Shear Value #8 Screw- Any Idea?

[PJB12]

Hello All!
I must attach 1-1/2" wide x 1" x 1" x 1/8" Hot Rolled Steel angels to the face of vertical wood 2 x 6. The largest fastener that can be used is #8 screw. (Hammer or nailer won't fit. Larger diameter screw won't work in the angles.) Only one screw per angle will fit. (Obstructions & access problems are plentiful in this area)

Do any of you know the shear strength for #8 screws with steel as the veritcally loaded member? The load is straight down.

The screws at this point will be Simpson #8 wafer head screws x 1-1/4" long. Simpson never answered my inquiry, not does the web site list the rating. On line, general shear rating for #8 wood screw in steel vary from site to site. (25 pounds to 457 pounds are ones I found) No #8 structural screws could be found.

For reference, ideally the fasteners would shear at 100 pounds or above. The load is static.

Thanks For Helping!
Paul

 

[Steve123]

Depends on the steel used for the screw, but load to shear a #8 screw will be towards the top end of the range you found.

That screw will pull out before it shears in two.
But keep in mind, that if this is for something where ugly or unsafe possibilities if it failed, an engineer or architect would work to a factor of safety of, I am not sure, but somewhere around 5.

 

[bud16415]

Great info above. In general if you take the minor dia. of the screw (tap drill size) and calculate the area x 60,000 psi for mild steel it would give you an approximate number. Then select a safety factor 5-10 depending on the damage failing would cause.



Deck screws are quite tough as are tapcon screws. Drywall screws are very strong but also brittle as they are hardened. Why they are great for drywall but not good for building framing structure where you need a combination of strength and flexibility.

 

[PJB12]

Thank You Steve & Bud for very helpful information.

I never thought of the safety factor reducing the published figure. Simpson replied to my inquiry, showing the screw is rated 50 pound shear in steel. That must be a safety factor figure. (Tried to shear one today in a test jig. I know I applied far above 50 & it didn't break.)

Thank You for the engineering formula, Bud. It'll save me a lot of searching for rated values.

Thank You also for mentioning Tapcon screws being tough. For some reason I never think of them when I have a fastening project that isn't masonry.

Thank You Both Again!
Paul

 

[bud16415]

Safety factors are a highly variable factor. You never want to design anything with only a factor of 1. Sometimes when weight is of utmost importance and risk can be assumed you can use a much lower number. Say you are bolting a connection where there are 6 bolts loaded but you know you only need 3. If one was flawed the remaining could carry the load. I used to design a lot of equipment used in locomotive production and I remember being given the job of designing a lifting device for the 16 cylinder engine that weighed about 50,000 lbs and had men working near the suspended load. I gave a lot of thought to safety factors on that job and went with a 10x factor along with a lot of testing. Even welding is now a very mature process but it still sometimes has some variability. In lifting devices we always tried to avoid welds in tension.

 

[Steve123]

That screw will pull out before it shears in two.

Notice any similarities in the loading ?