Capacity of existing wood joist

[bookowski]

I run into this problem fairly often and I'm wondering how others deal with it. I am evaluating existing timber floor joists. The joists are typically from the late 1800's to early 1900's and in the range of 2.75" x 11.75" true dimensions. I have had several people tell me that these are typically old growth wood and that design values far exceed typical modern examples but I have not found a reference to justify this. In some cases in the past we have had the timber tested/examined but this is typically not realistic for small projects. Does anyone have any reference or advice? Thanks.

 

[JAE]

Could you do a load test?

 

[flash3780]

Wouldn't the prudent approach be to use design strength of present-day lumber? It may be stronger, but that's not a bad situation to be in. Overestimating the strength is, however, a bad situation to be in.

 

[JAE]

I wouldn't rely on present day lumber unless I had some other evidence to support that assumption OR I was being very very conservative by using Hem Fir No. 6 or something (joke).

You can possibly set up and perform some simple load tests on a scattering of joists to see what the load-deflection relationship is (verifying E) and also to see what sort of maximum capacity a single joist can take - perhaps setting up what you NEED in terms of loading - follow IBC Chapter 17.

If you just assume a wood property for old lumber (old growth but possibly brittle) then you are simply taking all the risk and not basing your design on any standard of care.

 

[nhstruct]

The way I have always done it us to use the present value for the wood, typically the wood wood around here is either hem-fir or doug-fir.

 

[JAE]

But I think older wood can become brittle with time and have questionable tensile capacity. Unless you know the full history of the wood I'd be careful.

 

[JAE]

Here's another past thread on this topic:
thread337-141252


It refers to load tests also which, you pointed out, might not be feasible with smaller project situations.

 

[SlideRuleEra]

bookowski - I suggest that you take the advice that has been offered and not count on 19th century wood being any better in the 21st century than today's lumber.

To answer your question about references for timber strength in the 19th & early 20th centuries... "Trautwine, The Civil Engineer's Reference-Book". Publication began in 1872, I have the 1937 (21st Edition. The data presented in the attachment is from 1880.

You'll quickly notice that things were done differently back then. For starters, ultimate strengths (of questionable accuracy) are given. There are vague suggestions on what safety factor to use (1/4 to 1/8). Engineers then were aware of variation in strength depending on the quality of tree used for lumber... again more suggestions, not rules.

The bottom line, IMHO - Don't count on 19th century lumber to be superior.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[woodman88]

" In some cases in the past we have had the timber tested/examined ..."
In these tests were the lumber values better, the same or less than today lumber values?

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[jaypartain]

I have little experience in this area, but I will offer my penny. I would try to estimate the grade and species of material used and then determine an allowable capacity based on published data. IMO the grade and species is the most important part because it should be field determined. Based on that, I would review current capacities and then compare to historical capacities to determine my comfort level for an allowable stress. The historical capacities do not have to be from 1900. I would look for a Wood Handbook or look at an NDS from the 1940's, which is an era where data is published. I know that the 1944 NDS is on the American Wood Council Website as a viewable pdf file. I also recommend you look at ASTM D245ractice for Establishing Structural Grades and Related Allowable Properties for Visually Graded Lumber.

And as a couple of others mentioned, old growth may have been used at that time, but you do not know if it was used on your project. Without knowing the history of loads that the members have seen, it is harder to determine the true capacity.

Some of this may also boil down to engineering judgment. If it is a house, is the joist span and loading something you would expect to accomplish with sawn lumber.

Jason A. Partain, P.E.

http://www.myconstructionfinds.blogspot.com

 

[MiketheEngineer]

Even today's visually graded lumbers have wide variations when tested. Only the MSR or MEL tested lumber is reasonably consistent.

In the past 40 years - we have seen a couple of grade value declines. Supposedly, this was because lumber was being harvested quicker or the mfgs finally got their acts together.

If you can find E - by deflection testing - you can can then estimate a good Fb - theoretically.

Just be conservative.

 

[Lion06]

Mike-
How do you estimate Fb from E? Is there a direct correlation?

 

[hawkaz]

Load tests don't need to be cost prohibitive. Stack sheets of drywall, or buckets of water on the slab. In eithor case, the weight is easily calculated and the product is cheap.

Follow the deflection testing that Mike refered to above

 

[MiketheEngineer]

Not that I am aware of - but look at the charts. A lumber with a 2,000,000 E has a much higher Fb than one with 1,000,000 E

MSR lumber is rated by testing for the E value - so someone took the time to figure out some kind of correlation...I suspect..

 

[Brad805]

book, have you done any surveys to see how the floor has performed? A laser level is cheap to rent and simple to operate.

I agree that the load test does not need to be complex. They can be, but that is more common for large structures.

Brad

 

[bookowski]

Thanks for everyone's input.

I generally I agree with your suggestions. Unfortunately it's usually easier to come up with what you should do than it is to actually implement it.... or to convince others to implement it.

I'm typically dealing with turn of the century NYC brownstone type houses. Spans are in the 23'-25' range and joists are around 2.5 x 11. For typical residential use this gives bending stresses in the 1300-1400psi range. Roof joists are typically smaller and thus it's easier to make the argument that they are undersized.

Since these joists have been in use for well over 100 years the argument usually goes that past performance is sufficient to show that they are adequate. Most likely they have never seen their design live load so it's hard to say whether this argument holds water. At the same time it doesn't do our profession any credit to write them off as not working simply because of the calcs when clearly they have performed well (at least under service conditions).

Unfortunately I haven't had much luck convincing people of a load test every time we come across this situation. Usually the schedule is fast and it's hard to convince anyone that it is necessary. I have seen unintended "load tests" such as a bookstore that was packed to the ceiling with books and the joists performing just fine.

I was just wondering if anyone had specific knowledge of wood properties from this time period/location or has dealt with similar situations.

 

[hokie66]

bookowski,
Why do you need to know? Is the occupancy changing from residential to something else? If the occupancy is unchanged, I think the past performance argument is compelling.

 

[MiketheEngineer]

Performance over time is a good indicator... usually.

Now if it was used as a residential joist - I sure wouldn't OK it for a library. But keeping it residential is probably OK.

Would I seal it for this intended purpose. Assuming 16'' oc I too get about 1300-1400 psi range. Would I expect that to work. I would....unless there is some bad deterioration.

You are making a visual assessment based on your experience and valid engineering assumptions - and put that in your report. That is what we do.

I have done it many times.

 

[flash3780]

This isn't my area, but couldn't you just assess the design based on current timber strength and report your results?

If it were me, and my calculations show that the 100-year-old property was not designed to withstand the severe loading that modern buildings are designed to withstand based on best-practice strength ratings, that's what I report.

If someone claims that the older wood in the structure is stronger than newer wood, and they can offer proof, then you can recalculate with the revised material strength. Again, not my field, but I'd wager that there's a large statistical variance in the strength of wood. Since we assume th

I think that I'd probably say something like: "We should beef these joists up to meet current building standards."

You're not claiming that the old joists didn't work - they clearly did. Instead, you're recommending an improvement to the structure based on best practice for modern construction. That's a perfectly valid recommendation.

 

[flash3780]

^accidentally cut off my sentence.

If someone claims that the older wood in the structure is stronger than newer wood, and they can offer proof, then you can recalculate with the revised material strength. Again, not my field, but I'd wager that there's a large statistical variance in the strength of wood. Since we always try to assume worst case strength (-3sig or 95/99) it's very difficult to back up that statement.