Calculate deflection in a beam.

[Baskavoda]

I'm trying to size a structure where I have a simply supported beam. The design uniform load is 8.64kN/m, the span is 12 meters, and the maximum deflection allowed is 60 mm.

I need to calculate Iy to choose a beam that handles this.

Shouldn't it be solvable with:

formula: Wmax = (5*Qd*L^4) / (384*EI)

giving

((5*8.64*10^3*12^4)/(384*210*10^9*Iy)) = 0.06

I find that the moment of inertia is a very small number, which is incorrect.

Does anyone see what I'm doing wrong?

Best regards, Carl

 

[gorran]

Yes, but the Iy you get is in m^4 while tables over beams, etc., are often in cm^4. This means you should multiply your answer by 10^8. Does it work? / gorran

 

[Baskavoda]

That seems correct, yes. Alternatively, one might write L^4 in mm instead.

Thank you for your help!

 

[spendera]

Remember to also account for the weight of the beam itself. Normally, this weight can be significant for the deflection.

 

[Baskavoda]

It's included in the design calculation. Thanks for the help again!

 

[Granngubben]

With my calculator, the required moment of inertia I = 18500 cm4, thus e.g., HEB280.