Strength and stiffness of small glued-laminated beams with different qualities of tension laminations

Abstract

A total of 180 small Douglas Fir–Larch (DF-L) or Southern Pine (SP) glued-laminated beams were evaluated to determine the tension lamination quality necessary to obtain desired design stresses. The test beams had either the regular laminating grades of L1 DF-L/No. 1D SP or the special 302-24 laminating grade as tension laminations. Because an initial set of SP beams did not provide valid strength data due to a high frequency of finger-joint failures, a second set of SP test beams was manufactured. Although the initial set of SP beams could not be used to answer the objectives of this study, it did indicate that current strength requirements for daily fingerjoint quality-control bending tests need to be increased. The 60 DF-L beams and the 60 beams for the second set of SP beams show that specially graded tension laminations are necessary even for 4-lamination beams. A tension lamination grade between the regular laminating grades of L1 or No 1D and the special laminating grade called 302-24 may be adequate on a 4-lamination beam to obtain a 2,400 lb/in.2 design stress. The 8- and 10-lamination beams, however, appeared to require a 302-24 grade tension lamination to obtain that design stress. The strength of test beams with L1 or No. 1D grade tension laminations was about 15 percent less than the strength of test beams with 302-24 grade tension laminations. This suggests a 2,000 Ib/in.2 design stress for the test beam combinations with L1 or No. 1D grade tension laminations. The data will provide guidelines to industry committees in recommending design stresses and specifications for glulam beams.

Citation

Marx, Catherine M.; Moody, Russell C. 1981. Strength and stiffness of small glued-laminated beams with different qualities of tension laminations. Res. Pap. FPL-RP-381. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 48 pages