Showing 0 products
Frequently Asked Questions
What forces do these ties resist?
Uplift and lateral movement - directions a gravity-designed structure resists poorly.
Uplift is wind trying to lift the roof off the walls.
Lateral movement is ground shaking driving the structure sideways.
Gravity alone holds a roof down in ordinary conditions.
In a storm the uplift can exceed the roof's own weight.
At that point only the connections keep the roof in place.
How does seismic loading differ from wind?
It reverses direction repeatedly, within seconds.
Wind loading is largely sustained in one direction.
Ground shaking loads a connection one way and then the other.
That cyclic reversal is what distinguishes seismic design.
Seismic connectors are frequently heavier for the same size.
They also take more fasteners than a wind-rated equivalent.
What is a continuous load path?
A chain of connections from roof to foundation with no missing link.
A rafter tie achieves nothing if the wall is not tied to its base.
The base must in turn be tied to the foundation.
Force simply finds the weakest link in the chain.
A single missing connection makes the rest ornamental.
Ties are therefore specified as a system rather than as individual items.
How is the required capacity determined?
From the wind speed or seismic category for the specific site.
Those figures differ enormously between locations.
Coastal and cyclone-prone regions have far higher requirements.
High-seismic regions have requirements unrecognisable elsewhere.
The local code sets them and no general figure substitutes.
On engineered buildings the engineer specifies the connector and its capacity directly.
Do seismic ratings use different holes?
Frequently yes - specific holes are designated for the seismic value.
Filling only the standard holes gives the standard rating.
The seismic figure requires the additional designated holes filled.
Manufacturers identify these clearly in their literature.
It is easy to miss on site if only the part number is checked.
The schedule for the rating being claimed is what has to be followed.
Can one tie do both jobs?
Many are rated for both, at different values for each.
A connector commonly publishes uplift and lateral capacities separately.
The governing case depends on the site and the structure.
Some regions require both to be checked independently.
A tie adequate for wind may be inadequate for seismic.
The published table gives each figure rather than one combined value.
Where do these ties go?
At every junction in the load path, not only at the roof.
Rafter or truss to wall plate is the most visible one.
Wall plate to studs and studs to sole plate follow below.
Sole plate to foundation completes the chain.
Spacing is specified rather than chosen by eye.
Gable ends and overhangs frequently need additional attention because uplift concentrates there.