huangyhg

design build pemb foundations
we recently started working for a contractor that specializes in pre-engineered metal buildings. we provide the foundation design based on soils information and loads from the pemb manufacturer as well as other features from the architect. the first couple were cake.
with winter approaching we are feeling pressured to submit drawings based on preliminary reactions from the pemb supplier, not loads sealed by a structural engineer. we are contemplating adding a note to that effect and that "final footing sizes must be verified by the foundation engineer with sealed loads from the pemb".
when we design a foundation for a "design-bid-build" pemb, we get estimated loads to base our design on and note that the footing will be reviewed based on loads from the successful pemb contractor. in those cases we have some control over the contractor and can enforce the provision. in this case it will be more difficult.
how have others handled this situation? thanks for the help.
typically, whenever i do a foundation for this i clearly note in several spots that the design is based on loads provided by xxx and that anchor bolts, etc. are by others. in this case at a minimum i would say the same thing and note who has what responsibility if the loads or conditions change. maybe you should put the provided design loads right on the plan with a note to verify everything before construction.
thanks jjeng2. we do add the notes on the drawings. the question is: do i have a responsibility to see that they are followed by the contractor, our client, who i do not have control over like i would in design-bid-build? once these drawings go out the door i usually never hear from them again.
you may have answered your own question when you said "once these drawings go out the door i usually never hear from them again". you have a strong suspecion that they have ignored your instruction. obviously, this note is not effective. i would suggest a change in your technique or followup to reduce your liability.
a phone call to your client with a written followup would help. you cannot force the client to react but you can at least put him on notice.
do not bow to pressure from contractors. i have designed hundreds of pemb foundations. as stated above, once the plans leave your office, you will not hear anything from anyone.
do you think the contractor is going to hire and engineers to evaluate the footings based on final reactions from pemb? i have not met a contractor who will read most of my notes on my plans. they read them when they get in trouble.
as eor, i do my own frame calculations based on prescribed frame spacing. i determine the loads and i use load combination of wl + 0.6 dl to size my footings. this yields conservative results. i re-evaluate my foundations when i get the signed and sealed calculations from pemb supplier. i get them because i request them and stay after the contractor till i get them.
i am not a big fan of pemb and i do not care if they read my comments. they get by with lots of stuff that other structures had to meet and they do not. next time you check lateral drift, see what their allowable is (h/45!!!). they do not compete if you impose normal loads (over 20 psf reducible live load). may be they can support few psf of collateral loads such as sprinklers. you cannot support more than 500 pounds of concentrated load from the light gage 鈥渃ee or zee鈥?purlins.
after saying all of that, people love them because they are cheap and will go up fast.
regards,
lutfi
i do my own frame calculations and back-check when i get the shop drawings, just like lutfi. early in my career, i failed to recognize that most metal building designs are based on a very small dl so the uplift reactions were incorrect. luckily, i was conservative enough in the preliminary design for the first few designs it was not a problem. i use 3psf for dl calculations.
i too, do not care for pemb. there is only one fabricator in my area that follows the construction documents and provides what is performance specified for loads, lateral drift, location of x-bracing or portal frames. most pemb suppliers sell what they want to sell and force the owner to buy it.
i've always wondered: why is it that pemb manufacturers do not engineer the foundations for thier buildings?
jmiec,
at one time some of the pemb catalogs had footing and hairpin sizes for various loads and thrusts. i suspect they were removed for liability reasons. i think one local contractor still uses an old catalog for his foundation design.
all,
thanks for the input.
mwpc
pemb suppliers used to publish foundation details, tables and hairpins as well. i think they got away from doing it as stated above, liability.
previous editions of pemb manual also used to promote that only 70% of the wind load reaction will make it to the foundations!
on positive note, pemb association gets lots of credit for promoting low rise wind tunnel testing. most of the results of this testing can be found in building codes.
regards,
lutfi
ok, so if liability is the issue for the pemb supplier, then why isn't liability an issue for the foundation engineer? it seems that the liability/fee ratio is inordinately high for the foundation engineer, who often becomes the engineer of record.
when approached by a client to do a metal building project, i find out if a metal building fabricator has already been retained to design the framing. if so, i accept responsibility for the foundation and anchor bolt design only and tell the client that before i begin working. if i do the foundation only, i don't check shops for or review the framing. i have a note on the drawings stating i am not responsible for the framing design. my fee is lower for foundation/anchor design only. if the client plans to send the drawings out for bidding, i discuss my scope of work with the client and tell them i will be the eor for the whole project only if i write a performance spec for the framing, review the framing and shop drawings. my fee is a full fee if i do this. if the client doesn't want me to write the performance specs, i won't check shops or review the framing. the client is on their own for working with the pemb.

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