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Re: Manhole cap

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Fellows,

I may be wrong here; but it seems to me that you can buy cast iron or cast steel manhole covers complete with supporting rim as a stock item. It follows that you can cast a concrete "plate" top slab that is two feet (610 mm) thick for only a couple of hundred dollars worth of concrete.

It seems to me that it is a waste of valuable engineering time to do anything more than the most simple analysis and over design the thing. I would be inclined to assume a 12" wide beam on each side of the hole and design top and bottom steel for a five foot span with a point load at midspan. I would then wrap the bars with ties designed to resist shear/torsion as calculated by some equally simplified procedure. I would also use a similar pattern of steel rotated 45 degrees to fill in the "corners" around the round hole.

My above comments are intended for a "one of" design. if this is for a factory standard where many thousands will be mass produced then a much more sophisticated approach is required.

       Respectfully submitted.

Regards,

H. Daryl Richardson

----- Original Message ----- From: "Gary L. Hodgson and Assoc." <ghodgson(--nospam--at)bellnet.ca>
To: <seaint(--nospam--at)seaint.org>
Sent: Saturday, July 22, 2006 6:33 AM
Subject: Re: Manhole cap


Joe,
I have Roark's book but there are no examples of square or rectangular plates with holes in them. The closest is circular plates which are used a lot for washers, springs,trunnions, etc. You might consider a circle which would cover or slightly overlap your square. If you want to try one, I can fax or e-mail you the appropriate case--let me know. Also, there is a company called Arcon (Arkon?) Engineering which sells various software solutions at reasonable prices. One of them is the round plate with a round hole in the centre, with various loads. I didn't buy that one but looked at the demo. The three I bought were $40 each, several years ago.
Gary

Joseph R. Grill wrote:
is there anyone out there with a Roarks (spelling?) that might cover a square plate with a hole in the center?
Joe

    ----- Original Message -----
    *From:* Joe Grill <mailto:jgrill(--nospam--at)swiaz.com>
    *To:* seaint(--nospam--at)seaint.org <mailto:seaint(--nospam--at)seaint.org>
    *Sent:* Thursday, July 20, 2006 3:34 PM
    *Subject:* Manhole cap

    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 16^th 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.

    Thanks,

    Joe

    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

    *jgrill(--nospam--at)swiaz.com*


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