designing a slab with an opening subjected to moving loads

huangyhg

designing a slab with an opening & subjected to moving loads
hello everybody,
i would like to model a concrete slab (15" thick) with an opening. also the slab is subjected to moving loads (wheel loads). can anyone suggest me the best way to do this?
i am trying this:
1. modeling it as plate elements (15" thickness)with a opening & simply supported at all four sides. in that case i don't know how to use moving load option (i think i can't).
2. modeling it as beams on all four sides of the slab and also on all four edges of the opening (length of beams being the same as the slab length/or width). i will be having total eight beams. in that case i can use the moving load option of staad. but...will this model represents the actual slab???
i am using staad-pro. the slab is actually lid of a manhole. the opening is for placing the grate. so....actually the wheel load can be placed on opening (grate) too!
sorry for making it long. my intention was to make it clear.
thanks in advance.

i don't think that you need to use the moving load at all.  if this is an inlet for drainage then chances are the structures isn't of the area that would allow an entire truck to influence the design.  as such you need only look at the maximum wheel load that can be applied and assume a single concentrated load.  
also, you need to see aashto on the analysis and design of slabs simply supported on four sides in section 3.  this will greatly simplify the work.
lastly, as a reality check, depending on the overall dimensions, you should really have light steel (#5 bars) in the slab with some additional reinforcing around the actual hole.
noopur!! (someone familiar????!!!)
well qshake was right in that maximum wheel load would influence the design rather than moving load ..owing to the fact that the manhole slab predominantly behaves as a thick plate (or beam rather) and shear is the main criteria governing it...if it is a thick plate..when bending is not that predominant why at all bother on variation on bending moment with respect to position of wheel load...also possibly the entire wheel load placed at the center of the plate ( or rather beam ) would be critical...
in case its a thin manhole plate then better model using plate elements with static loads that should suffice..
hope this helps
regds
chari
raj
thanks qshake and drraj for your reply,
i am concerned...because this manhole is under runway of a airport....
and....the maximum size of the inlet/slab that i need to design is 9'x7.5'.
and for a typical model md-11 (aircraft)....the wheel loads are like this....
four wheels spaced 54''x64" , each wheel being 75 kips.
looking at this.........there may be a worse case when all the four wheels may lie on the slab.
now....there will be huge moments as well as punching shear that i need to consider.
and i would like to see how does it affects the slab....if the load is moving.
so.......again my question is.....how would u model it & then design it??
waht do you think may be the maximum reinforcement in both direction at both levels??
please suggest
noopur
the size of the slab is small so i wonder whether it will make too much of difference if you design the slab as beams only. choose your smaller span 7.5' as span of a beam and run two wheels of 75 kips as moving loads. this will simplify your problem to one dimensional one. this is a conservative approach, but landing aircrafts deserve some conservative approach due to unpredictibility of their impacts.
if i had to face a similar problem and 2-d analysis were a must, i would have wriiten a program myself to work out max. moments by iterative process. if you can program, try this. its fun and useful.