Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Old Concrete

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Thanks Mike.
----- Original Message -----
Sent: Wednesday, November 29, 2006 9:34 AM
Subject: Old Concrete

Daryl Richardson replied:
3.) Repeat of option 2.) but place the posts directly over building =
columns. This will take a little more structural steel that option 2.) =
but will eliminate any need to check out slabs and beams.
Hope this helps.
H. Daryl Richardson
----- Original Message -----=20
From: Brian K. Smith=20
To: Seaint(--nospam--at)
Sent: Tuesday, November 28, 2006 2:53 PM
Subject: Old concrete
I am working on adding some roof top units to an existing =
building. Date of original construction is unknown. The roof framing =
consists of a 4.5" concrete slab on 8"x14" monolithic beams with square =
columns. In a few areas, the slab was demo'd previously for added =
vents, etc. The rebar is 1/2" square and twisted, 7"oc each way. =
Anyone ever seen such?
Brian K. Smith, P.E.
I write:
if you don't have any data on the concrete strength to evaluate the shear strength of the beams, Daryl's option 3 looks to be the best.  I have at times assumed, say, fc' = 2000 psi and evaluated shear strength with that.  It's a judgement call, which I make only if I have to.  I'd get some tests.
By the way, I like to make mechanical platforms tall enough so that the roof can be re-roofed at some point.  Depending on how big the platform is, I like to leave them at least three feet clear under it.  That's not much room to bend over with a hot mop.  Always use round pipes for the legs, as they are by far the easiest to clamp a boot around.
On the rebar, the standard in the 1920's and 30's was round bar up to 1 inch, then square bar.  In, I think, 1939, the larger square bar was changed to round bar with the same areas.  That's why #9, #10, and #11 bar aren't exact multiples of eighths of an inch.  Anyhow, since your bar is less than 1 inch and square, it's older than this standard.  Check the CRSI historical document as Roger Davis suggests.  You're probably dealing with rebar Fy of 33 ksi.
I have a similar situation on a project.  The building was originally built in 1929.  The mix design on the drawings is 1:2:4 (one part portland, two parts sand, four parts gravel; add water to taste).  I have rebound hammer tests ranging from 5800 to 8600 psi.  I have 3 inch diameter cores finding 3/4 inch aggregate (round river rock), with strengths of 3400, 3500, and 3600 psi.  I have 3 inch cores finding 1 inch and 1 1/4 inch aggregate  with strengths of 1600, 2000, and 2400 psi.  The rebound hammer tests were on the same columns the cores were taken from.  Obviously I have some size effect with the small cores and large aggregate, but these results are whacked.  Can anyone help me understand this huge inconsistency?
Mike Hemstad

Michael Hemstad, P.E.
Meyer, Borgman and Johnson, Inc.
12 South Sixth Street
Suite 810
Minneapolis, MN 55402
(612) 338-0713 (main)
(612) 604-3621 (direct)
(612) 337-5325 (fax)

No virus found in this incoming message.
Checked by AVG Free Edition.
Version: 7.5.430 / Virus Database: 268.14.19/556 - Release Date: 11/28/2006 3:22 PM