Frame Depth and Why a Leaf Sags

Sag is geometry rather than a fault that develops, and a cantilever leaf drooping at the nose was built too shallow for its span. The frame calculations covered by Pearland Sliding Gates Freesizer begin with depth rather than wall thickness, since on wide sliding gates the depth of the truss does far more work than the gauge of the steel in it, which is the shape most long openings in Pearland end up needing.

Depth Beats Thickness

Stiffness rises with the cube of the depth of a beam and only linearly with the amount of material. Doubling the height of the frame is worth vastly more than doubling the wall of the tubing. A gate that flexes is nearly always a shallow frame in heavy section, which is the expensive way to build a bendy gate.

The Truss

Top rail, bottom rail and a diagonal web between them makes a truss, and the diagonal is what stops the rectangle folding into a parallelogram. Vertical members alone do not do this. On a long leaf the diagonals are arranged to work in tension toward the supported end, which is where the load wants to go.

Welding and Distortion

A frame welded flat on a bench and then lifted is a different shape from one welded in the air. Heat pulls metal toward the last weld laid, so the sequence is planned and the joints are staggered from side to side. A leaf that arrived straight and sits crooked on its rollers was distorted at the bench, not damaged in transit.

Measuring an Existing Sag

Close the gate, run a string line from the top corner at the supported end to the top corner at the nose, and measure the gap at the middle. Do it again fully open. A leaf sagging only when open has a stiffness problem; one that sags in both positions has usually lost its alignment at the roller mounts instead.

What Can Be Fixed

A shallow frame cannot be made deep, but a diagonal can be added, a bottom rail can be replaced with a deeper section, and a nose can be given a light truss of its own. All of that is welding on a hung gate, which is awkward work. Getting the depth right at drawing stage costs nothing.