Saturday, June 10, 2017

What Is A 'Continuous Load Path'?

Most people think of the structural failure of a house as collapse. But falling down is only one way a structure can fail, and gravity is not always the culprit. A building can also fail upwards, when a hurricane creates a pressure imbalance that literally sucks off the roof; and it can fail sideways, when high winds cause lateral pressure that makes a gable end wall hinge away from the rest of a wall assembly and fall over. A earthquake can also put severe lateral and uplift loads on a house.

Nails are the traditional connectors for the wood structural components of a house, and they are excellent at keeping wood framing snug and aligned so that the weight of the structure, its furnishings, and occupants can be reliably transferred down to the ground without anything shifting. But nails do not work as well for uplift and lateral loads because they are often in what engineers call “withdrawal,” which means that when these unusual loads are applied they are typically in the opposite direction of the way that the nail was hammered into the wood and the nails simply “pull out” with little resistance.

To avoid failure of a house structure from uplift and lateral loads each piece, from the roof down to the foundation, must be securely connected together like the links of a chain. This is called a continuous load path and, also like a chain, it is only as strong as its weakest link.

Both metal connector plates and structural wood panel sheathing are used in a wood stud-framed house to create a continuous load path. The illustration below shows examples of the typical metal connectors to create a secure load path from the base plate up to the roof trusses.

A concrete block house utilizes concrete columns and concrete-filled cells with steel reinforcement running from the foundation/floor slab to the tie beam, then metal connectors to the wood components above it, to tie everything together. Constructing a continuous load path is a building code requirement.

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