Precast plank camber
Precast plank camber
(OP)
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!
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






RE: Precast plank camber
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".
RE: Precast plank camber
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.
RE: Precast plank camber
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.
RE: Precast plank camber
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.
RE: Precast plank camber
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)
RE: Precast plank camber
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.