Designing built-up columns? Now there’s a way to mechanically laminate multiple 2x members to meet the specifications in the National Design Specifications for wood. Simpson Strong-Tie evaluated Strong-Drive® SDW Truss-Ply screws for attaching multiple laminations with easier installation methods. With these screws, there’s no longer a need to nail from both sides of the column, or to use not-so-common 30d nails as specified in the NDS, or to pre-drill for bolts. Instead, installers can now install all the screws from one side of the built-up column, which provides time and cost savings.

Introduction

Columns can be classified into solid columns, built-up columns and spaced columns. Solid columns are single members or individual members glued together to act as one solid member. Mechanically laminated built-up columns are formed by fastening two or more members with bolts, nails or screws. If built-up members have spacer blocks between the members, they create a spaced column. The design of built-up columns is different from the design of solid columns.

These three classes of columns have differing load capacities. The capacity of a built-up column can be expressed as a percentage of the strength of a solid column of the same dimensions and made with material of the same grade and species. The ratio of the built-up column compression capacity to that of a solid column is defined as efficiency (1). The efficiency of built-up columns is 1.0 in the strong axis and between 60 and 75 percent in the weak axis depending on the type of fastening. The loss in capacity in the weak axis compared to a solid column is due to the slip between the laminations.

Column failure is due to crushing, buckling, or a combination of both modes. Short columns more often experience crushing failure, and long columns tend to fail more often by buckling. According to the NDS, the efficiencies are generally higher in short and long columns than in intermediate columns. The NDS assigns nailed built-up intermediate columns a 60% efficiency and bolted built-up intermediate columns a 75% efficiency. Even though long and short columns would have higher efficiencies, all column lengths are assigned a single efficiency. Note that provisions in NDS 15.3.2.2 allow short columns to use full design values when designed as individual columns.

Whole-section engineered wood products are recommended for higher compression loads, although they can add cost.

Solid Columns

Design of solid columns is addressed in Section 3.7 of the NDS. The main difference between solid column and built-up column capacity is in the calculation of C_p, the column stability factor. The column stability factor adjusts for the buckling effect on the column capacity. If the column is completely braced in all directions, then C_p can be taken as 1. For all other conditions, C_p should to be evaluated for both strong-axis and weak-axis bracing conditions. In solid columns, the column stability factor is calculated as follows:

In this calculation, l_e/d represents the larger of the ratios l₁/d₁ and l₂/d₂ as shown in Figure 1. The slenderness ratio of solid columns, l_e/d, shall not exceed 50. Higher slenderness ratios have a lower C_p factor, which means that a slender column can buckle more easily and has lower compression capacity than a similar column with a lower slenderness ratio. The same holds true for built-up columns.

Built-up Columns

Built-up columns fastened with nails or bolts are addressed in Sections 15.3.3 and 15.3.4 of the NDS. However, fastening built-up column members with screws is not addressed in the NDS. For built-up columns, the only difference in design compared to solid columns is the addition of K_f, a column stability coefficient, in the calculation of C_p. See Figure 2 for built-up column notation. For built-up columns, C_p is calculated as follows:

The C_p value is calculated for slenderness ratios based on l₁/d₁ and l₂/d₂, and the smaller C_p is used to calculate the adjusted compression design value parallel to grain. In the strong axis, K_f = 1, and the design is similar to solid columns. However, in the weak axis buckling is affected by the slip and load transfer that occurs at fasteners between the laminations, and the K_f factor changes with the type of fastener.

NDS Section 15.3.1 provides the limitations for built-up columns based on these design attributes:

• Each lamination has a rectangular cross section and is at least 1-1/2” thick,
• All laminations are of same depth and faces of adjacent laminations are in contact,
• All laminations are full column length.

These limitations apply to laminations fastened with nails and bolts. In Simpson Strong-Tie design method they also apply to Strong-Drive SDW screws.

Nailing

The spacing and end distance requirements for nails are covered in Section 15.3.3.1 of the NDS. The nails need to be driven from opposite sides of the column and need to penetrate at least ¾ of the thickness of last lamination. If all of the requirements are met, K_f of 0.6 can be used in the calculation of C_p, when l₂/d₂ is the limiting ratio for calculation of F_cE. The NDS does not provide a table for built-up column capacities fastened with nails. The designer has to run through calculations and follow the provisions of NDS Section 15.3.3 to determine the capacity of a nailed built-up column.

Let’s figure out the nail length needed to connect 3 – 2xmembers. For a 3-ply member the nail length needs to be a minimum of 2 x 1.5inches +3/4 x 1.5 inches = 4.125 inches. Only 30d or higher nails are available in these lengths. Since these nails are not commonly used in the job site and do not fit the regular nail gun, installers may need to use a special nail gun. The NDS provides some typical nailing schedules that are shown here in Figure 3.

Bolting

NDS Section 15.3.4.1 has end, edge distance and spacing requirements for bolts. Also a metal plate or washer is required between the wood and the bolt head and between the wood and the nut. The nuts should be tightened to ensure that the faces of adjacent laminations are in contact. Figure 4 is a detail showing the typical bolting schedules. If the requirements of the NDS Section 15.3.4 are met, K_f of 0.75 can be used in the calculation of C_p, when l₂/d₂ is the limiting ratio for calculation of F_cE. The NDS does not provide built-up column capacities fastened with bolts. Again the designer has to determine these capacities by calculation.

New Option – Fastening with Simpson Strong-Tie® Strong-Drive® TRUSS –PLY SDW Screws

Simpson Strong-Tie® tested column assemblies as shown in the Figure 5 to determine Column Stability Coefficient, K_f, for SDW screws. The limitations of NDS Section 15.3.1 have been found to also apply to Strong-Drive Truss-Ply SDW screws. The spacing and end distance requirements for the screws are shown in Figure 5. One huge advantage of using SDW screws is the screws can be installed from one side of the column or from both sides of the column. Installation from one side or both sides affects the K_f factor used in the calculation of C_p. If the screws are installed from one side of the column, then K_f of 0.6 can be used in the calculation of C_p when l₂/d₂ is used to calculate F_cE. If the screws are installed from both sides of the column, then K_f of 0.7 can be used in the calculation of C_p when l₂/d₂ is the limiting ratio for calculation of F_cE.

Let’s work on a design example for built-up columns fastened with Strong-Drive® Truss-Ply® SDW screws:

Example: Calculate the capacity of a 3-2×6 built-up member attached with SDW screws with a) installation of screws from only one side b) Installation of screws from both sides of the column.

Column Type:                              Built-up                 

Column Length:                          10 ft.     

Bracing:                                      Completely unbraced in both directions                 

Size if Column:                            3 – 2 x 6

Wood Species:                           SPF         

Grade:                                       #2               

Load Duration Factor, C_D:         1              

Temperature Factor, C_t:            1              

Wet Service Factor, C_M:           1

 

Per Table 4.3.1 (table shown below) of NDS:

F_c^’ = F_c x C_D x C_M x C_t x C_F x C_p

Per Table 4A or 4B of NDS, Compression parallel to grain, F_c = 1150 psi

E_min =                                                              510000 psi

Size Factor C_F =                                             1.1

Fc^* = F_c x C_D x C_M x C_t x C_F                           1265      psi

Now let’s calculate C_p in both directions:

C_p in Strong-Axis –

Where:

F_c^* = reference compression design value parallel-to-grain multiplied by all applicable modification factors except C_p (see 2.3 of NDS)

F_cE = 0.822 E_min^‘/ (l₁/d₁)²

l₁ =           120 in

d₁ =          5.5 in

F_cE =         881 psi

K_f = 1.0 for solid columns and for built-up columns where l₁/d₁ is used to calculate F_cE and the built-up columns are either nailed or bolted

c = 0.8 for sawn lumber

Substituting the values above,                  C_p =          0.557

C_p in Weak-Axis –

Where:

F_c^* = reference compression design value parallel-to-grain multiplied by all applicable modification factors except C_p (see 2.3 of NDS)

F_cE = 0.822 E_min^‘/ (l₂/d₂)²

l₂ =           120 in

d₂ =          4.5 in

F_cE =         590 psi

K_f = 0.6 for built-up columns fastened with SDW screws from one side of column

K_f = 0.7 for built-up columns fastened with SDW screws from both sides of column

c = 0.8 for sawn lumber

Substituting the values above with K_f = 0.6,        C_p =          0.246     (Screws installed from one side)

Substituting the values above with K_f = 0.7,        C_p =          0.287     (Screws installed from both sides)

For screws installed from same side, minimum C_p = Minimum (0.557, 0.246) = 0.246

Column Capacity = F_c^* x C_p x d₁ x d₂= 7690 lbs.

For screws installed from both sides, minimum C_p = Minimum (0.557, 0.287) = 0.287

Column Capacity = F_c^* x C_p x d₁ x d₂= 8970 lbs.

To avoid these long calculation steps and to help the designer, Simpson Strong-Tie compiled a table with allowable compression capacities for built-up columns made with several typical combinations of No. 2 visually graded lumber and fastened with SDW screws. Now that you have a new and faster way of fastening multiple plies using SDW screws along with an easy to use design table, go ahead and design away!

If you have any questions or comments about fastening built-up columns with Simpson Strong-Tie fasteners, pass them along to us in the Engineering Department.

References:

1. Malhotra, S.K and A.P Sukumar, A Simplified Procedure for Built-up Wood Compression members, St. John’s, Newfoundland, Annual Conference, Canadian Society for Civil Engineering, June 1-18, 1989.
2. Malhotra, S.K and D.B. Van Dyer, Rational Approach to the Design of Built-Up Timber Columns, Madison, WI, Forest Products Research Society (Forest Products Society), Wood Science, Vol. 9, No. 4: 174-186, 1977.
3. National Design Specification for Wood Construction (NDS), ANSI/AWC NDS-2012. 2012. American Wood Council, Leesburg, VA. 282 pp