Example Timber Design Compression Member by dRBI

Summary

This video provides a comprehensive tutorial on designing timber compression members. It covers the steps for determining the maximum axial load a timber column can support and checking its adequacy for applied loads. The script walks through the calculations using a timber column of strength class SG5, with a focus on geometrical properties, slenderness ratio, and modification factors. The process includes determining the permissible stresses, evaluating axial load capacity, and assessing the interaction between compression and bending stresses. Ultimately, it concludes that the column cannot adequately resist the applied loads, highlighting the importance of these calculations in timber design.

Takeaways

• 😀 The timber column in question is 4 meters tall with a rectangular cross-section of 97 mm by 145 mm, and is of strength class SG5.
• 😀 The first objective is to determine the maximum axial long-term load that the timber column can support.
• 😀 The second objective is to check if the column can withstand an applied long-term axial load of 12 kN and a bending moment of 1.0 kN·m about the X-axis.
• 😀 The effective length of the column is 4 meters, as it is restrained at both ends in position but not in direction.
• 😀 Geometrical properties like the radius of gyration and section modulus are crucial to determining the column's load-bearing capacity.
• 😀 The slenderness ratio of the column should not exceed 180, which is confirmed to be satisfactory in this case.
• 😀 Timber grade stresses for SG5 are given, with permissible values for bending and compression parallel to the grain (12.1 N/mm² and 10.8 N/mm², respectively).
• 😀 Three modification factors (K1, K2, and K8) are applied to adjust the grade stresses based on load duration, load sharing, and member behavior.
• 😀 The permissible compressive stress is calculated to be 1.73 N/mm², leading to a maximum axial long-term load of 24.33 kN for the column.
• 😀 The column is unable to resist the applied load and bending moment, as the calculated interaction quantity exceeds the acceptable limit of 1.0.

Q & A

What is the strength class of the timber column in the example?

The timber column in the example is of strength class SG5.

What is the cross-sectional size of the timber column?

The timber column has a rectangular cross-section of 97 mm by 145 mm.

How is the column restrained at both ends?

The column is restrained at both ends in position but not in direction.

What is the effective length factor for the column, and how is it calculated?

The effective length factor is 1.0, and it is calculated as the length multiplied by 1.0, resulting in an effective length of 4 meters.

What is the slenderness ratio, and what is the limit for compression members?

The slenderness ratio is calculated by dividing the effective length by the radius of gyration. For compression members carrying dead and imposed loads, the slenderness ratio should not exceed 180.

What is the permissible compressive stress for the column, and how is it calculated?

The permissible compressive stress is 1.73 N/mm², calculated by multiplying the grade stress by all relevant modification factors (k1, k2, k8).

What is the maximum axial long-term load that the column can support?

The maximum axial long-term load the column can support is 24.33 kN.

What is the modification factor k8, and how is it determined?

The modification factor k8 is used to adjust the permissible stress for compression members. It is determined by looking at the intersection of the slenderness ratio and modulus divided by grade stress in table 6, or by using an equation in Annex C. For this case, k8 equals 0.16.

How do you check if the column is adequate to resist the applied loads?

To check the adequacy, the interaction quantity formula is used. If the sum of the ratio of applied compressive stress to permissible compressive stress and the ratio of applied bending stress to permissible bending stress is less than or equal to 1.0, the column is adequate. In this case, the interaction quantity is 2.12, which exceeds 1.0, meaning the column is inadequate.

What are the applied loads and bending moment in the second part of the question?

The applied loads are a long-term axial load of 12 kN and a bending moment of 1.0 kN·m about the x-axis.