wrote:

On Aug 29, 11:24*pm, "Rick" wrote:
wrote in message


om... 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.

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 -

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?

Well, for one thing, you're using the wrong axis for the moment of
inertia...- Hide quoted text -

You are absolutely correct.

I had looked it up in a table, which didn't specify the axis. Since I
wasn't sure of the axis I did actually check it with bh^3, which
coincidentally gives 24.1 in^4, which caused me to stop right there,
forgetting of course that the actual formula is bh^3/12 !!!

So correcting my original post, the allowable load on a single 2x6
spanning 36" would be 247 lbs which might be a little close for
comfort.

The Sagulator sag calculator at
http://www.woodbin.com/calcs/sagulator.htm
(assuming Ponderosa pine) says you'll have a 0.09" sag with a 300 pound
dead load. Of course, a walking person is a live load, so doubling to
600 pounds says the sag will be 0.17". I'm ignoring the second 2x6,
since almost all the walker's weight is on the edge of the front board.

You would definitely be able to feel a step that bounced by almost a
quarter of an inch. Whether you would care is something you'll have to
decide for yourself.

--
Steve Bell
New Life Home Improvement
Arlington, TX