Drywall screw pull-over strength

[ajk1]

ajk1 (Structural)


9 Aug 13 14:05

I had an earlier string with the question as to what the "pull-over" strength of a drywall screw might be in the drywall. There were no specific answers received to this question, so I did an at-home approximate test of a couple of drywall screws in 1/2" thick drywall and thought people might be generally interested in the answer, although the test was not conducted precisely.

I set up the test with a bathroom scale at one end of a specimen of drywall and a wood block at the other end, on top of a work table in the garage. I placed a thin strip of wood on to of the bathroom scale so that the load would be applied to the scale only at the wood strip location. The span of the drywall was 11.5". I placed a wood block under about the centre of the drywall specimen and then clamped this wood block to the work table. Then I zeroed the bathroom scale and screwed the drywall down into the wood block below. The scale gave me the reaction when the screw head punched through the drywall. I then calculated the load that would cause that reaction, based on actual measurements of the screw location. The calculated value is I believe the pull-over strength of the screw fastener.

I repeated the above procedure for a second screw, but reduced the span of the drywall to 7.3".

The results were failure loads of 12.9 pounds and 13.7 pounds respectively.

Dividing by a saftey factor of 4 gives an allowable capacity of about 3.3 pounds.

This allows me to calculate the required screw spacing when supporting a new drywall ceiling that is placed tight to an existing questionable old plaster ceiling to restrain it from falling (part had fallen).

Prologue: We actually removed the entire old plaster ceiling, and the work was completed about a week ago, but if we ever get another similar situation, the foregoing experiment will give us a bit of a "feel" for what might be doable, although more precise and extensive tests should be done.

 

[XR250]

Those numbers seem awfully low just based on my experience messing around with drywall.
Was it Chinese :>
Is it possible you divided by two instead of multiplied by two to get the load?
Also, unless you pre-drilled the drywall, the thread engagement would add to the pull- thru force but not be registered on the scale (unless I am mis-understanding your test rig)

 

[concretemasonry]

Chinese, American or presidential is not a factor since it could be made for an American company by a Chinese supplier in a plant in Mexico.

What was the method of attachment and how does it relate to real engineered applications? What was the size of the "screw" and what was the orientation if the test assembly?

Was the bathroom scale calibrated? - Any time you try to do a DIY test, you are misleading yourself. There are well established tests for similar connections and materials that are conducted under controlled situations (normally ASTM standards), which may not really reflect an individual use or project.

It is a "cute" test, but not worth using for anything, except a science project in school.

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[ajk1]

Thanks for the comments. They are thoughtful and helpful and much appreciated.

The load was determined by standard beam analysis.
i.e. Screw load = span / a * R where R is right end reaction and "a" is distance to load (screw) from left end. So no, I did not divide by 2 instead of multiplying by 2.

You are right that the threads engage the drywall when the hole is not predrilled, but this indeed does register on the scale. However I could repeat the test with a predrilled hole and see what I get. but does that correspond with what is actually done when drywall is installed.

You raise a good point about the scale calibration. I did not calibrate my bathroom scale but I can calibrate it against a precision electronic scale that I have and see how that affects things (the precision scale measures only to 2.2 kg though; that is why I did not use it in the test).

The position was horizontal with the screw being screwed down instead of up as it would be in a ceiling. The drywall was normal 1/2", not the special high strength drywall. I do not know where the drywall was made.

I am interested in why you think that the results are unreasonably low, and waht you would have expected.

They seemed reasonable to me, and correspond reasonably well with the required screw spacing required by the Code to support the weight of 1/2" drywall. Although I have 49 years of experience as a structural engineer, that experience is not in drywall. I lack your experience with drywall. I take to heart your counsel that I am probably deluding myself with this test. If you have other results or can direct me to where I could find such results, I would be very grateful, or if you can tell me if there is an ASTM or CSA or DIN test procedure for determining the pull-over strength with drywall. I have spent a lot of time searching for the pull over strength of screws in drywall but came up empty.

Again much thanks.