Point load or uniform area load? 2

[us4thorns]

At 24" o..c. rafters and floor joists create a uniformly distributed load. I was informed today that 4x8 porch rafters with 3/4 T&G pine decking @ 36" o.c. should be calculated as point loads. I was hoping someone could tell me what the cut off is that changes a UDL to a series of point loads.

Thanks,
Doug

 

[JAE]

No official cutoff...just engineering judgement. At some point the shear and moment diagrams will differ enough to make a difference in the final design unless you are being a bit conservative in your sizes, etc.

 

[Ron]

JAE is exactly right. It's your call as an engineer. Judgment....the thing that sets apart as an engineer from a non-engineer, fundamentally and legally!

Do it both ways and see the differences...then apply your judgment.

 

[msquared48]

Sometimes, if you use point loads to beams, some of the load goes directly to the columns, with the beam never seeing that load. Consequently, if the reduction is enough, the beam will be smaller.

Mike McCann
MMC Engineering

 

[BAretired]

If loads are equally spaced between supports, the bending moment diagram is a polygon which inscribes the parabolic curve of the uniform load bending moment. Each point on the polygon hits precisely on the parabolic curve.

The assumption of a uniform load is always conservative but if your design is governed by shear, you may wish to consider point loads.



BA