Hi Timber Wrestler & Derek,
I agree Derek's comments and now find myself trying to answer two questions inside two days.
One of the reasons that I got into 2D and 3D timber frame CAD, plane frame (2D) and space frame (3D) structural modelling was to be able to provide quantified answers to the very question raised by TW.
Observations made on old buildings that have failed in a particular way led me to survey and record old buildings and then to try and better understand the processes at work. As mentioned elsewhere on this forum the timber frame frame is the primary structure and usually its structural performance is suplemented by other building materials and in some cases even by adjacent buildings so it can be difficult to prove positive the actual loads and moments being experienced on any given component. That said, the plane frame (2D) structural model can and does provide a quick and dirty indication on how any particular frame is likely to behave. By measuring actual historic timber dimensions from a building and then entering these into the model and then applying loads as per today's standard building design codes it is then possible to establish a set of outcomes for each component that provide a historic and empirical benchmark set of acceptable (or not) design load and stress levels that might help a designer to be more confident with his (or her) predictions for new buildings. That's what I do !
Most building failures and especially those related to the timber frame elements rarely lead to the immediate collapse of a building. This is more likely to be a long drawn out affair progressing until the point of no return is reached with a partial or full collapse then happening. This type of extreme event would most likely be be occassioned by a very heavy snow storm or hurricane wind.
Having said all that the the problem still remains for the designer / engineer to demonstrate to all concerned that a building is likely to be able to safely perform its intended function and hence the plane and space frame structural programmes go a long way toward convincing others of the appropriateness or otherwise of design configurations and timber sizing.
This week I have been working inside the scissor braced roof of the Bishop's Camera in Farnham, Surrey. This roof is about 600 - 700 years old and comes with more than its fair share of problems. We took the unusual step of performing dendro testing on the roof timbers and found that the wall plates and primary tie beams were likely to date to 1306 (A. Moir, 2007) but rather surprisingly we then discovered that the rafters and scissor braces above dated to 1381 (A, Moir, 2007), so it would appear for whatever reason that this roof has been subject to a major rebuild from wall plate up. Why ? Did the first roof not perform adequately ? Did the black death that hit Farnham in 1349 resulting in the death of between one third and one half of the population have some kind of impact on the roof ? Did the Bishop just decide to remodel his roof ? Though it is important to try and understand the events that have ocurred over the life of a timber frame structure it may prove almost impossible to establish proof positive as to why exactly why some historic buildings fail or are replaced.
This topic would certainly provide a fantastic scope of work for a Masters dissertation or Doctoral thesis.
Sounds like you are up for it TW.
Regards
Ken Hume P.Eng.
http://www.kfhume.freeserve.co.uk
