Timber trusses do not have a 'neutral-axis', as such, in the chords, ties or struts, the principal stresses are in tension or compression tied and triangulated into to the joint plates - [ which is why it's called a truss! ]. There are additional bending loads across most of the chords, due to the weight of roofing, sheet-rock or man-loads at access, of course. There are therefore NO safe zones to drill holes for bolts or studs in a truss's members. The allowances for material defects, multiple-trusses and all other loads are already used up at the design stage - so there is little additional redundant margin of strength available for modifications. Any hole or notch not only reduces the section area, it introduces a stress raiser [ = bad]. The reason nails or small calibre screws don't ordinately affect the strength of the timber members is that they tend to push the wood-fibres aside rather than shearing them off, thus the continuity of strength of the timber member is retained, [ imagine pushing a knitting-needle through a rope's strands ].
Within certain weight limits then, nailing a timber rail on the upper edges of a set of bottom chords is an acceptable way to provide an anchor for studs to support light loads such as ducting or wiring trays. The chord supports the load, so the nails are just there as locations. A horizontal rail can be drilled for studs/penny washers and nuts, as it's not a critical truss member. In most roofs there will be just such rails, [ longerons ] fitted in order to give the roof lateral stability. 1" x 4" is the usual sort of calibre. I'd consult the/a truss maker anyway to cover my butt; no big deal, the computer programs used to design trusses can rapidly give a yeh or nay to any such proposed changes in minutes.
Alan
[This message has been edited by Alan Belson (edited 01-05-2007).]
