Modeling Reduced Rafter

Tips & Tricks

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In a purlin roof structure, the rafters are generally placed on a continuous purlin. Due to the roof inclination, an angle is formed between the roof rafter and the purlins. Hence, in most cases, the rafter is triangularly notched. This notching is to be equaled with a reduced cross‑section and should be considered in the rafter design.

To consider this cross‑section reduction in RFEM and RSTAB, you must specify the location of the reduction. The easiest way to do this is during the modeling of the members. For example, a rafter was modeled with a continuously constant cross‑section with the according supports on the neutral axis and the mentioned “notch” is to be considered in the area of the supports. For this purpose, you can use the function “New Inserted Member”, allowing you to insert a new member with a different cross-section between two x‑locations.

The cross‑section of the new inserted member should then correspond to the remaining cross‑section after the notch. A proposal for the support of the member with the reduced cross‑section is shown in the picture. Here, the lengths “overlap below” and “overlap above” describe the corresponding member lengths. The reduced zone should then be aligned flush at the top edge of the rafter by means of a member eccentricity.


birdsmouth cut Notch Weakening Reduction Chevrons


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Structural engineering software for finite element analysis (FEA) of planar and spatial structural systems consisting of plates, walls, shells, members (beams), solids and contact elements

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