safety factors

[justbuildit]

how many safety factors does an engineered problem include? by the time the calculation is done what really is the safety factor. from the material properties that the manufacture uses to provide a piece of wood at some safety factor to the last 1.5 factor at he end.

 

[jike]

I am not sure what you are asking but in structural work, we have uncertainty of loading, the likeihood or probability of combining certain loads together, the uncertainity of material strengths and the inaccuracy of our analysis. We seem to focus alot on the first (load factors) and the second (load combinations and LL reductions) and some on the third (phi factors) but none on the fourth that I am aware of.

I guess we always assume that we accurately analyze it no matter whether we use an approximate or refined method. The engineer should be aware of this but I know of no code provision that gives any direction in this regard.

 

[UcfSE]

A better question would be "what is the probability of failure". Finding an overall safety factor implies that we have exact knowledge of materials and loads, which of course we don't.

 

[jdonville]

justbuildit,

Wood is a strange beast as it is not a manufactured material in the engineering sense like steel and concrete are. In this respect, it is similar to soils.

Wood is also generally "graded" by visual means. This leads to (I would suspect) greater inherent uncertainty in the strength and performance properties of each member.

There is a code for designing with wood - the AFPA/AWC NDS. You should refer to this code and commentary to answer specific questions regarding methods of calculation for structural resistance to loads for wood elements.

The various structural design codes seek to provide a standard way of evaluating the resistance of structural elements to specified loading conditions. Additionally, each code may permit the use of differing (although similar) approaches, such as Allowable Stress Design (ASD) or Load and Resistance Factor Design (LRFD). Additionally, they present formulae for the evaluation of member capacity that are tractable using hand calculation methods, rather than using overly complicated computer-only relations that yield an "exact" answer.

Each design approach views the interaction of loads, load combinations, material properties, inherent errors in the analysis and resistances slightly differently. I suspect that there are good introductions available on the internet that describe the subtleties of each design approach and compare them with each other. I suggest that you spend some time to review this literature and come back with questions.

Jeff

 

[BigH]

Also remember that we are assessing a "computed factor of safety" based on codes/accepted analyses/uncertainty and reliability - whether dealing with soils, or wood, or building frames or . . . This does not imply an equivalency with the absolute factor of safety which, of course, is unknown and will likely always be that way.

 

[COEngineeer]

well, simpson hangers for example. The number they give you is usually 1/4 of the result of their test.

Never, but never question engineer's judgement