Timber beam moment/deflection question

[blackbird2552]

Hello everyone,

I have a question regarding the moment and deflection calculation of this handrail beam that I have. I am attaching a picture to give you guys a look at what I am working with.

The hand railing is 4" x 4" and it is not attached half way, its just resting on that beam, it is however attached with a bolt at the bottom. My question is, when im looking for the resisting moment would I take 89/2 in the section modulus calculations or the full 89mm. Same goes for deflection, when I do the moment of inertia, would I do the full 89 mm or 89/2. If it is 89/2 then how would I incorporate the rest of the beam in the resisting moment/deflection or would it have no effect on it?

any help would be greatly appreciated, thanks!

 

[woodman88]

Your question is so basic and a handrail can be a very big safety issue that I reccomend you hire someone with structural experience to do the calculations. If you decide to do them yourself, then get an engineering book on it and read it completely. It is what you don't know or look at in the overall structural system that will cause a failure.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[blackbird2552]

thanks for the reply, I have looked at the books and examples, but all they have is completely fixed end or completely free ends not a partial fixed/free end. Thats why im getting stuck. Since it is only connected half way, I would believe that the resisting moment would be caused just by that half fixed end. But for deflection, im thinking the free side would have some effect on the overall deflection, I just don't know how to calculate that.

 

[pelotoner]

There are mechanics of material methods to calculate a changing moment of inertia (summation methods, I believe).
The moment should be taken from the bolt as the fixed end. Make sure the bottom-extended end is attached firmly, but is long enough. The bending and bearing should be checked on the bolt and the extension (ie, the bolt is in tension, the extended end in compression - pushing on the framing)

 

[MiketheEngineer]

Section modulus for a rectangular beam is always bd^2/6. Now, it is up to the engineer to determine the moment it will "see".

Not only are hand rails a BIG safety item, the codes are very explicit on sizes, locations, etc.

Hire some help or get Real involved in this!!

 

[blackbird2552]

Thanks for the help everyone. Since it is only fixed at about half the width of the beam it will only "see" that moment and provide a reaction based on that width.

 

[bill30206]

The sketch indicates that the post is notched to fit over the beam. Beyond the question of net versus gross section to use in the flexure formulae is the matter of lumber grade degradation. The notch may remove the "cream" from the section of wood, leaving behind a defect-filled area. You can not routinely assume that the design properties of the specified lumber can be counted on in a notched piece. Put another way, thinning down a graded piece of lumber can make a piece of No 1 into No 3 or worse, based on what the resulting net section has in the way of defect size and population. NDS has provision for notched is beams, with 1/6 or 1/4 of the original depth permitted to be notched depending on the location. You are talking about half the post, so that is not covered. I do not know any great way around this problem other than to use a built-up post composed of separate, unaltered pieces instead of a single piece to design the post. the Good luck.

 

[BAretired]

It is just a bad detail for the reasons stated by bill30206. Run the post through full size and notch the deck around it. Move the edge joist to the left to clear the post. (Incidentally, if the edge member is a 2x8, the metric dimensions should be 38x184).

BA

 

[shobroco]

Why assume that the beam is rigid? If we are looking at a 2x8 (or multi-ply) beam, the torsion from the post will make bending in the post irrelevant because the beam will deflect far beyond acceptable limits even if the top edge is restrained by the deck. This is pretty elementary stuff but without knowing a lot more about where this is located the whole discussion is either trivial or life-threatening.