Load Testing Wood Structure

[McSEpllc]

I am working on a historic timber mill structure of questionable capacity. There has been loss in capacity because of termites and fungal decay, and increased loads due to a 4" non-structural topping slab.

Question: I have worked on load testing concrete structures in the past, but can not find any reference materials for load testing wood structures. Does anyone know of procedures or literature? Per NDS, live loads are assumed to act on a structure for 10 years, which is not feasible for load testing. I was wondering whether it is acceptable to increase the load by a factor of 1.6 for a ten minute loading (wind/seismic) and reduced the testing duration?

Appreciate it.

 

[SlideRuleEra]

McSEpllc - IMHO, the short answer to your question is "No"

The long answer: Wood structures are a lot different from concrete. Since the structure has "questionable capacity", how do you know what normal (10 year) load it can safely withstand? Testing at 160% of a load you have guessed is not a good idea. I suggest you bring in someone who has done this type work before.

With that said, here is one approach:

1. If you don't have plans for the building, make your own.
2. Decide what repairs are essential, then make it so.
3. Compute the existing dead loads.
4. Establish what live load and load combinations are needed for the "new" use of the building.
5. Considering the deterioration & that only KNOWN essential repairs have been made, start your load test a LOW value (maybe 20% of your "new" requirement - maybe less).
6. Increase the test load in increments, while monitoring member deflection, settlement, etc. Closely watch the joints - they are usually the weakest point - If there are problems with the structure - Stop and find out what is happening, then do something about it.
7. Don't get in a hurry, or take short cuts - you could collapse the building... or worse. That's why this type work is not cheap.

Here is a link to a basic article on "New Loads For Old Structures"

http://www.jrclancy.com/Downloads/LoadArticle2002.pdf

BTW - NDS does not make the assumption that live loads act for 10 years. The "Load Duration Factor" is just one of many factors that allows the Engineer to compute the affect of loads on wooden members. The Engineer uses this factor as a mathematical tool to size the members to carry the loads for the required time.

http://www.SlideRuleEra.net

 

[McSEpllc]

Thank you SlideRuleEra for your quick response.

The various load testing projects I have been involved been were done in 25% load increments up to the total required design load. The total required design load for the concrete structures have been always 1.7 times the required live load and for site retaining walls 1.0 times the design live load. After each 25% load increase the deflection was measured. The total test load was left in place for 24 hours and deflections again measured. The deflection of the structure was again measured after 24 hours had passed since the test load was removed. Failure was not possible as there was shoring 1 inch below the structure (we estimated the test deflection for this purpose). I was visualizing a similar attempt for testing the timber structure.

NDS 95, Section 2.3.2 says:..." Normal load duration represents a load that fully stresses a member to its allowable design value by the application of the full design load for a cummulative duration of approximately 10 years". Per NDS, table 2.3.2, occupancy live loads is listed specifically as a ten year load duration. This is based on test data showing the characteristics of wood structures to be able to support substantially higher loads for short term durations. The load testing needs to happen in a lot less than ten years, hence my reasoning to change the static long term load test to a short term duration test with higher magnitude loads relative to NDS's load duration factors. The load duration factors are not arbitray, but based on actual test data.