I-beam

I-beam
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Posted by: damille2 ^Ž

02/20/2004, 12:52:52

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what size I-beam would it take to support 2400 lbs in the middle across 36 ft supportted on both ends.

would any of these be strong enough:

8x4 18.4 lbs/ft center web .271

8x4.2 23 lb/ft center web .441

10x4.66 25.4 lb/ft center web .331

10x4.944 35 lb/ft center web .594

could you also tell me how strength compares between the 23 lb 8x4 and the 25.4 lb 10x4.66.

thanks

dave miller

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Re: I-beam

Re: I-beam -- damille2 Post Reply Top of thread Forum

Posted by: gumbo ^Ž

02/22/2004, 09:57:16

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The beams you have are S-Shapes. Are you designing for deflection or strength and what grade of steel are you using?  I will assume a simply supported condition. If I also assume they are made of A36 (36 ksi yield)steel and AISC design criteria for stress of 0.6Sy (static) is used then the allowable stress  level, Fa, should be below 21.6 ksi. The minimum section modulus is then M/Fa where the maximum moment is PL/2 for the center load. The beam weight also needs to be included so using the lightest beam weight the maximum moment will be P/2 + WL^2/8. Using the lightest beam you specified of 18.4 lb/ft the maximum center moment is 294,970 in*lbs. This requires a minimum section modulus of 13.65 in^3. The S8x18.4 has a strong axis section modulus of 14.4 in^3 so it will work.   Note that your deflection will be approximately 2.8" at the center under these conditions which may not be acceptable.

If we use the same moments from above and just ratio the section moduluses and MOI's we can tabulate the conditions for each of your beams. Note that the stress and deflections will be slightly higher than shown below because the moments are not increased for the dead weight of the beams.

Beam                       Stress                       Deflection

S8x18.4                    20.5 ksi                        2.8"

S8x23                        18.2 ksi                        2.5"

S10x25.4                   11.95 ksi                      1.3"

S10x35                      10.0 ksi                        1.1"

Hope this helps.

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Re: I-beam

Re: I-beam -- damille2 Post Reply Top of thread Forum

Posted by: Cragyon ^Ž
Bart
02/20/2004, 14:16:18

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Dave,

You can access a structural beam calculator at https://www.engineersedge.com/beam_calc_menu.shtml and do the calculationd yourself.

You need to know the following:

Moment of inertia of the cross section.

Modulus of elasticity of the material.

Distance from the center of the beam to outer edge of beam.

I suspect all of the beams you indicated will work. The calculator calculates your resulting stress and deflection.

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Re: I-beam
Re: I-beam -- damille2	Post Reply	Top of thread	Forum