Load path of timber collared roof 1

[stevemorters]

High 4.3m, hipped roof to loft conversion. Collars at 3m to form ceiling. Always assumed load on/above collars is transmitted as axial load down rafters. B.I. insists all load is taken via purlin wall. Never been questioned on this before. Can anyone give me a difinitive answer or ref to a book/journal with answer.
Urgent, Thanks.

 

[FSS]

Collars/ceiling joists are used to resist outward thrust caused by geometry of rafters.

 

[bylar]

The ceiling joist will carrry the horizontal thrust but i assume from your post that the ceiing joist are non existing or removed. The collar ties are usaully in the upper third of the span of the rafters and do not carry the horizontal thrust. Ther are there to stop the opening of the rafters with uplift. If you think they carry the horizontal thrust analysis the rafters using the collars to restrict the movement and you normally will find a rafter that works based on the span only is over stressed when the coolars act as a horizontal restaint. The makimum bendin occurs at the collar ties.

 

[RARWOOD]

I am having a hard time visualizing your problem, however by relating it to basic rafter systems hopefully I can shed some light on your problem.

In the basic rafter systems you have rafters sitting on a stud wall sloping up to the ridge where the rafters oppose each other usually with a 1 x nailer in between. On the low end you have ceiling joists nailed to the rafter forming a simple truss. The rafters are often designed as bending members only based on the horizontal span. However under vertical loads the rafters are actually subjected to combined bending and axial load.

To understand the system you need to remove the ceiling joists creating a cathedral ceiling. You are then left with a three hinged arch. In this case the rafters may not be of adequate size to resist the applied loads.

The biggest problem often, however, is that the exterior walls can not resist the thrust developed by the rafters.One basic requirement of a three hinged arch is that it has pinned supports which does not occur if your exterior walls are moving outward. The pupose of the ceiling joist is to resist the thrust. You also can solve the problem by adding a ridge beam.

Now as you move the ceiling joist up the rafters towards the ridge you no longer have a simple truss. Instead you have a tied frame. As you move the ceiling joist up higher your horizontal thrust at the wall increases.

Ussually there are two main problems when you do this, the first is that the exterior walls can't resist the thrust. The second problem is developing a reasonable connection between the rafters and ceiling joist.

Expanding this though to a hip system, when you have four hips sloping up and meeting in the middle of a square with no support except at the low end of the hips; you have two three hinged arches perpendicular to each other.

This type of construction is very common in churches. The system then requires a tension ring at the hip low ends or some very large piers to resist the outward thrusts of the hips.

There are two refernces, which I am familair with, which treat the case of rafters with raised ceiling joists. The first is Simplified Design of Building Structures by James Ambrose,Copyright 1979. The second is the Timber Design & Construction Handbook by the Timber Engineering Company copywrite 1956.

Hope this helps you out.