Load path continuity

Why this is the most important check

A structure is not good because every member passes its check but because the members form a coherent system. The load path review checks that: whether every load can be traced from where it arises to the foundation.

The question matters especially in a thin-walled structure, because:

• it consists of many small members, so there are many connections,
• the members individually have small capacity, so there is no large reserve,
• the connections are often what governs.

The typical load paths

1 roof structure · 2 upper-floor load-bearing walls · 3 floor · 4 ground-floor walls · 5 foundation

Vertical load

Roof → roof structure → walls → floor → walls → foundation.

The critical point: walls stacked above one another. If the load-bearing wall of the upper storey does not sit over the one below, the load has to be transferred within the floor — possible, but it takes a purpose-made structure.

Horizontal load

Façade → floor or roof plane → shear walls → anchorage → foundation.

The critical points: the connection between the floor plane and the shear wall, and the anchorage.

Uplift

Roof → roof-to-wall connection → walls → anchorage → foundation.

Here every connection also has to work in tension — a different action from the one an ordinary bearing is made for.

Where it most often breaks

1. At the floor-to-wall connection, if there is no proper connection to the shear wall.
2. At the anchorage, if only the usual track fixing was installed.
3. At a change of plan between storeys, where the wall does not continue downwards.
4. Around an opening, if there is no proper support beneath the jamb.
5. At a later alteration, when someone removes a member.

How it is checked

It does not start with calculation but with tracing: pick a load and point along its path to the foundation. Wherever you cannot point to the next member, the load path breaks.

This is the check a design review performs first. The design checker.

The role of the contractor

The load path can be continuous on the drawings and still break in reality: an omitted screw, a missing hold-down, bracing left half done. These faults do not show in a finished wall. Inspection points.

The point

A structure is as strong as its weakest connection. Sizing the members is routine — carrying the load path through is the engineering.