DIYbanter - View Single Post - Do I need a third stringer for my deck stairs?

aemeijers

wrote:
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?
I'm no engineer, but my SWAG answer would be because it isn't a static
load. Hence the term, 'springy' feeling. I know from experience that an
end-supported 2x6 3 feet long will indeed flex under a walking man,
probably not enough to be dangerous, but enough to feel creepy.
(temporary construction steps while the thicker pretty custom treads
were safely stored away till us jackbooted thugs were done with the
messy work.)

If I was OP, I'd change the design to either 2 'underneath' stringers
inset from the end, or use the full-width treads with the grooves that
drain water and resist splitting, or use a center stringer. If
appearance is an issue, a dark stain on the center stringer before it is
installed can make it visually blend into the background more when seen
from a low angle. Same trick as painting the rocker panels on a car
black to make it seem sleeker.

--
aem sends...