Frequently Asked Questions
My structural evaluation of an existing structure indicates that my glulam beam is insufficient to support additional
loads. Can I add laminations to the bottom of the beam to make it deeper and increase its capacity? Are there issues of
which I should be aware?
This type of repair can be performed, but it poses multiple challenges including (1) lamination quality, (2) bond quality,
and (3) notch restrictions at the end of the beam. Additionally, (4) transformed section analysis will be required for the
structural evaluation of the repaired beam.
(1) Typically, full-length laminations of 302 tension lamination quality with AITC-certified finger joints are not available
to the contractor performing the repair. The most critical part of the beam is the tension lamination. Unless it is properly
graded and manufactured with certified end joints, the strength requirements of the beam will not be met. This alone, typically
precludes adding laminations to the bottom of the beam.
As an alternative, adding new laminations of laminated veneer lumber (LVL) may be considered. The additional lamination
should extend the full length of the beam with no field splices attempted. The LVL should have a 5th percentile tension
strength of at least 1.67 times the nominal glulam bending strength. This translates into a minimum published LVL tension
strength of 80% of the published glulam bending strength – for a 24F beam, a minimum LVL tension value of 1920 psi is needed).
If this strength of LVL is not available, the allowable stress must be reduced proportionately.
(2) If high-strength laminations can be obtained, another issue remains: assuring bond quality for the added laminations. It
is difficult to ensure the proper cleaning and surfacing of the bottom of the beam and adequate application of pressure to the
bond line in the field. Southern Pine generally also requires fresh surfacing to ensure adequate bonding. Sanding is not
recommended, because it reduces bond strength.
LVL typically does not have a smooth surface and often has oils or other contaminants on the surface that may interfere with
bonding. Lightly planing the LVL to remove surface irregularities (such as lumps caused by lap joints in the veneers) and
contaminants is advised.
A fully-waterproof, gap-filling adhesive should be used after careful preparation of the surfaces to be bonded. The gap-filling
adhesive should meet the requirements of ANSI/AITC A190.1-2007 section 4.5.3. Additionally, some bond line specimens should be
made for testing at the time of the repair as an additional quality check. Standard delamination tests (AITC T110) and shear
tests (AITC T107) should be conducted on the specimens to verify acceptable bond quality.
(3) A third issue that needs to be considered with this type of repair is the possibility of creating a notch that doesn’t meet
code requirements. Typically, it is impossible to add laminations over the supports for a beam in service. This consequently
results in a notch at the end of the beam that must be accounted for in design. A structural analysis must be performed using
the notched beam formula from the National Design Specification® (NDS®) for Wood Construction, Section 18.104.22.168. The flatwise
shear value of the LVL or new lamination should be used in this calculation. The notch must also meet the prescriptive requirements
of being no more than 10% of the beam depth or 3 inches, whichever is less.
(4) Outer fiber (tension) stresses in the new lamination(s) (based on transformed section analysis) should be limited to a maximum
of 10% above the nominal bending strength of the glulam beam. For the transformed section analysis, lay-up details for the glulam beam
can be obtained from AITC 117. Modulus of elasticity (E) values for the laminations in the existing beam can be determined using the
lamination grades from the lay-up tables and the E values for uniform-grade glulam timbers of the corresponding grade.