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- To: <seaint(--nospam--at)seaint.org>
- Subject: Manhole cap
- From: "Joe Grill" <jgrill(--nospam--at)swiaz.com>
- Date: Thu, 20 Jul 2006 15:34:47 -0700
O.K. Guys and Gals,
Please be patient while I try to explain. And please understand that I am not a bridge engineer and no bridge engineers in the office. I have been stuck with this little problem.
I have to do a design for a cast-in-place manhole. The manhole plan dimensions are 5 ft square (clear dimension at the interior). I have a cast-in-place top slab which has a 24” manhole frame and lid centered in the slab. I am assuming that I have to use the AASHTO loading and, where I am confused, the AASHTO load factors. I have a copy of the 16th edition of AASHTO. The slab, so far, has been designated as 9” thick, with a “d” depth of 7”.
I think I have only to look at AASHTO “Load Factor Design Group IA” which has a gamma=1.3, beta d=1.0 (for flexural members) and an (L+I)n=2.2. Go ahead and tell me if I am wrong with this so far.
Since the slab span (assuming one-way because of the 24” hole in the middle) is about 5 ft. I have calculated an impact factor of 1.3.
Here is where I really start getting confused.
Is the factored live load 1.3x2.2(16x1.3)=59.5kips (using a 16kip wheel load, HS20-44)? Wow, that’s 3.7 times the service load. I get a wheel imprint of 8”x20”.
With this information, I have placed the wheel imprint at a corner of the slab (“d” dist. From the two faces). For beam shear I get a “b” length of 20+8+(3x7”)=49” giving me an allowable shear phi*Vn=32.5 kips using 4000 psi concrete. I think shear is a problem, but my boss ( a civil guy) will say that he has seen many manhole caps like this out there.
This is just the shear question of the problem. The flexural questions I have yet to get to. Basically compounded due to the hole in the center.
Hope this all makes sense.
Joseph R. Grill, P.E. (Structural)
Shephard - Wesnitzer, Inc.
Civil Engineering and Surveying
P.O. Box 3924
Sedona, AZ 86340
PHONE (928) 282-1061
FAX (928) 282-2058
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