Page 539 - Forensic Structural Engineering Handbook

TIMBER STRUCTURES 14.29

dowel reinforcing are covered in AITC 404-92, 2005, which is an appendix to Inspection
Manual AITC 200-92, 2004. 4
Since radial tension stresses are a function of radius, reinforcing is required in the
curved portion only. A side effect of radial reinforcing is that member dimension is locked
in perpendicular to grain when it is fastened or bonded to the steel dowel. This prevents
slight expansion and contraction of the timber due to moisture cycling. When the wood
does shrink due to moisture loss but is restrained, checking occurs in the reinforced portion.
Stitch bolts or stitch clamps (eliminating section loss) perpendicular to grain may also be
used to resist tension.
Shear Reinforcement. Load-induced shear separations are rare in wood as a rather large
safety factor separates actual ultimate shear strength and tabular design stresses in hori-
zontal shear. Current allowable shear stresses are much higher than 15 years ago. If delam-
ination occurs in glulam, reinforcing against a potential shear failure may be appropriate.
The potential failure mode for this situation is separation of the beam into two segments
along a horizontal plane and subsequent flexural failure due to reduced section modulus.
If a separation parallel to grain due to delamination, checking, etc., has the potential for
shear failure of the member, then shear reinforcing is appropriate. The reinforcing must
transfer shear between the separated segments, which may be accomplished by embedded
dowels or exterior side plates. The dowel shear transfer or side plate connectors must be
engineered to transfer the calculated shear. Lag bolts or epoxy-embedded dowels, placed
perpendicular to grain, may be used with the added advantage of the dowel’s being able to
resist any perpendicular to grain forces. A precise evaluation of beam net section contain-
ing reinforcing dowels would be with a transformed section using full section for the com-
pression side of the neutral axis, and the full section less dowel area removed for the tension
side of the neutral axis. A quicker, more conservative assumption would be the full dowel
area removed from the entire net section. (See Case Study 3, Tinora High School.)
It is very difficult to mechanically reinforce multiple laminations (as in a nail lam beam)
for shear resistance as they are all tending to slip in the same direction. In this case, steel
side plates on each side of a beam connected with timber rivets have been used to transfer
shear between separated wood segments. The total shear transfer required has to be calcu-
lated with the resulting component forces resolved.

Clamps and Stitch Bolts. Clamps and stitch bolts are particularly useful to prevent prop-
1
agation of timber separations in truss chords and webs. Usually a stitch bolt of / 2'' diame-
ter with washers is adequate to close a separation. Stitch bolts should only be used at
stressed members if the loss of section by the additional bolt hole may be tolerated. A ver-
tical stitch bolt adjacent to truss lower chord-web connections may be used to resist the
cross-grain tension stress created by ceiling loading on the lower chord below the connec-
tion or by parallel-to-grain loading on the round connector. Clamps consisting of steel
plate, angle, or channel above and below a member and fastened by bolts each side of the
member may be used to close separations or resist cross-grain tension forces without loss
of section. (See Case Study 2, Rainier Beach Library and Case Study 11, William Shore
Memorial Pool.)

Field Installed Addition of Laminations. In rare instances glulam beams have had lam-
inations added in place to improve capacity. This requires careful monitoring and control
of the moisture content in existing beam(s) and the new laminations. The bottom of the
beam(s) in place needs to be accurately planed. The beam(s) needs to be jacked the appro-
priate amount to avoid overstress of the existing laminations when the new ones are
applied. The new presumable high-grade laminations need to be factory finger-jointed,
planed, shipped and carefully handled at the job site. The appropriate adhesive needs to be