Hold-down and uplift anchorage

Where the tension comes from

From the shear wall tipping

The horizontal force in the plane of the wall wants to turn it over: a downward force arises at one end and uplift at the other. The narrower the wall segment, the larger that force. Designing a shear wall.

From wind suction

On the roof and at the edges of the façade the wind does not press but sucks. In a light structure that force can exceed the self-weight. Resisting uplift in light structures.

Why the bottom-track fixing is not enough

The usual track fixing is there to hold the panel in place and transfer the horizontal force. The concentrated tension, though, appears at the end of the wall, at a point — it is not spread along the track.

If only the usual fixing is present there:

• the track tears around the fixing,
• or the fixing pulls itself out of the foundation.

Either way the bracing cannot work.

The parts of the anchorage

1. The hold-down at the end of the wall, connected to the stud or the boxed jamb with the right number of screws.
2. The anchor in the foundation: a chemical or mechanical anchor, or a component cast in at the pour.
3. Verification on the concrete side: edge distance, concrete grade, pull-out and cone breakout resistance.

All three steps have to be carried through. The most common failing is to design the steel side and not the concrete side.

What happens over several storeys

In a multi-storey building the tension does not arise anew on each floor — it accumulates downwards. So the anchorage has to be continuous from storey to storey: the tension from the shear wall on the top floor has to reach the foundation too. Load path continuity.

On site

Anchorage is an element that:

• is decided before the pour, if cast-in anchors are used,
• becomes concealed once the wall is finished,
• cannot be retrofitted at any reasonable cost.

So its position belongs on the drawings, and it is checked during erection, with a record kept.

The point

Bracing is worth what its anchorage is worth. This is the point where a light gauge steel building differs most from heavy construction — there the weight held it down, here the connection does.