To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
Subject: RE: Cost Analysis
From: "Sprague, Harold O." <SpragueHO(--nospam--at)bv.com>
Date: Fri, 23 Mar 2001 13:33:40 -0600
If you haven't had this problem and you design office buildings, you will
have this problem.
Getting accurate camber is one thing, getting it to come out as our
computers tell us with + or - 1/4" tolerance is another. Look at the
problem from the contractors point of view.
There are residual stresses in any rolled shape that can act as prestressing
forces. If a flat elevated floor is something that you need, I would
suggest 2 placements. Leave the first one rough and then top it with a
The architect and engineer want a flat floor. Contractor response - when?
The floor slabs are placed long before all of the dead load is placed. The
flatness is a moving target depending on the dead load.
As the concrete is placed, the contractor will start at a corner somewhere
and work his way across the floor plate. He will be placing dead load and
removing camber as he goes. Double screeding causes more finish problems
than it cures in flatness, and will make an enemy of your flatwork
The flat work contractor will not place the slab to a level plane in one a
one course placement. He will place it to a nominal slab thickness. He will
set his screeds to the deck not to some theoretical level plane like he can
with a slab on grade and a laser screed.
I would advise anyone to sit down with a flat work contractor and discuss
the options and weigh the costs. This issue is particularly critical with
composite slabs because the LRFD has given us the ability to skinny up the
WF members and we have not given proper consideration to "construction"
serviceability. It is an interesting exercise to evaluate the actual loads
on a structural system with a true horizontal flat floor. The added slab
thicknesses and weights will surprise you.
My advice is to heavy up those WF composite members and evaluate the
deflection of the deck. This became more of a problem when LRFD came on
board and we engineers saw we could knock off a couple of pennies per square
foot for the structural system. The problem was exacerbated by the longer
spans the architects wanted to get rid of those nasty columns and work to
more realtor friendly longer office modules. Those pennies "saved" cost
many times that when we tried to get a slab as flat as in the old ASD days.
Again we met the enemy and it is us.
I admit it. I have made this same mistake once. OK maybe twice before the
flat work contractor knocked some Socratic wisdom into me. I didn't have to
spit out a tooth, but it was close. I have had this debate before, and on
this one the flat work contractor is correct. He may not have known all the
engineerin' but he knew sumpin' was rong with the design.
Harold O. Sprague
> -----Original Message-----
> From: dcarroll [SMTP:dcarroll(--nospam--at)bfmengr.com]
> Sent: Friday, March 23, 2001 11:04 AM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: Cost Analysis
> I too am very interested in this framing type for a number of reasons. We
> too have used this type of constr. many, many times with pretty good
> success. The typical problems we see is green load deflection of the beam
> (purlin) and girder. We recently switched to RAM and can design an entire
> bldg in short order.
> Since no one likes shoring and is variable by definition, we now are
> using a minimum camber for the purlins and designing for maximum 1/2 "
> green load defl in girder.
> We are very concerned about getting more camber in the purlins than what
> we specified, and thereby getting a too thin slab where the conc is
> screeded level(using a lazer screed), or a wash-board slab if the conc is
> screeded 'constant thickness'(using screed rails).
> Incidentally for our most current job our bay size is 32X40 and we are
> using 2"X20 ga comp deck with purlins @ 8'.
> So my question to the readers at large is this: >>
> HAS ANYONE ENCOUNTERED A MAJOR PROBLEM WITH CAMBERING ONE DIRECTION FOR
> COMPOSITE BEAM FRMG??___________________
> thx David