# Question on calculating Bending Moment for Beam

1. Jun 9, 2010

### joeykeys

I would like to know ,
given with the 2 attachments below,

are there any ways to find the bending moment (as circled) of the bridge?

or it is not possible to calculate simply because the "load" data is not given?

Several attempts have been tried but the circled data can not be reached.

Thanks a dozen for the help!

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2. Jun 9, 2010

These load values could be given as ultimate resistance values for the structure.

3. Jun 9, 2010

### joeykeys

However, this "ultimate" value is not currently provided so it is not possible to calculate the bending moment as shown..

is it?

4. Jun 9, 2010

### Studiot

I think you need to provide further information about the context.

The first bridge is stated to be an arch. Arches have no bending moments, being pure compression structures.

The second bridge is stated to have a span of 98m and a (presumably) max bending moment of 134627 kN-m.

A simple RC beam with such a span would experience a max bending moment of 2*6*2400*10*98*98/8 = 345744 kN-m. This is nearly three times as much.

But the picture shows a prop at about 1/3 of the span. Such a prop would reduce the max moment and reposition it. The exact figure would depend upon the assumptions about continuity in the beam over the prop. If we assume the beam is fully continuous over the prop, can you calculate the approx moment? - it is possible to estimate if we can locate the prop along the 98 m overall span.

5. Jun 9, 2010

### joeykeys

Could you tell me which formula have you used for calculating the bending moment?

say is it: (b*d* ? * ? * span^2 ) /8 = bending moment

what does 2400 and 10 stand for?

Thanks again for the clarifacation.

Last edited: Jun 9, 2010
6. Jun 9, 2010

### Studiot

reinforced concrete weighs about 2400 kg/cubic metre.
10 approximates the acceleration due to gravity to convert this to Newtons

the weight per metre run of beam is depth x width x1 x above density

the maximum moment in a simply supported beam is wL2/8

where L is the span length and w is the weight per metre run.

But you didn't answer my question so I can't help further