Jack Truss Connections To Girder
Question:
In a hip roof application where jack trusses connect to a girder truss using pressure blocking (power blocking), what is the proper installation and the span that is acceptable before a hanger is required?
Answer:
If the gravity and uplift loads on the jack trusses are known, values for this type of installation can be calculated based on the shear value of the nails in the various components. The gravity loads are given on the truss design drawing (TC dead & Live, BC dead & Live) in psf and need to be calculated for half the load on the end attached to the girder truss, or the maximum reaction at each bearing location should be shown on the truss design drawing. The uplift for the end attached to the girder should be indicated on the truss design drawing.
Each block should be nailed sufficiently to carry the load from both ends. According to table 11N of The American Wood Council’s National Design Specification for Wood Construction (NDS), the shear value of a single 0.131 in. diameter nail in SPF is 82 pounds, assuming a penetration of 10 diameters (1.31 in.). The connection of each jack truss to each block would be calculated in a similar manner. If toenailing is used, the shear value of each nail is reduced by multiplying it by 0.83 (82x0.83 = 68 lb).
Note that this application is not uncommon but is not prescriptively addressed by the building codes, so local approval of the practice should be confirmed.