Sometimes trusses must be fabricated with lumber that has been treated. Some of these treatments include, but are not limited to, fire and weather protection on lumber. Sometimes trusses will be used in environments where extra corrosion protection is required for the plates. With some of the products used there are design reductions in plates and/or lumber values. These reductions can be applied in the MiTek 20/20 engineering software by adjusting the Duration of Load factors (DOL), or by using the treated lumber in the software.
The most user friendly way to take the necessary reductions when using treated lumber is to use the treated lumber in the inventory of the engineering program. Lumber with a “T” following it is fire
A common type of treatment is intended for fire protection. There are many different brands of treatments that may be used, and the manufacturer of these products will have all design reductions documented. A few common brands are Firepro, Hickson Dricon, Hoover Pyro-Guard, and CSI/D-Blaze. Trusses built with any of these treatments may be fabricated with our standard G60 galvanized metal connector plates. The grip value of the plates must be reduced by 20% when using fire treated lumber and this may be done automatically or manually. To manually reduce the plate values by 20% you will need to multiply the original DOL by 0.80
EXAMPLE: 1.15 * 0.80 = 0.92
In the above calculation 1.15 is the standard DOL. Input 0.92 as the new DOL in standard loading. Lumber values must also be reduced in these designs as well. Firepro has an 18% reduction to lumber values, making the new standard DOL 0.94 if starting with a DOL of 1.15. Hickson Dricon, Hoover Pyro-Guard, and CSI/D-Blaze have a 35% reduction in design values, making the new standard DOL 0.74 from 1.15.
These same percent reductions would also have to be applied to the wind and snow loading DOL. Again, all of these reductions are taken into account automatically by the MiTek 20/20 engineering software when the included fire treated lumber is used in the design.
Preservative treated lumber is another common material used in trusses. Again, different brands and types have different reduction values. When certain types of chemicals are used plates must either be coated with approved materials to prevent corrosion, or stainless steel plates are to be used. Borate and Micronized Copper Quaternary (MCQ) also known as MicroProTM type treated lumber do not require any reductions to design values as long as the lumber is re-dried to a 19% moisture content prior to truss fabrication.
When certain types of chemicals are (or may be) present in a structure where trusses are going to be used, the plates must either be coated with approved materials to prevent corrosion, or stainless steel plates are to be used. Please note that stainless steel plates come in limited sizes and G185 plates are only available in 20 gauge. Check with your sales representative
In any of the cases where special plates are to be used, other materials used in the job must also be protected from corrosion. Any nails/screws/bolts in contact with the lumber must be protected. Hangers and tie-downs connections must also be protected.
View and/or download additional information from TPI and WTCA on types of treatments. Please contact your MiTek engineer with any questions or concerns you may have.