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Structural Screws vs. Lag Screws - Definition
Structural screws and lag screws are two commonly used screw types in the construction field. This article explains the design, production, costs, and other considerations to make when choosing to use structural or lag screws.
Lag screws, also known as lag bolts, are similar to wood screws. They have a larger diameter and length, however, and often have coarse threads. Lag screw heads are most often hexagonal, which is why they are also called lag bolts. They must usually be installed with a wrench, not a screwdriver. The screws are most often used to secure heavy pieces of wood to each other or to other construction materials, such as concrete.
When using a lag screw with concrete or masonry walls, a lag is often used. A lag is an insert for the screw that is made out of a soft metal alloy. As the screw threads into the lag, the lag expands outward somewhat and deforms around the screw threads. This creates a watertight seal around the screw, preventing corrosion and increasing strength.
Image: Lag Bolts - Flickr - James Lee
Structural screws are a relatively new type of fastener. They are often used in place of lag screws because they are much more convenient to work with. The increased convenience comes from the design of structural screws, which eliminates the need for pilot hole drilling and lags. When using with metal, though, the material should still be pre-drilled to avoid damage to the structure's coating. Structural screws are often stronger than lag screws and create a more resilient connection between materials.
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Design and Production
Lag screws are stocked in a wide variety of sizes and lengths. Most lag screws are generic, though, meaning that they do not have a rated stress tolerance readily available. Some companies do make lag bolts with a structural rating, but these can be hard to find and expensive. Still, it is not difficult to calculate required values from bolt diameter and length.
For instance, the bending yield strength of a bolt with diameter .375 inches and unthreaded diameter .265 in. is 45,000 PSI. Maximum stress and moment for such a bolt can be calculated from equations or from tables following ASTM guidelines. These table values are fairly conservative. For a lag screw calculator, follow this link: Connection Calculator. For full tables, see the link at the bottom of the article entitled "Design Aid for Application of Technical Report 12 for Lag Screw Connections."
Structural screw design is more complicated than lag screw design because structural screws are driven through the construction material themselves, without the use of a pilot hole. Some screws have mini-drill bits attached to the end, which helps clear wood as the screws are being drilled in. Structural screws are good in shear, having typical maximum allowable shear of about 250 to 400 pounds. Structural screws are designed to specific standards, which are often listed for each type of screw. Designers use many different types of treatments to increase the strength of structural screws, including heat treating and galvanizing.
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When to Use
Structural screws are designed to be a replacement for lag screws, so they can often be used interchangeably. Each type has its own advantages and drawbacks. For instance, a user needs to drill two holes for every lag screw to prevent the wood from splitting. This is extremely time consuming in applications requiring a large number of fasteners. In addition, more lag screws may be required because of their lesser strength.
If shear-offs are a concern, structural screws are often a better choice. Structural screws are designed to meet engineering standards, unlike some generic lag screws, and are made of quality hardened steel.
Though it may seem like structural screws are the better choice, they have several disadvantages. The biggest drawback of using structural screws is the cost: about three times as much as that of lag screws. For instance, a 6" long, 5/16" diam. structural screw costs about 85 cents, while a comparable lag screw comes in at around 30 cents. Of course, the price difference is less when labor costs are considered.
Another issue with structural screws is selection. Though suppliers typically have a good size selection, they may only carry one or two brands of screw, which can be overly limiting. The same providers carry lag screws in a great variety of size and make.