Flitch Plate Beams - DeStefano & Chamberlain, Inc.

1 From Experience how did you do that ? . Flitch Plate Beams Design Guide By Jim DeStefano , The wood side pieces provide lateral support Beams since these members contain three Flitch Plate Beams have been to the slender steel Flitch Plate and brace the wood pieces. T. commonly used with wood frame steel against lateral buckling. The shear capacity indicated in Table 1 is construction for over 75 years. With a Flitch Plate beam, the structural the shear strength of the wood alone since Despite the popularity of these hybrid F. load is shared between the steel Plate and the full beam reaction must be transferred structural elements, structural design the wood side pieces proportionally to their through the wood side pieces at end bearing t.

2 Guidance has been conspicuously relative stiffness. y r ighIn order to structurally ana- supports. A. absent from engineering literature. lyze a flitchp Co Plate beam, transformed section The values in Table 1 are based on the steel That is, up until now. properties are used that treat the composite Plate being A36 steel. This is a reasonable R. section as an equivalent wood member. The assumption since steel plates are not readily F. litch Plate Beams are composite mem- section properties indicated in Table 1 are available in Grade 50 steel. Be sure when spec- bers which combine the strength and based on the wood members being composed ifying Flitch plates to indicate that the Plate D. stiffness of structural steel with the ver- of Douglas Fir - Larch #2 as well as laminated is made of steel.

3 Some builders believe that satility of wood. A Flitch Plate is a steel Plate veneer lumber (LVL). Flitch plates can be made out of plywood. that is sandwiched between pieces of framing The flexural strength of Flitch Plate Beams lumber and bolted together. They are used in using Douglas Fir side pieces is controlled Bolting of Flitch Plates a similar manner to built-up wood girders or by the bending stress in the wood, while the Adequate bolting is crucial to the perfor- headers in residential and light commercial strength of Flitch Plate Beams using LVL side mance of Flitch Plate Beams . Since the load is construction. Flitch Plate Beams are capable pieces is controlled by the bending stress in applied to this composite beam through the of achieving greater spans and supporting the steel.

4 Wood side pieces, the bolts must distribute higher loads than built-up wood members. No load duration factor was applied to the that portion of the load that the steel plates Unlike engineered wood Beams , Flitch Plate allowable bending stress. The allowable stress will be carrying. At the beam end bearing Beams can be flush framed with dimension was adjusted by the appropriate size factor. A supports, only the wood side pieces rest on lumber joists without causing shrinkage re- repetitive member factor was applied to the the supports. Consequently, bolts at the ends lated distortions to the structure. allowable bending stress of double Flitch Plate of the beam must transfer the end reaction from the Plate to the wood.

5 Flitch P. L Section Properties lTR (in4) STR (in3) M(ft-lb) Msteel/M Vwood (lb) n Wood Steel (2) 2x8 PL dx7 289 7,173 67% 2,610. E = 1,600,00 psi Fb = 900 psi P. (2) 2x8 L 2x7 354 8,793 73% 2,610. Douglas Fir - Larch #2. (3) 2x8 (2) P. L 2x7 661 18,851 78% 3,915. (2) 2x10 P. L dx9 611 10,890 68% 3,330. (2) 2x10 P. L 2x9 748 13,340 74% 3,330. ASTM A36 Fy = 36 ksi (3) 2x10 (2) P. L 2x9 1,397 28,649 79% 4,995. (2) 2x12 P. L dx11 1,109 14,790 68% 4,050. (2) 2x12 P. L 2x11 1,361 18,142 74% 4,050. (3) 2x12 (2) P. L2x11 2,543 38,994 79% 6,075. (2) LVL 1:x92. 715 20,776 64% 6,320. E=1,900,000 psi P. Fb = 2,600 psi L 2x9. (2) LVL 1: x11f 1,337 31,784 64% 7,900 P. L 2x11. (2) LVL 1:x14. 2,549 47,593 69% 9,310. P. L 2x14.

6 Table 1. STRUCTURE magazine 56 June 2007. Load is transferred between the steel Plate and the wood side pieces by a combination of shear through the bolts and friction between the steel and wood. The magnitude of the load transferred through friction is a function of the tightness of the bolts. Washers should always be used under the nuts. The nuts should not be tightened T. to the extent that the wood crushes under the bolt head or washer. As the lumber seasons F. and shrinks, the bolt tension and the friction forces will t . y righ A. diminish. Because of this and p the fact that determining the Co magnitude of load transfer due R. to friction between the wood and steel cannot be calculated with certainty, this bolt friction D.

7 Contribution is usually neglected in the calculations. Carriage bolts with either or e-inch diameter are commonly used with Flitch plates. Larger diameter bolts are not readily available through lumber yards or hardware stores. Figure 1: Empirical bolting pattern The bolt holes should be drilled or punched in the steel Plate . Flame cut bolt Bolts should not be positioned closer than standard bolting pattern for single and double holes do not allow uniform bearing for the 2 inches to the end or edge of the wood Flitch Plate Beams . The 16-inch spacing of bolt. Bolt holes should be 1/16 inch larger members for -inch diameter bolts and not bolts insures that the bolt heads and nuts will than the bolt diameter.

8 Closer than 2 inches for e-inch diameter not interfere with joists which are also spaced Where wallboard or wood trim is applied bolts. at 16 inches. directly to the face of a Flitch Plate beam, it There are two methods for determining the Rational Method: The required bolt size is desirable to counterbore the nuts into the required bolt size and spacing, the empirical and spacing is determined from structural wood side pieces. Since counterboring reduces method and the rational method. calculations. The allowable bolt capacities the amount of bearing for the bolt, the depth Empirical Method: A standard bolting are calculated based on the National Design of the counterbore should not exceed the pattern is used which has performed Specification for Wood Construction (NDS).

9 Thickness of the nut and washer. adequately in the past. Figure 1 indicates a The load carried by the steel Plate is deter- mined and uniformly spaced bolts are pro- vided to transfer this load. The end reaction Design Example on the steel Plate is then calculated and bolts are provided at the end of the beam to resist this reaction. This method neglects the con- (2)-2x10 & Plate -inch x 9 inches - Douglas Fir - Larch #2 tribution from friction between the steel and Span = 12 feet Uniform load = 700 plf the wood and, as a result, will yield conser- Calculate the capacity of a -inch diameter bolt, with the load vative results.. perpendicular to the grain, per NDS 2005: NDS Eq. z = 945 lb Jim DeStefano , , is the senior partner Calculate the uniform load on the Plate : with DeStefano Associates located in Fairfield, CT.

10 Jim can be reached at 700 plf x 74% (from Table 1) = 518 plf Calculate the uniform bolt spacing: (945 / 518) x 12 inches = inches >>>> Space bolts at 20 inches Calculate the end reaction on the Plate : 518 plf x 12 feet/2 = 3108 lb Calculate the number of end bolts required: Visit STRUCTURE on-line 3108 / 945 = >>>> Provide 4 bolts at each end of beam STRUCTURE magazine 57 June 2007.