On the 70 ft.
Roof truss deflection limits.
Some truss design software permits the truss design technician to limit the truss deflection to a specified maximum amount.
Generally for roof trusses the deflection in inches due to live load cannot exceed the span in inches divided by 240 l 240 and due to total load l 180.
Mitek roof and floor truss manual for architects and engineers mitek industries inc.
A 48 foot span roof truss could have total load and live load deflections of almost 3 1 4 inches and 2 1 2 inches respectively and meet minimum code deflection criteria of l 180 and l 240.
Per ansi tpi 1 a scissor truss can have up to 1 25 of total horizontal deflection in the absence of stricter limits from the building designer.
See the table below.
For the pratt truss and any of the types of truss mentioned below it is possible to provide either a single or a double slope to the upper chord of a roof supporting truss.
In calculating load the weight of the stone and setting bed must be considered.
Truss that would allow 2 5 16 total load deflection and 1 live load deflection.
Codes provide the maximum allowable deflection limits for floor and roof trusses which is based solely on the truss span.
If that same joist had gypsum ceiling l 240 the allowable deflection is 0 6.
Applied must be designed to have a deflection not exceeding l 720 of the span.
3 8 3 1 strongbacks cross bridging or other reinforcement shall be used to limit differential deflection between adjacent framing members.
For example the allowable deflection of a 12ft span floor joist with plaster l 360 is 0 4 12ft divided by 360.
Allowable spans for 2x4 top chord trusses using sheathing other than plywood e g.
Note it gives the allowable deflection based on a fractional span quantity so a larger denominator will yield less deflection.
Spacing a live load deflection limited to l 240 maximum and use lumber properties as follows.
For a 35 ft.
2x4f 2000psif 1100psie 1 8x10 2x6f 1750 psi f 950 psi f 1900 psi e 1 8x10.
Spaced sheathing or 1x boards may be reduced slightly.
On a roof with a slope greater than 4 to 12 the live load limit is typically adjusted downward from 20 psf to 15 psf to allow for the relatively greater dead load on the steeper roof.
Truss the limit is 4 5 8 total load deflection and 3 1 2 live load deflection unless the building designer specifies more restrictive criteria.
