precast plank camber

huangyhg

precast plank camber
i have a project in construction using 6" precast concrete hollow core planks.  the planks have a camber of 1 3/4" for a 21'-8" clear span, and 1" camber for a 12'-5" clear span.  the manufacturer claims it is based on the design loads (100 psf live + 15 psf dead for the 21'-8" span, and 40 psf live + 15 psf dead for the 12'-5" span) plus temperature, humidity and storage time.  the pci manual lists a 0.3" and 0.2" camber respectively for the specified loading conditions during erection.  now there are problems placing conduit in the specified 2" topping slab, besides insufficient depth at the crest of the planks for a regular topping slab.
any thoughts on this?
i would like to know how many of you had similar problems and how many had no problem with camber.
we are considering using a self leveling cementitious and gypcrete based topping slabs depending on floor finishes.  did anyone have problems with that on precast floors?
thanks!
eric mcdonald, pe
mcdonald structural engineering, pllc
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camber is determined by the design load, strand cover (fire rating), plank depth and span. an architect and engineer are essentially specifying the camber when they determine the above criteria. the precaster does not have a choice in what he provides as camber, you have already determined it for him!
there maybe long term affects (creep) that will increase the camber. humidity and temperature will also affect the camber. all these things are very hard to predict so archtects and engineers should make allowance in case the camber is not what you expect.
i would recommend using 3" topping, not 2".
we have problems on virtually every job. pci doesn't set the max camber but rather tells the design team to better communicate the project's needs with the precaster. it's a bit of a cop out. i'm often surprised that we continue to have the same problems they had 30 or 40 years ago. increasign the topping thickness is a distasteful but sometimes prudent measure.
in the end the precaster does have some control - how the planks are stored, how they are designed (keep reinforcement to a minimum), and how they are reinforced (top bars can be added), but these all cost him time and money. we write a max camber into the spec so at least we have some leverage when the planks get to the site.
if you have a pricing system that is based on low bid, you have to accept the manufacturing process of the product you specify. if you want specific camber range, just enforce the specifications.
since hollow core planks are made in lengths of up to 800', adding variables will increase the cost and possibly the delivery time depending on the job size and schedule.
in many cases, a single production run of plank could be 6000 sf, so you see how this impacts the cost and delivery schedule since plank are not normally drawn from a general inventory.
i had this exact condition on a multi-story building - 6" hc with about 18 to 20 ft. spans.  planks delivered to the site had too much camber - way more than the pci tables.  we told them to take them back and reconstruct with a more appropriate camber, which they did.
there is the ability to control the camber by adjusting the vertical location of the strands.  
there is a trade-off between number and prestress of strands and their depth, d.  keeping the camber down isn't always corresponding to optimizing/reducing numbers of strands.
looks to me the live load cambers are excessive and unnecessary. usually the camber (unless otherwise specified)shall include the effects from the dead load, the superimposed dead load and all applicable losses (relaxations). it puzzles me that who decided to use different live loads (100/40) for different span lengths in the calculations?
for the remedy, just make sure the bond between the materials, and the strength limit of the hollow core slabs.
(the added weight might reduce live load allowrance)
sometimes late decisions by the project architect may also have an affect on the camber. often the first thing that they want to know is what the plank thickness is before they have researched the necessary fire rating (which affect strand cover), room functions (which affect load) or even the weight of supported partion walls (which also affect load). these things will affect camber along with undefined openings.
getting this info. early on in the project or making reasonable assumptions about these things will help pick a more reasonable plank thickness which will have less camber.