Spliced Mechanically-Laminated Posts

common to construct the post so that an interior laminate is left shorter than the surrounding laminates. When the post is installed with this feature located on the top of the post, the truss can be set in the resulting pocket, enabling a double shear connection between the post and truss. The interior laminate is generally signifi-cantly shorter (approximately 1 foot) than needed to accommodate the truss. This is done to compensate for varying depths of embed-ment. After posts are installed, a spacer (or block) of the same cross-sectional size as the shortened laminate is placed in between the shortened laminate and the truss. A schematic of this procedure is shown in Figure 7.3.

Figure 7.3. On-site truss placement in a me-chanically laminated post.

7.6 Spliced Mechanically-Laminated Posts

7.6.1 Types. A spliced post is any post in which at least one laminate contains one or more end-joints (i.e., is comprised of two or more individual pieces of lumber). Major end-joint types used in spliced mechanically-laminated posts include:

simple butt joints, reinforced butt joints, and glued finger joints. Butt joints are generally reinforced by pressing metal plate connectors into one or both sides of each joint.

1. Post set, bottom of truss marked, and block height measured

3. Block nailed into place and top of outer layers cut off.

Block

Figure 7.4. (a) Treated portions of 3-layer spliced posts are embedded in the soil. (b) Top of treated portions cut so that tops at same elevation. (c) Untreated post portions spliced to treated portions.

7.6.2 Use. Virtually all mechanically-laminated posts with overall lengths exceeding 20 foot are spliced posts.

7.6.3 Advantages. Splicing enables the fabrica-tion of long posts from shorter, less expensive lengths of dimension lumber. Splicing also enables the construction of posts with preserva-tive treated lumber on only one end. This re-duces the quantity of treated lumber used in a building, which in turn reduces the number of special corrosion-resistant fasteners needed to join treated lumber.

With simple butt joints, the attachment of non-treated lumber to non-treated lumber is sometimes done in the field. This attachment is done after the treated pieces have been laminated and embedded in the ground (figure 7.4a). Prior to attaching the untreated top-portion of each post, the embedded treated portions are all cut so that their tops are at the same elevations (note:

because of differing depths-of-embedment, the top of each embedded section is generally at a different height above grade). With the embed-ded portions at the same elevation (figure 7.4b),

the upper portions will have the same overall length (figure 7.4c). This eliminates cutting and blocking like that associated with the special construction shown in figure 7.3.

7.6.4 Disadvantages. Spliced mechanically-laminated posts have the same disadvantages as unspliced mechanically-laminated posts (see Section 7.5.4). In addition, a simple (non-reinforced) butt joint can significantly reduce bending strength and stiffness in the vicinity of the joint. If a post contains a simple butt joint in each laminate, and these joints are all located within 1 or 2 feet of each other, engineers will often model that portion of the post as a hinge connection.

7.6.5 Design Properties. Design properties for spliced mechanically-laminated posts are highly dependent on the type and relative location of end joints, and on the type and relative location of mechanical fasteners, especially those lo-cated in the vicinity of end joints. Procedures for designing and determining the bending strength and stiffness of spliced nail-laminated posts are outlined in ANSI/ASAE EP559 (ASAE, 1999).

(a) (b) (c)

Level line of sight

The design portion of EP559 includes require-ments for joint arrangement, overall splice length, nail strength, nail density, nail diameter, and nail location. If these design requirements are followed, the bending strength and stiffness of the nail-laminated post can be calculated using the equations in the EP. It is important to note that the intent of the EP559 design re-quirements is to maximize the bending strength of the splice region, while minimizing overall splice length. Overall splice length is defined as the distance between the two farthest removed end joints in a post that contains one end joint in each laminate. Reducing overall splice length generally reduces the amount of preservative treated lumber used in a post.

7.6.6 Laboratory Tests. Engineers must gen-erally rely on laboratory tests to determine design properties for spliced posts that do not meet the design requirements of ANSI/ASAE EP559. In recognition of this, a laboratory test procedure specifically for spliced mechanically laminated posts is outlined in ANSI/ASAE EP559.

7.6.7 Computer Modeling. Discontinuities at butt joints result in a post with a varying bending stiffness along its length. If the overall splice length is rather short (i.e., all joints are located within a distance equal to 1/4^th the post length), the post is generally sectioned into three ele-ments for computer frame analysis: a middle element that contains all the joints, and two

“joint-free” outer elements. The joint-free ele-ments are treated like unspliced mechanically-laminated posts with flexural rigidities calculated as described in Section 7.5.7. The element containing the joints is assigned an effective flexural rigidity that will cause it to deform like actual laboratory tested posts. A procedure for

“backing-out” an effective flexural rigidity from bending test data is given in ANSI/ASAE EP559.

The EP also contains an equation for calculating the flexural rigidity of the splice region of any nail-laminated post that meets the design re-quirements of the EP.

7.7 References

American Forest and Paper Association (AF&PA). 1997a. National Design Specifications for Wood Construction (NDS). American Forest and Paper Association, Washington, D.C.

American Forest and Paper Association (AF&PA). 1997b. NDS Supplement - Design values for wood construction. American Forest and Paper Association, Washington, D.C.

American Institute of Timber Construction (AITC). 1983. Structural glued laminated timber.

ANSI/AITC A190.1-1983. Englewood, CO.

American Institute of Timber Construction (AITC). 1985. Design standard specifications for structural glued laminated timber of softwood species. AITC 117.85. Englewood, CO.

American Institute of Timber Construction (AITC). 1988. Manufacturing standard specifica-tions for structural glued laminated timber of softwood species. AITC 117.88. Englewood, CO.

ASAE. 1999. ANSI/ASAE EP559: Design re-quirements and bending properties for mechani-cally laminated columns. ASAE Standards, 46^th edition. ASAE, St. Joseph, MI.

Bohnhoff, D.R. 1992. Modeling horizontally nail-laminated beams. ASCE Journal of Strucutral Engineering 118(5):1393-1406.