Bolts: Glulam Steel Bolts Connections
Glulam connections with steel bolts are commonly used to attach structural members, providing a practical, effective and economical way to transfer loads. When designed properly, steel bolts will enact to maintain structural integrity throughout the life of the structure; however, poor design and low reinforced glulam structural members could decay and lead to premature failure of bolted connections. Glulam bolted connections that fail or could not withstand design charges will require costly repairs or even further, replacement of the structure. Bolted connection should be designed accordingly with the National Design Specification (NDS) for Wood Construction.
Glulam: Bolt Size
When using bolted connection on glulam structures, there are several requirements that must be met to ensure structural integrity. The maximum bolt sizes allowed by the National Design Specifications are one inch in diameter. Recent studies have shown that ‘negative effects of drying and fabrication imperfections were more pronounced with large, stiff fasteners than for smaller diameter fasteners’ (Suddarth S.K., Test Performance of 1-1/2 Inch Bolts in Glulam—Row Effect and Effect of Subsequent Drying, Wood Design Focus Issue 1, Volume 1, 1990).
Other researchers (Khushefati, W.H. Performance of Bolted Joints Comprised of Glue-LaminatedWood Members Connected with Large Diameter (1-1/4 inch) Bolts. Master’s Thesis, Cornell University, 1985) have found that 1-1/4 inch diameter blots used on 4-1/2 inch wide glulam with ½ inch or 2-1/4 inch wood side members compare favorably with fastener capacities determined using the NDS procedure.
Glulam: Bolt Strength
Bolts used in glulam construction are typically ASTM A307, grade A or SAE J429, with a yield strength of 36,000 and ultimate strength of 60,000ksi. Using these bolts on glue laminated lumber provides you with bending yield strength of 45,000 pounds per square inch, based on the NDS fastener capacities of this bolts. Some contractors also tend to install high strength bolts connecting glulam members following standards from ASTM F1575 or averaging the ultimate tensile strength and the tensile yield stress to determine its bending yield strength.
Glulam: Bolts and Nut Tightening Tips
- Tight the nut sufficient to bring the faces of glulam members into contact.
- When using U-shaped hangers, nuts should be tightened only to bring the nut and bolt into contact with the side plates. The same installation and tightening force should be used in saddles.
- Nut tightening on bolts installed in slotted holes, should be tightened by hand to allow the members to slide past each other with minimal resistance
- No minimum torque or tension is required even when using high strength bolts.
- Remember that blot tension will diminish due to shrinkage of glulam members due to drying through time.
- Never try to over tight a nut connection to avoid localized crushing of glulam member fibers or producing damages to the steel bolt.
Nuts uses in glulam members should be tightened to the following recommendations:
Glulam: Bolts and Washers
Steel washers are commonly used to secure glulam members. However, it is not necessary to use steel washers if a steel plate or strap is installed between the bolt head and the glulam member. Nonetheless, if a washer is used it must be sized properly at least as a standard cut washer. It is important to note that hardened washers and not required on glulam member connections.
Glulam: Bolt Holes
Bolt holes in glulam members are typically dimension 1/16 inch larger than the bolt size diameter. However, it is commonly standard practice to prepare oversized or slotted holes in the metal members to allow for cross-grain shrinkage of the glulam member. These holes are not the same holes used in steel construction for slip-critical connections. Washers are not required between the bolt head and the steel plate when slotted holes are designed in glulam connections.
