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RE: seaint Digest for 28 Apr 2004[Subject Prev][Subject Next][Thread Prev][Thread Next]
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- Subject: RE: seaint Digest for 28 Apr 2004
- From: "Rick Stone" <rstone(--nospam--at)madisonconcrete.com>
- Date: Thu, 29 Apr 2004 09:24:11 -0400
Re: Cambered Slabs Gail, as noted in other thread responses, it is not uncommon to camber 2 way slabs. It is usually called out as a positive number on the plans or in a schedule. The amount of camber required is, in my opinion, subjective because there are so many variables (actual dead load vs. design load, Modulus of Elasticity, time of removal of shores/reshores, early age loading, etc) that determining the amount seems to be an art. As David and Jim have noted, the formwork can usually be configured to make this happen. In "stick" form systems (frame/stringer/joist) the joists must be arranged to allow a change in elevation without "breaking their back" or crippling them. In table systems, the same approach applies with either a 2 table bay with shims off the trusses to create the camber (so the camber is actually planar not parabolic in the slab soffitt) although as David points out there are cases where double framing is used. If the camber is not too great, it is possible to "turn the screws up" on shore frame or post legs in the center of the bay even if a random joist arrangement is used, as the aluminum joists are pretty flexible and can accept a fair amount of deformation and remain elastic in response. Finally, Titan and Peri have post and panel systems that can do this also. The deck should ALWAYS be checked for alignment (Usually done from below with a laser and set targets) prior to a pour to verify that the deck is where it is desired to be, and should ALWAYS be monitored during the pour because parts of the shoring system always have some take-up as load is applied. We task a 2 man crew to monitor form elevations throughout a pour and they turn screws up or down as needed during the pour. The top of slab is set "hot" at the slab edge and construction joint bulkheads, and screed lines are set at the columns on the vertical steel with paint lines; the rest of the floor is usually controlled for thickness using wet pads or by "dipping" (a rod or bar painted to show the slab thickness) as it is struck off. Random dipping and striking with a long straightedge mitigate the planar aspect of the deck form and results in essentially parabolic top profile. As far as cost is concerned, there is some small cost incurred if the camber required demands more form/shoring material or an arrangement of material that is "extraordinary"; as far as additional survey to check final elevations that is usually a 2 man crew per pour on the day after the pour. Just like floor FF and Fl profiles, it must be checked directly after the pour before shoring is removed. Finally, there is always the condition that results after the shores are removed; will it come out? Is it affected by the maturity at loading with self weight? Is the camber intended to offset creep? There are a lot of variables and one way to control them is to set minimum maturities for form removal, shore removal, reshoring duration, allowable superimposed loading from reshores for floors above, etc....these are time related issues that really end up as incurred costs because it means less efficient form, shore, and reshore cycling and more form, shore, and reshore material and just plain "going slow"-so to sum up, camber is not uncommon, but be aware of the effects to schedule and cost. Richard W. Stone, P.E. Project Executive Technical Services/Project Management/Estimating Concept Design/Engineering/Quality Assurance Madison Concrete Construction Company 130 Quaker lane Malvern, PA 19355-2479 email: rstone(--nospam--at)madisonconcrete.com Voice: 610.695.8800 Facsimile: 610.695.8678 Nextel: 610.496.5764 Visit our webpage at www.madisonconcrete.com !!!! ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
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