one story box, a PEMB might be initially cheaper. But how about servicability? If you're running fork lifts around, pushing pallets of widgets, will there will be a lot of physical damage to the skin? How about heating and cooling? Does the client want a certain architectural "look" not achievable with metal siding?
If initial cost was the only factor, we would all be driving Yugos.
Don't get me wrong, we use a lot of PEMBs. But your job is to make sure the end user gets exactly what they want, which isn't always the cheapest.
Bare bones PEMB's with typical bay sizes and no finishes are cheaper. Once you have atypical stuff, it isn't.
A lot of people seem to think they can have a normal building clad in brick and beautiful finishes and just because it's a PEMB it'll be cheaper. That's not the case.
Watch out for the sales tactics some PMB companies use. We've had several architects hire us with an owner who has already signed a contract with a PMB only to find out that the base bid could not have mezzanine, brick exterior, or anything else other than to be an empty shell. In the end, the buildings cost more for them.
271828, thanks for you comment. I responded to this post in ignorance to the real question at hand. My only experience has been with pre-fab metal buildings of a much smaller scale-along the lines of storage sheds. I did some research, and I never realized all of the shapes and sizes that these PEMBs come in. I learn so much on these forums and it's all due to the people that participate on them! Thanks!
My big problem with PEMB is that the lightest structure is typically the cheapest structure and wins the bid. Therefore PEMB mnfr’s typically design to a stress level of 1.0 and take every reduction possible by code. They usually have little redundancy and if one member fails the whole building fails. One of the major engineering magazines even reported a case of a PEMB used as a hanger for corporate jets collapsed because the sprinkler installer installed the clamps upside down and flattened out the bottom flange of the “Z” purlin. This caused the purlin to fail, then the unbraced length of the main frame doubled which caused it to fail and then the whole building collapsed on several planes.
We received several calls from the January ice storm in the midwest and this months ice storm. Between the two storms we have looked at 23 buildings that collapsed, every single one was a PEMB. The mnfr’s state that the loads from ice and snow are above code loads and that is why they failed. They are correct, but I must point out that we have not inspected a single block and bar joist that failed under the same loadings.
If a building is designed for a LL of 20 psf it should not fail at 23 psf, unless it is a PEMB. A PEMB done correctly is a great building, however I would offer these tips when ordering one.
1) Add 10 psf to the code minimum LL or SL, do not allow any reductions.
2) Don’t use the PEMB DL of 2 or 3 psf, the owner will also add more.
3) Make sure the main frames are designed to handle a point load if the PEMB. We inspected a 40,000 sq ft building that collapsed because they attempted to lift a 700 lb engine out of a Ford with the hoist attached to the main frame.
4) Do not add a mezzanine or 2cd story, many PEMB mnfr’s software programs can not handle this and they hire a real Structural Engineer to design it for them.
5) Stay away from small mom and pop operations if it is going to be a commercial building.
6) Get in writing that if one purlin fails the main frame will not.
7) Double check wind and seismic calcs.
8) Make sure you have wet signed and sealed plans and calculations.
9) Require the mnfr to inspect the building upon completion and sign off of it.
Next time you complain about the changes to ASCE 7, just remember this. Most of the significant changes to the building code came because of all the failures that PEMB had when first introduced. Just look at the early building sketches in all the codes.
Why is it that a PEMB mnfr can design complex moment frames but can not calculate the size of a footing? I don’t mind I get to cash in on their inabilities all the time, but really the footing is the simplest part of the building.
BRGENG, that's because they're not in that business. It's a free country, so if one of them decides that they can secure profit from doing that, I'm sure they will. They so far have chosen to remain focused.
BRGENG typed: "My big problem with PEMB is that the lightest structure is typically the cheapest structure and wins the bid. ...."
You make a lot of good points in your post. I'd personally adopt a few of your bullet points if I were to specify a MB, especially #5. I dare say that that would fix most or all of the others.
However, there's no reason to be down on the manufacturers. Owners almost universally want the cheapest possible structure when they come looking for a MB. MBMs provide what the owners want. If they don't, then the owner will find someone who will. Blame them if there's a desire to blame somebody.
If someone wants a beefier building, he can do as you recommend--higher loads, etc.
As someone within the "industry", I would like to respond to some of the comments but I hope I don't sound too defensive. Mainly because there are a lot of steel buildings fabricated by business men whose brother-in-law owns a welding machine.
271828 is correct in that each building is designed individually and that the PEMB manuf’s are just supplying what the owner or the owner’s rep. asks for. There was an article in Modern Steel Construction several years ago where the writer (an engineer) complained because a top running crane could not be added to an existing PEMB without adding reinforcing the frames. Well, duh. If the owner had the foresight to know that a crane was to be added, then the building could have been designed for it. Why increase the initial cost of the building if the owner has no plans for a future crane? Or mezzanine? This also goes for beefing up columns where forklifts will be operating or something similar to that.
BRGENG makes several good comments. However, I am not aware of any large PEMB manuf. that cannot correctly design a steel building with a 2nd floor or mezzanine. They all employ plenty of “real” structural engineers that can modify their software or use RISA, Staad, RAM, etc. Also, for a 40,000 sq ft steel building to collapse because of the addition of a point load would be highly unusual, unless there was already 150% of the design roof snow on the building. Because of the inherent ductility of steel, PEMB’s, like other steel buildings, allows loads to be transferred from damaged areas to undamaged areas without collapse. And no properly designed PEMB should fail under a 23 psf roof load if the design load is 20 psf (see note above about businessmen and brother-in-laws). His point about the flattening of the purlin lips is correct. Cold-formed steel is a funny animal. Field modifying the shapes can produce significant reductions in the capacities. All of the manuf’s that I am aware of note this on the drawings and show how hangars should and should not be attached to purlins.
PEMB’s have their market and it is expanding. However, I don’t want metal panels on my house.
And speaking of house, that’s where I’m headed. Everyone have a Merry Christmas!
Tonight I had to design the foundation for a PEMB, the client was not happy at all. The PEMB salesman told him he would save money buy not needing to hire a structural engineer for the project because the design fee was included in the PEMB package. Needless to say the client was surprised when they were told by the City that they had to have a foundation plan signed and sealed by a professional engineer before they could get a building permit. The PEMB mnfr, which is one of the larger PEMB mnfr in the US put the following note on their cover sheet.
“It is recommended that the foundation of the building be designed by a Registered Professional Engineer experienced in the design of such structures.”
Typical note seen on every set of plans, but not what the advertise.
Luckily for the client the PEMB mnfr salesman stated in an email that a Structural Engineer would not be required, they might be able to get my fees paid by the PEMB mnfr. The client called the PEMB mnfr after hearing their commercial on the radio. And there lies the problem, PEMB is a very competitive business with a very aggressive marketing campaign. They often market directly to the owner who are not educated enough to understand what they are ordering from the PEMB. You should always consult with an Architect or Engineer before starting a project, but the PEMB mnfr wants to get them under contract before contacting the consultants. They will promise them a building at half the cost of convention construction. Based on interior columns, 28 gage metal, cheap roll up doors, short warranty’s if any, etc. and anything else to lower the price. Then they change order everything out that the Engineer or Architect specify and in the end the savings are not there.
I have dealt with PEMB companies for over 25 years, I wrote the design programs that many use in their office. I believe they have a product that is great in certain situations but is terrible in others. I seen to many clients regret building a PEMB, and the longer they are in them the more often they regret them. This applies to churches, schools, shopping centers, etc. My industrial clients love PEMB and would never consider anything else.
You bring up a good point that should be in your list of tips. Be wary of buying a building from a "manufacturer" that advertises on two national radio shows. That company is a broker of metal bldgs, does not manufacture anything, and they have no engineers. The "company" is actually a roomful of telephone operators who take orders. You can google their name and see numerous sites where they have been accused of deception and have even been sued by the state's Attorney General.
The major players in the PEMB industry are represented by the MBMA. See the March issue of Structure magazine for an article written by MBMA which states the case for why an owner should hire an Engineer of Record, such as yourself, when using a metal building.
May be cheaper if you count only the initial cost of enclosed space per square footage and ignore plumbing, heating, energy saving code requirements, interior and exterior cladding, usability with future modifications for change in use, and resale value of various types of construction.
