Glulam Beam Installed Upside Down

The 24F-V4 glued-laminated timber beam (glulam) is intended for simple span framing where the transverse load (causing the beam to bend or flex downward) produces tension stresses on the bottom of the beam and compression stresses on the top. Thus, laminations especially selected for tension are used in the bottom of the beam, and laminations of other (less expensive) grades are placed in the remaining regions of the beam. This deliberate use of higher quality material where it is needed results in a more efficient and more economical beam. Where a V4 (`unbalanced layup') beam is installed upside down, laminations that were originally intended to be in the compression zone of the beam end up being in the tension zone of the beam. The lower tensile capacities of these laminations are reflected in published design values as "compression zone stressed in tension" values, and they are lower than the "tension zone stressed in tension" (how the beam was intended to be used).

Positive Bending

As an example, let's consider a case where a DF/DF 24F-V4 beam has been installed upside down. The design calculations for the beam indicate that the beam would be stressed at 72% of capacity in positive bending under design loads had it been installed upside up, as intended. (Positive bending or positive moment refers to a beam loaded to produce tension stresses on the bottom - perfect for the unbalanced (V4) layup.) From AITC 117-04 the design value for the 24F-V4 beam installed correctly (tension zone stressed in tension) is 2400 psi. Installed upside down, we need to use the compression zone stressed in tension value, which is 1850 psi. Thus, with regard to positive bending, by installing the beam upside down, it has 1850 / 2400 = 0.77 or 77% of it's capacity. So, with regard to positive bending, this beam is still acceptable upside down, since the available capacity is 77% and the needed capacity is 72% .

Negative Bending

Since `higher grade' material ends up in the compression zone (where it wasn't necessarily needed), negative bending (tension zone stressed in compression) generally isn't an issue.

Shear

The shear capacities of the laminations in glued laminated timbers are not taken to vary between grades; as such, the shear capacity of the beam as a whole is not affected by upside down installation.

Deflection

The stiffness of a glued laminated timber is not affected by upside down installation. Hence, the amount the beam will flex (bend) under various loading conditions is unchanged. What may become a problem with upside down installation is whether or not the beam was manufactured with camber. Camber is the amount of upward curvature manufactured into the beam to offset some or all of the deflection anticipated for Dead loads. As such, the equilibrium geometry of the beam subject only to Dead loads (weight of floor framing, floor and ceiling coverings, whatever) ends up being `flat', or perhaps curved slightly upward. Whether or not this `pre-sag' should be called a `deflection' could be argued, but, either way, a cambered beam installed upside down will have camber `adding' to deflection, instead of being intended to cancel some of it out.

Stock beams such as the DF/DF 24F-V4 are generally manufactured to pre-determined radius values that produce various cambers for different spans. For example, if the beam discussed above was manufactured at a radius of 5000 ft, it will have a camber of 0.19 inches at a span of 25 ft (equation 3.8 of the Timber Construction Manual). Installed upside down, the beam is already sagged (deflected) this 0.19 inches before it even carries any loads (including its own weight). If the anticipated total deflection (upside up) was calculated to be 0.66 in. (for the 25 ft span), it should now be anticipated to be 0.66 + 0.19 = 0.85 in. In terms of a fraction of the total span, this 0.85 in. would be 0.85 in. / (25 x 12 in.) = 1/353. While this fractional total load deflection is in this case less than the generally accepted limit of 1/240, and as such the deflection check is still `good', the increased deflection should still be `accepted' by the Owner before the overall condition of the upside down beam be deemed good. If the Owner was actually looking forward to the camber of 0.19 in., then the upside down beam will be twice the 0.19 in. or 0.38 in. (3/8^th in.) more sagged than anticipated.

If the beam is a roof beam, and the roof is flat, then installed upside down the beam could cause ponding. This is NOT a good situation and may require specific detailed analysis.

The upside down condition is not taken to affect Live load deflection, as live Load deflection is the net deflection is taken to be the net response to the Live load only.

Bearing

Upside down installation is generally not considered to affect bearing conditions.

Conclusion

Unbalanced glulam beams installed upside down pose two problems. The first problem is that of safety, where laminations intended for resisting compressive stresses are required to resist tension stresses, for which they have less capacity. For this condition the "compression zone stressed in tension" design values must be used in the design check for the beam. And the `answer' will be either `yes', or `no'. If the design check with the beam installed in the lower capacity configuration is not good, then the beam must be replaced or reinforced. If the design check is `right at the knife edge', it might be possible that a more detailed evaluation of loads (Dead and others), and /or more precise geometry information, may tip it into the good category (as loads and span dimensions are often overestimated a bit).

The second problem is that of deflection, and more particularly camber. If the beam was straight (not cambered) then it will sag the same in either configuration (upside up or upside down). If the beam was cambered, the camber will add to the deflection produced by the loads instead of diminish it. The Owner must be made aware of this (new) condition. This is a `serviceability' issue. Safe or not, or within Code limits or not, will the Owner accept the additional sag? Finally, in the case where the beam is installed in a flat roof, where camber may have been intended to help shed water, the upside down beam may cause the roof to collect or `pond' water. Ponding is very much a safety issue and may require specific analysis.

References

Standard Specifications for Glued Laminated Timber of Softwood Species, AITC 117-2004, American Institute of Timber Construction, 7012 South Revere Parkway, Suite 140, Centennial CO, 80112.

Timber Construction Manual, 5^th Edition, American Institute of Timber Construction, Published by John Wiley & Sons, Hoboken, New Jersey.