In this post, I will discuss the second example in our timber / wood design course covering the analysis and design of tension members. The goal of this example is to analyze a timber/wood glulam member subject to tension loading. We are using the National Design Specification (NDS) 2012 Code, and we will focus on the ASD approach.

The problem statement states,

Calculate the actual and allowable tensile stress parallel to the grain of the tension member given the following parameters:

Wood Type: Glulam Axial Combination 5DF

Wood Size: 2 1/2 x 6 (net finished dimensions)

Tensile Force : Dead + Snow Load, 10000 lbs

Moisture Content: >16%

Temperature: Normal

The tension member has a row of 5/8″ diameter bolts with 1/8″ oversized holes

The first step is to calculate the net cross-sectional area, A_{n}. It is equal to the width of the timber member minus the bolt hole diameter all multiplied by the thickness of the member.

The second step is to calculate the actual tensile stress, f_{t}, acting on the member. It is equal to the tensile load divided by the net cross-sectional area.

The third step is to identify all the adjustment factors that will be needed to calculate the allowable tensile stress per NDS 2012. The adjustment factors needed to analyze a timber tensile member include the wet service factor, load duration factor, and temperature factor. The wet service factor can be found in Table 5B of the Supplement National Design Specification 2012. The load duration factor and temperature factor can be found in NDS 2012 Tables 2.3.2 and 2.3.3 respectively.

The fourth step is to compute the allowable tensile stress, F’_{t}, per Table 5.3.1 in NDS 2012. It is equal to the reference tension design value parallel to the grain times the wet service factor times the temperature factor times the load duration factor for snow load. The reference tension value parallel to the grain can be found in Table 5B of the Supplement NDS 2012.

The last step is to compare the allowable tensile stress to the actual tensile stress. If the actual tensile stress is less than or equal to the allowable tensile stress, then the sawn lumber member is adequate. If the actual tensile stress is greater than the allowable tensile stress, then the sawn lumber member is adequate.

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