O.K. Guys and
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
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
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
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
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
R. Grill, P.E. (Structural)
- Wesnitzer, Inc.
Engineering and Surveying