On Aug 11, 3:44*pm, stratford1 wrote:
On Aug 11, 9:52 am, Harry K wrote:





On Aug 10, 9:56 pm, stratford1 wrote:

I'm building a set of stairs on my deck and CANNOT seem to find a
straight answer on this. I'm using closed outside stringers spaced 36"
and will use metal tread brackets to attach two 2x6 treads for each
stair. Some info I've found says I DO need a third stringer in the
middle for support...others have suggested I DO NOT need a third
stringer as long as my span isnt longer than 36". *Any suggestions on
this? Will my 2x6s hold up ok or should I do the third stringer
regardless to make my stairs stronger?

So why are you debating using one? *The cost of adding a third
stringer is minimal and you already know in your mind that without it,
it will be 'springy'.

Add the third stringer. *It won't need any fancy cleats, etc, just cut
to fit under the treads.

Harry K

Thanks for all the input! Yes, some of you are asking the right
question...why am I set against not doing the third stringer, since
its minimal cost and work to add it and will make it more safe? Well,
originally I wasnt crazy about the asthetics of having the third
stringer visible between my steps, since they will be spanning from
closed stringers on the ends and will not have a riser board attached,
but its not the end of the world. An injury on my deck would be MUCH
worse. Anyway, thanks for all the help!- Hide quoted text -

- Show quoted text -

I struggle with this issue from an engineering perspective and I'm
hoping another engineer can set me straight.

Deflection of a simply supported beam with a concentrated load is: y=
(F L^3) /(48EI)

Using an L/480 stiffness criteria gives an allowable deflection of 36"/
480 = 0.075"

A standard 2x6 has an I = 24.1 in^4 and E = 1.6 e6 psi

Therefore the allowable force on a SINGLE 2x6 would be: 2975 lbs !
That's a 10X safety factor for a 300 lb man !

Spread the load over two boards and the allowable load would double.

Given these calculations, why would a third stringer be even remotely
necessary?