# How to calculate screw pullout strength?

I am designing a system where a beam is acted on by a horizontal force. I intend to hold the beam in place with a set of screws, although I am uncertain how much force they will be able to withstand.

What method can I use to determine the holding force of screws?

• Can you tell us what materials do the screws go into? What kind of screws are they? The design methodology is very different for lag screws into wood or thread forming screw into plastics for example. Commented Apr 17, 2015 at 5:39
• I imagine it depends enormously on how the screws are manufactured: the accuracy of their size, the alloy, if/how it was hardened, how it was machined and no doubt more. While your question is a good one, if you want help with your specific problem, I think you need to give more information. E.g. you say the beam is acted on by a horizontal force, but with no mention of how the rest of it is oriented, or what shape the beam is, what material it is, if nuts will be holding it or if it will be itself threaded, etc etc! Commented Apr 17, 2015 at 12:09
• The other reason I'll add as to why we need more information is because there are two ways the screw can pull out: the screw threads can break or distort, or the material they're driving into can break around them, giving them nothing to grab on to. The second is definitely going to be more common, but that means it's more dependent on the material than the screw itself (apart from thread geometry) Commented Apr 17, 2015 at 12:30
• Pullout isn't all you need to worry about. I've broken the heads off more cheap wood screws than I can count...
– Air
Commented Apr 17, 2015 at 16:18
• Could you supply a diagram? Or at least specify whether the force is transverse to the screws or along the screw axis? Commented Apr 17, 2015 at 22:20

In theory (remember this is theory, and I think that's what you want) you would find the part of the screw that would fail first due to the applied force.

The screw could fail in three ways:

1. The screw head is pulled off and then there is nothing holding the plate to be screw. (It could theoretically break other ways, but the largest force will probably be perpendicular to the head face compared to some force applied to a cross section).
2. The threads shear and the screw becomes a pin basically.
3. The screw shaft snaps.

So you use the screw material to get your elastic modulus for tension and for shear. Then you calculate the total area the force would be applied against (the screw shaft area for tension or the combine area of thread contact for shear). You need to know the applied force and then you calculate be resulting pressure experienced by the screw.

If the pressure is greater than the modulus, it will fail in that method. That's basically a high level summary. There could be torque applied to the head as well if the plate isn't pulling straight against the screw head bottom face.

Many cities have either their own code on allowable load for nails and bolts or have adopted a uniform building code.

In Los Angeles for example the building department has published these codes on their site, or give them to you as a hand out for free.

American Wood Council has many pages of guidelines and information on their site.their calculator The allowable load for wood connections depends on many factors:
wood structural rating, its humidity content, species, storage history as well as mechanical strength and type of fastener (screw, bolt, lag bolt, prefabricated fasteners and kits) and the mode of their failure( shear, bending moment, pull out, etc..) and alos the function of the connection, there are load factors for seismic loads or shear loads, etc.

For nails there are concerns as to the ductility of nail, its size and length, the distance between nails, the grain of wood and its angle and many more factors. All of these cases have been extensively tested and tabulated and are available from your City or the hardware store or the manufacturers site.
A deputy structural engineer inspecting the construction site is responsible for checking the nailing and proper application of their use and will ask the builder to correct or remove not complying nails, fasteners or members and re-installing them correctly.
You have to discuss your case with an engineer in your building department. In most cases it is free consultation.