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Re: Which method do you like for calculating deflection

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I would start with the heaviest crane.   You know the wheel loads for the
cranes and with that you can also calculate the lateral loads.  You will
also need to know the frame size of the crane trucks and how far the
bumpers are away from the wheels.  This will set the closest distance the
two bridge cranes can approach.

Find the CG of the single crane wheels.  From the AISC steel manual (black
book Table 3-23, last frame), the maximum bending moment produced by moving
concentrated loads occurs under one of the loads when that load is as far
from one support as the center of gravity of all the moving loads on the
beam is from the other support.  This is for a simply supported beam.

With multiple cranes, try that same evolution with the cranes butted
together, then vary the load back and forth to verify you have the maximum

For a continuous beam, I'd jump to a  program like STAAD that allows a
moving load across the continuous beam.  If your runway beam is overloaded
or has excessive deflection, adjust the minimum spacing between the two
bridge cranes.  Have them weld new bumpers on the cranes to ensure that
minimum spacing is not violated.

As far as saying the other crane is 50% loaded, I wouldn't go there.  I
recommend that you design for the CAPACITY of the crane.  In the US, to
design for a lesser load requires the crane to be derated to that load and
the new capacity permanently posted on the crane.  I've had to repair
bridge crane runway systems due to field decisions to overload the crane.
Remembers Murphy's law.

Good luck!
Phoenix, AZ

On Thu, Mar 21, 2013 at 8:39 AM, Gary L. Hodgson and Assoc. <
design(--nospam--at)> wrote:

> I don't see how influence lines can help me with rolling loads. I know
> bridges are subject to rolling loads but how do you choose the worst
> location for 2 or 3 cranes each with different wheel loads and variable
> spacing between them.
> I have a customer now who has a 15 ton crane and 7.5 ton crane and wants
> to know how close he can operate them while fully loaded and then only 
> 50% loaded for one or the other.
> Gary

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