Unspliced Mechanically- Laminated Posts

7.5.1 General. The majority of posts used in post-frame construction with an overall length less than 18 feet are unspliced, mechanically-laminated posts. An unspliced post is any lami-nated post that does not contain end joints. This means that each layer is comprised of a single uncut piece of dimension lumber.

7.5.2 Fasteners. As previously noted, a me-chanically laminated post is a laminated post in

which nails, screws, bolts, and/or shear transfer plates (STPs) have been used to join individual laminates. Nails are the most commonly used mechanical fastener and posts that only feature nails are often referred to as nail-laminated posts. STPs are medium-gage metal plates that are stamped such that teeth protrude from both surfaces.

Mechanical fasteners that connect preservative treated lumber should be AISI type 304 or 316 stainless steel, silicon bronze, copper, hot-dipped galvanized (zinc-coated) steel nails or hot-tumbled galvanized nails.

7.5.3 Advantages. Unspliced mechanically-laminated posts generally cost less than solid-sawn posts, and they are stronger than similarly sized solid-sawn posts when bent around axis V-V (figure 7.1a). As previously noted, this is due to the fact that strength reducing defects are spread out in laminated assemblies. Also, pressure preservative treatment retention is more uniform in the narrower laminates of a mechanically-laminated post than it is in wide solid-sawn posts.

7.5.4 Disadvantages. When mechanically-lami-nated posts are bent around axis H-H (figure 7.1b), there can be considerable slip between laminates. For this reason, the bending strength and stiffness of mechanically-laminated assem-blies bent about axis H-H is relatively low. To compensate for this weakness, mechanically-laminated posts are generally only used where:

(1) there is adequate weak axis support (i.e., the posts are part of a sheathed wall), (2) cover plates can be added to increase bending strength and stiffness about axis H-H (figure 7.2), or (3) the bending moment about axis H-H is relatively low or non-existent.

Figure 7.2. Cover plates used to increase the bending capacity of a mechanically laminated post about axis H-H.

7.5.5 Bending About Axis V-V. Allowable design stresses for bending of unspliced me-chanically-laminated posts about axis V-V are calculated in accordance with ANSI/ASAE EP559 Design Requirements and Bending Properties for Mechanically Laminated Columns (ASAE, 1999). The procedure outlined in ANSI/ASAE EP559 is identical to procedures outlined in the NDS (AF&PA, 1997a) with the exception of two adjustment factors: the repeti-tive member factor, Cr, and the beam stability factor, CL.

7.5.5.1 Repetitive Member Factor. ANSI/

ASAE EP559 allows the use of the repetitive member factors in Table 7.3 when: (1) each lamination is between 1.5 and 2.0 inches, (2) all laminations have the same depth (face width), (3) faces of adjacent lamina-tions are in contact, (4) the centroid of each lamination is located on the centroidal axis of the post (axis V-V in figure 7.1a), that is, no laminations are offset, (5) all laminations are the same grade and species of lumber, (6) concentrated loads are distributed to the individual laminations by a load distributing element, and (7) the mechanical fasteners joining the individual layers meet the criteria in table 7.4. Note that if one or more of these criteria are not met, the NDS repetitive member factor of 1.15 should be used if it applies.

7.5.5.2 Beam Stability Factor. The beam stability factor, CL, is a function of the slen-derness ratio, RB, which in turn, is a function of: beam thickness, b; depth, d; and effec-tive span length, Le. ANSI/ASAE EP559 states that for mechanically-laminated posts being bent about axis V-V, thickness, b, shall be equated to 60% of the actual post thickness, and depth, d, to the actual face width of a lamination. The effective span length, Le, is a function of the unsupported length, Lu. The unsupported length shall be set equal to the on-center spacing of bracing that keeps the post from buckling laterally.

7.5.5.3 Design Values. Tables 7.5a and 7.5b contain design values for assemblies fabricated from visually graded and machine stress rated dimension lumber, respectively.

The design bending stresses have been ad-justed for repetitive member use. They must be further adjusted to account for stability,

wet use, load duration, temperature, and in certain cases, special preservative and fire treatments.

Table 7.3. Repetitive Member Factors*

Number of laminations 3 4 Visually graded 1.35 1.40

Mechanically graded 1.25 1.30

* For mechanically-laminated dimension lumber assemblies with minimum interlayer shear capacities as specified in Table 7.4. From ANSI/ASAE EP559 (ASAE, 1999).

Table 7.4. Minimum Required Interlayer Shear Capacities*

per interface per unit length of post, lb/in.

6 12

8 15

10 19

12 24

* For unspliced mechanically-laminated posts.

From ANSI/ASAE EP559 (ASAE, 1999).

7.5.6 Bending About Axis H-H. When all laminates are the same size, species and grade of lumber, the allowable design bending strength about axis H-H is conservatively taken as the sum of the bending strengths of the individual layers. The bending strength of an individual layer is equated to the product of the “flatwise”

section modulus of an individual laminate and the NDS adjusted design bending stress. For flatwise bending, the NDS adjusted design bending stress, Fb’, is equal to tabulated design bending stress, Fb, multiplied by the appropriate flat use factor, a repetitive member factor of 1.15, and all other applicable factors. Note that the beam stability factor is equal to 1.0 for flatwise bending.

Table 7.5a Design Values for Unspliced Mechanically-Laminated Posts in Bending About Axis V-V.

Extreme Fiber Bending Stress*, psi Nominal Width of Individual Layers, inches

6 8 10 12

Number of laminations Grade

3. 4. 3. 4. 3. 4. 3. 4.

Modulus of Elasticity,

x 10⁶ psi

Douglas Fir-Larch

Sel Str 2540 2640 2350 2440 2150 2230 1960 2030 1.9 No. 1 & Better 2020 2090 1860 1930 1710 1770 1550 1610 1.8

No. 1 1760 1820 1620 1680 1490 1540 1350 1400 1.7

No. 2 1540 1590 1420 1470 1300 1350 1180 1230 1.6

Hem Fir

Sel Str 2460 2550 2270 2350 2080 2160 1890 1960 1.6 No. 1 & Better 1840 1910 1700 1760 1560 1620 1420 1470 1.5

No. 1 1670 1730 1540 1600 1410 1460 1280 1330 1.5

No. 2 1490 1550 1380 1430 1260 1310 1150 1190 1.3

Southern Pine

Dense Sel Str 3650 3780 3310 3430 2900 3010 2770 2870 1.9 Sel Str 3440 3570 3110 3220 2770 2870 2570 2660 1.8

Non-Dense SS 3170 3290 2840 2940 2500 2590 2360 2450 1.7

Dense No. 1 2360 2450 2230 2310 1960 2030 1820 1890 1.8

No. 1 2230 2310 2030 2100 1760 1820 1690 1750 1.7

Non-Den. No. 1 2030 2100 1820 1890 1620 1680 1550 1610 1.6 Dense No. 2 1960 2030 1790 1960 1620 1680 1550 1610 1.7

No. 2 1690 1750 1620 1690 1420 1470 1320 1370 1.6

Non-Den. No.2 1550 1610 1490 1540 1280 1330 1220 1260 1.4

* For dry posts under normal load duration. Size and repetitive member factors applied. For other appli-cable modification factors, see NDS (AF&PA, 1997a).

Table 7.5b Design Values for Unspliced Mechanically-Laminated Posts in Bending About Axis V-V.

Extreme Fiber Bending Stress*, psi Extreme Fiber Bending Stress*, psi Grade

3 Laminates 4 Laminates Grade

3 Laminates 4 Laminates

900f-1.0E 1130 1170 1950f-1.5E 2440 2540

900f-1.2E 1130 1170 1950f-1.7E 2440 2540

1200f-1.2E 1500 1560 2100f-1.8E 2630 2730

1200f-1.5E 1500 1560 2250f-1.6E 2810 2930

1350f-1.3E 1690 1760 2250f-1.9E 2810 2930

1350f-1.8E 1690 1760 2400f-1.7E 3000 3120

1450f-1.3E 1810 1890 2400f-2.0E 3000 3120

1500f-1.3E 1880 1950 2550f-2.1E 3190 3320

1500f-1.4E 1880 1950 2700f-2.2E 3380 3510

1500f-1.8E 1880 1950 2850f-2.3E 3560 3710

1650f-1.4E 2060 2150 3000f-2.4E 3750 3900

1650f-1.5E 2060 2150 3150f-2.5E 3940 4100

1800f-1.6E 2250 2340 3300f-2.6E 4130 4290

1800f-2.1E 2250 2340

* For dry posts under normal load duration. Repetitive member factors applied. For other applicable modification factors, see NDS (AF&PA, 1997a).

7.5.7 Flexural Rigidity. To calculate deflections due to bending requires that the flexural rigidity of the member be known. The flexural rigidity of a solid-sawn member is equal to its modulus of elasticity times its moment of inertia about the axis it is being bent. The flexural rigidity of an unspliced laminated post when bent around axis V-V is simply equal to the sum of the flexural rigidities of the individual laminates about axis V-V. In other words, the flexural rigidity about axis V-V is not dependent on the properties of the mechanical fasteners. This is not the case with respect to bending about axis H-H. The bending stiffness about axis H-H axis is highly dependent on the shear stiffness of the mechanical connec-tions between the individual laminates. A high bound for flexural rigidity about axis H-H is obtained by assuming complete composite action between layers (no interlayer slip). A lower bound is obtained by assuming no com-posite action (no interlayer connections). In the latter case, the total flexural rigidity is equal to the sum of the flexural rigidities of the individual laminates. Special analysis procedures, such as that developed by Bohnhoff (1992) are available for more accurate estimates of deformation due to bending about axis H-H. Use of these pro-grams requires knowledge of the shear stiffness properties of the mechanical connections.

7.5.8 Compressive Properties. The allowable compressive load for an unspliced mechanically laminated post is typically calculated by treating the individual laminates as discrete columns.

This method conservatively assumes no com-posite action between laminates. An allowable compressive stress is first calculated for each laminate for buckling about axis V-V. This allow-able stress is then multiplied by the cross-sectional area of the laminate to obtain an allowable load for buckling about axis V-V. This calculation is repeated for each layer, and the resulting individual laminate loads are summed to obtain a total allowable column load for buck-ling about axis V-V. The entire process is re-peated to obtain a total allowable load for buck-ling about axis H-H.

The NDS (AF&PA, 1997a) presents methods for calculating a compressive load capacity that accounts for some composite action; however, connectors used in fastening the laminations must meet criteria outlined in the NDS.

7.5.9 Field Fabrication. A distinct advantage of mechanically-laminated posts is that fabrication can be performed using tools and equipment readily available on the job site. With unspliced