huangyhg

concrete slab and truck load during construction
this is something that i would be very concerned with.
yes it can be done, but keep in mind, once you give a contractor an inch they'll take a mile (no offense to anyone - i know that they just want to get the job done too).
the slab was initially designed to span say 6 meters. now when you introduce props, each span may be cut into four, or six (whatever). the top and bottom steel in the rc slab will not be placed in the correct location to take the stresses in the concrete.
i would definately get a shoring engineer involved, and have them hired by the contractor. this takes (more of) the responsibility off the building engineer.
there are also a huge variance of truck sizes (this gets back to the give an inch comment). i was once involved in a project where we okayed them to drive small (6m) concrete trucks on the slab. when i arrived for a site review there was a 10m truck off loading. when we gave them the okay to drive on the slab they ignored the limitations simply because "the concrete came in a bigger truck".
this issue arises often, and supports my theory that builders should also be involved in the design stage such that the (easy) change to support construction loading could be made during the design stage.
just my two cents.
cottageguy
good note cottageguy.
that is my experience as well.
i would caution you as a design engineer or client representative from becoming involved in any engineering on the part of the contractor. this contractor needs to hire an engineer to evaluate his means and method and ultimately sign and seal plans, calcs, specifications. it is up to you to approve or disapprove the means and method based on the integrity of the structure and any short/long term influence. also, once approved it is up to the owner, or firm employed to ensure compliance by the contractor to the specifications of the project, to see that the contractor doesn't take any risks beyond what his engineer prescribed.
why allow driving the truck on the slab when concrete conveyors and pumps are available? you have answered your own question, the slab isn't designed for it. shoring the slab to allow the truck traffic will cost more than the conveyor or pump rental! of course, there is also the option of concrete buggies. they could be filled at the truck and driven onto the slab with much less of a load effect. but, i still believe the conveyor or pump would be less expensive.
jheidt,
as a designer, i completely agree. pumps, conveyors and buggies will all work for offloading. but thinking as a contractor, i just want to get the concrete out of the truck as quickly and easily as possible. the closer they can get to the pour or crane the quicker (and cheaper) the work will be done.
if shoring is required, i have found that the contractor easily accepts the fact that they just can't drive on the slab, without spending more money and time. i don't blame them for asking.
i had a design-build project in a downtown core, where the contractor asked to do this during the design stage. we simply changed the load and beefed up the slab at his request. might be something to keep in mind at those perliminary design meetings.
cottageguy.
if there is a problem after the fact, you may not be covered by your professional liability insurance either in a situation like that, since it sounds like an out of scope/contract situation. i have worked on several bridge analyses where contractors want to drive a crane across an existing bridge. in such a case we are the contractor's "specialty" engineer, hired and paid by the contractor.

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