pre eng pre fab metal bldgs

huangyhg

pre eng pre fab metal bldgs
are pre eng pre fab metal buildings cheaper over standard
construction?
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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 a. manges, e.i.t.
structural engineer
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!
namanges
my big problem with pemb is that the lightest structure is typically the cheapest structure and wins the bid.  therefore pemb mnfr鈥檚 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 鈥淶鈥?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鈥檚 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鈥檛 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鈥檚 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 re  
why is it that a pemb mnfr can design complex moment frames but can not calculate the size of a footing?  i don鈥檛 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.