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Manhole cap

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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