Truss and Rafter Uplift Connector Demand
How much uplift the hurricane tie at one end of a truss or rafter carries.
Example
You enter
- Truss or rafter spacing (in) 24
- Span, bearing to bearing (ft) 32
- Overhang beyond this bearing, this end only (ft) 0
- Net design uplift pressure, strength level (psf) 28.4
- Roof dead load (psf) 12
- Connector allowable uplift from its evaluation report (lb) 585
- Connectors per end (1 to 4) 1
You get
- Roof area the end carries 32.00 sq ft
- Net uplift (lb) 314.88
- Utilization 0.538256
- Connectors of this rating needed 1
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeHow much uplift the hurricane tie at one end of a truss or rafter carries, and whether the connector you picked holds it. The end carries the spacing times half the span plus the overhang, and a little more because the overhang levers on the bearing; the wind on it is the strength-level net uplift pressure times that area, the roof's dead load resists, and the ASCE 7 ASD combination 0.6D + 0.6W lets only 0.6 of each count, so a 12 psf roof resists as 7.2 psf. Trusses at 24 in on a 32 ft span at 28.4 psf with a 12 psf roof put 32 sq ft on each end, 908.8 lb of wind against 384 lb of roof, and 314.9 lb of net uplift on a 585 lb tie (54%). At 45 psf on 36 ft with a 10 psf roof it is 756 lb, so the same tie is overstressed and two are needed. The pressure is entered (the wind components and cladding calculator gives it) and the allowable is the connector's evaluation-report value. The rest of the load path below the tie is not checked; the ASCE 7 ASD load combinations calculator covers the other combinations.
Formula or tablearea carried by the near bearing = (spacing / 12) x (span + overhang)^2 / (2 x span) (the reaction of a uniform load on a simple span with the overhang on that end); W = pressure x area; D = dead load x area; net ASD uplift = max(0, 0.6 W - 0.6 D); per connector = net / n; utilization = per connector / allowable; connectors needed = ceil(net / allowable).
Source and editionASCE 7-22 Section 2.4.1, basic ASD load combination 0.6D + 0.6W, by name; the net design uplift pressure (ASCE 7 Chapter 30) and the connector allowable (manufacturer's ICC-ES evaluation report) are the reader's entries.
Free accessASCE 7 is sold, not free to read; connector evaluation reports are free at icc-es.org and from the manufacturers.
What governsEstimate. AHJ and licensed structural engineer govern.
API field namesspacing_in, span_ft, overhang_ft, uplift_psf, dead_psf, allowable_lb, connectors, tributary_sqft, net_uplift_lb, utilization, connectors_needed
- Simple span, overhang on the checked end length carried = (span + overhang)^2 / (2 x span)statics
- Strength-level pressure entered ASCE 7-10 or later net uplift, before the 0.6 factorASCE 7-22 Section 2.4.1
- Top-of-wall tie only plate to stud, stud to sill and sill to foundation need their own connectorscontinuous load path practice