|Disclaimer: This is one of those cases where a little knowledge can be a dangerous thing. Entire courses in engineering school are devoted to learning about structural loads, so obviously the superficial notes you find here cannot provide much knowledge. If you have a situation where failure of fasteners can cause serious problems you should consult a knowledgeable specialist.|
Note: It's an oversimplification and not technically correct but, in the following, you can think of the words force, load and weight as meaning the same thing.
Important: When reading the strength (load bearing, weight carrying ability) given in the tables at the bottom of this page you must keep in mind these values are "ultimate" loads. This means they are the weight/force/load that will cause the fastener to fail/break. They are not the amount of load you can use safely. You must apply at a safety factor of at least 4 for your application/use. That is, if the table says a fastener can support 100 pounds it means you must divide this by 4, and thus not apply more than 25 pounds of force to the fastener.
The term "wallboard" used in the tables means gypsum wallboard/drywall/sheetrock.
The tables will give two strength (load/weight carrying) numbers. One called "tension" and the other called "shear." See the figure below.
Tension means the force trying to pull the fastener straight out of the wall. Shear means the force applied to the fastener pulling straight down, along the wall. Usually fasteners can hold more weight/force in shear than in tension.
How Much Load is on Each Fastener?
The shear and tension forces are independent -- they do not add to one another. You assess them separately. That is, you calculate both of them to make sure the fastener is secure in shear and also in tension.
Usually the shear force on a wall fastener is simply the weight of the object hanging on the fastener/screw. If you have two fasteners supporting the object, each is carrying half it's weight. The weight of an object hung on a fastener in the ceiling is pulling on the fastener and thus is tension. If the object is flat against the wall you only need to consider shear. If it's hanging directly down from the ceiling you only need to consider tension.
But, if the object hangs away from (stands out from) the wall [called cantilevered] it also pulls on the fasteners (tension). In this case there's no simple answer to how many fasteners are needed. See figure below. The shear force will still be the object's weight, but the tension force with a cantilevered load (weight) depends on the relationship between the lengths A and B. As A gets longer tension increases (more fasteners are needed). As B gets longer tension decreases (fewer fasteners are needed). If you think of the L shaped part in the Figure as a claw hammer pulling on the screw you can see how it can apply a lot of pulling (tension) force on the screw -- it's how you pull nails..
Normally you want to put the fasteners near the top of a cantilevered object to make the length B as long a possible, thus minimizing the tension force.
There's no good place to stop this discussion. I'll finish by explaining that the tension force on the screw in the above illustration is the length A divided by length B, multiplied times the weight of the object. So, if A = 18 inches and B = 3 inches the tension created by the 75# weight would be 450#
3 = 6
Assuming 1/2" drywall, it would take:
1/4" diameter toggle bolts, or
to support the object securely.
|Click here for strength of Toggle Bolt|
|Click here for strength of Molly Bolt|
|Click here for strength of Nylon Toggle|
|Click here for strength of Nylon Wall Driller|