Allowable Stresses in old timbers

[VTPE]

I am adding a new load to an existing carriage house built with wood timbers. I would like to find a reference to help me define the allowable stresses in a timber when the age, grade, history is not known.

This is for residential construction. My boss wants to park his Porshe Boxster under the timber, so I would like some good info prior to proceeding.

Thank you in advance.

 

[dicksewerrat]

Do you know what kind of timber it is? Oak, fir, maple, box elder?

 

[mirekp]

It should be not easy to predict the allowable stress of old wood. Maybe the best way will be to test it. Simply try to load the timber with for example 2-3 times higher load which will be normally applied.

Mirekp
Mechanical and engineering calculations

http://www.mitcalc.com

 

[jheidt2543]

Many times old timbers in trusses and beams are still carring their intended load, so a load test is not the easiest thing to perform. I've taken small plug samples of old timbers and had them tested to verify the wood species. Also, checking the member in non-destructive ways to make sure there is no dryrot or other deterioration of the wood or connection hardware. Then, I try to find out the original allowable fiber stresses from old codes at the time of construction and compare those to present day allowable stresses. Lastly, I make an assesment of the current loading conditons based on the actual dead and live loads vs. the code required loads.

Most of the time there is no definitive answer, that is what engineering judgement is -- an educated, conservative guess.

 

[griffengm]

VTPE,
There are too many factors to take into account without someone knowlegable in wood technology taking a look at the beams. Older lumber may actually be stronger than recent cut due to area harvested and type of timber in the forest. The grade of the lumber also has a tremendous impact on the strength e.g. how many knots and their location, type of cut to make the board, etc..
There are some good texts available but prime on your list should be The Wood Handbook from Forest Products Labratory.
While wood has several advantages over metal, conservatism in strength calcs is the rule.

Griffy

 

[vmirat]

I would like to reiterate what griffengm stated, in that there are too many variables to account for in timber design of that age. In addition, the connections need to be considered. Since wood will shrink over time, the connections may have come loose or no longer effective to carry the critical load condition. I suspect the cost of adding structural members to the building is at least as much as the cost of a Boxer. I would forget using the existing structure. This way, you safely protect the Boxer and your job!

 

[3dboy]

By far your most challenging task will be check the connections of the wood members. estimating a conservative value for the timbers themselves should not be too difficult, but you should do (or have done) an inspection to determine the condition of the wood. I am in the middle of a job where a 100+ year barn has been dismantled and is going to be rebuilt as a community center and we have gone through all these questions (although granted, it is a commercial project).

Have fun and don't ruin the boxter