----- Original Message -----
Sent: Wednesday, November 29, 2006 9:34
Subject: Old Concrete
Daryl Richardson replied:
3.) Repeat of option 2.) but place the posts directly over
columns. This will take a little more structural steel that
option 2.) =
but will eliminate any need to check out slabs and
Hope this helps.
H. Daryl Richardson
----- Original Message -----=20
From: Brian K. Smith=20
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.
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CRSI historical document as Roger Davis suggests.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
Meyer, Borgman and Johnson,
12 South Sixth Street
Minneapolis, MN 55402
(612) 338-0713 (main)
(612) 604-3621 (direct)
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