Drag Strut Forces

Calculation of Drag Strut Forces

Starting at one end of the shear line, the unit shear in the diaphragm at the top of the wall is accumulated. So is the design shear in the sheathing segments. The absolute value of the difference in the accumulated values at the drag strut locations is the axial force in the drag strut force at that location.

The drag strut force integration for segmented walls is therefore given by

F(x) = ∫ ( v(x) – V/L ) dx ,

where

• v(x) = V_i / L_i within full height segments.
• v(x) = 0 over openings
• V_i = force on segment
• L_i = length of segment
• V is the force on the entire shear line = SV_i
• L is the length of the line, including openings and gaps between walls

If the accumulated diaphragm shear force is greater than the accumulated design shear, a tension force develops; if the design shear is greater than the diaphragm force, there is compression. On a wall with one opening and shear line force from left to right, there is tension to the left of the opening as the strut pulls force into the sheathing segment to the left, and compression at the right of the opening as it pushes force into the segment at the right.

The calculation of drag strut forces is predicated on the method in Shearwalls of determining of shear line extents, and depends on the assumption of diaphragms covering the entire plan area of a level.

For seismic design, the force F is amplified as described in Seismic Drag Strut Force Amplification.

Applied Shear vs Shear Capacity

You can choose whether to use the applied shear load or the shear capacity in calculating drag strut forces. The default value is applied shear, which is most commonly used. You should change this when designing connections if your local design code specifies shear strength must be used.