When a member fails the design checks, the easiest solution is to select and assign a heavier section to that member. This is true in most cases.
However, there are some peculiar cases where this is not true. The best way to illustrate this phenomenon is by using an example.
The members considered in the example are diagonal members in a truss which are all subjected to tensile forces.
Note that the 50 x 50 x 4 L does not meet the L/r requirements:
The 50 x 50 x 3 L does meet the requirements:
The length of the members remain the same, while the radius of gyration of the heavier section is smaller than that of the lighter section. In other words, the heavier section is more slender. This can be seen in the extract from the Prokon Section Database.
The slenderness ratio of a member is the ratio of the effective length, to the corresponding radius of gyration.
The maximum L/r ratios can be set in the Input tab, as shown below.
Guidelines from SANS 10162-1: 2011
Section 10.4 of the code states the following:
- The maximum slenderness ratio of a member in compression shall not exceed 200.
- The maximum slenderness ratio of a member in tension shall not exceed 300. This limit may be waived if other means are provided to control flexibility, sag, vibration and slack in a manner commensurate with the service conditions of the structure, or if it can be shown that such factors are not detrimental to the performance of the structure or of the assembly of which the member is a part.
The user can choose to waive the limiting tension slenderness ratio or choose to use the lighter section.