Hi Michael,
This is a good thought provoking question.
On your recent trip to England you will have noted that most domestic buildings are about 16 feet 6 inches wide and have their cross frames (bents) set at between 8 to 12 feet apart. Very few are in the region of 16 feet except double bay open halls which would more likely be 2 x 8 foot wide open bays.
Very large structures are different. For example the roof on Westminster Hall has a 69 foot span with 18 foot spacing between the main trusses. If this same cross frame spacing to span ratio were employed on a 16 foot span building then cross frame spacing should be set on 4 foot bay centres but from our observation this is clearly not the case in England but this level of tying can be seen on Dutch frames.
Cross frame spacing is probably driven more by the common availability of timber in fixed lengths and the efficient utilisation of same.
The propensity of the building to spread must also be the other factor taken into consideration and on wide span roofs with the huge increase in roof loads experienced then obviously a more conservative spacing approach can be seen to have been adopted.
Practicality also plays a part and since continuous wall plates are not generally available then scarf joint will usually be set fairly close to a cross frame where they can be clamped in place and outward bending due to rafter thrust on the plate can be kept to a minimum.
The North American bent system which employs interupted wall plates effectively makes a joint at every cross frame and hence achieving 16 foot spacings between cross frames should be possible however the sag developed on a common purlin roof might give rise to problems in fitting and securing SIP's.
A good way to avoid truss spacing problems in a roof might be to use reverse assembly where the wall plate sits on top of the tie beams and then adopt a common rafter roof system where rafter spacing is no longer tied to cross frame spacing.
Regards
Ken Hume
