Attic truss design

[Roscoff]

Hi all, was just looking at some attic trusses and now I'm confused (although in fairness it doesn't take much to confuse me!). The truss in question is 47 x 220 mm, approx 8m wide. the distance between the vertical ends on the bottom chord is 4.8m, how is this possible when according to the TRADA tables the maximum clear span for a 47 x 220 floor joist is 4.04m?

 

[^woody^]

TRADA tables are quite conservative, and if calculated (as the truss maker would do) spans would work out greater.

Also trusses act differently to cut roof timbers in terms of load sharing

 

[Roscoff]

Hi Woody, thanks for taking the time to reply. I appreciate that a true triangulated truss does work diifferently in terms of load sharing but the attic truss is more framing than true truss. I'm not an engineer (obviously) but I cant see what forces are acting on the centre section of that bottom chord that wouldn't be acting on a normal joist secured at both ends to a wall plate and rafter.

If it's simply that the TRADA tables allow a 20% margin of error then what's the point in i joists as it would seem that normal timber outperforms them?

 

[^woody^]

Joists act in a load sharing scenario, so if worked out with more detail then timbers can span longer than the tables allow

We've got an extra .5m or so for some joists in the past, but I don't know if there is a general 20% extra span for all the timbers in the tables

 

[mikric]

Bear in mind , trusses are made using TR26 quality timber, span tables do not cover this grade of timber.