![]() 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). Most likely they have never seen their design live load so it's hard to say whether this argument holds water. 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. Roof joists are typically smaller and thus it's easier to make the argument that they are undersized. ![]() For typical residential use this gives bending stresses in the 1300-1400psi range. Spans are in the 23'-25' range and joists are around 2.5 x 11. I'm typically dealing with turn of the century NYC brownstone type houses. Unfortunately it's usually easier to come up with what you should do than it is to actually implement it. I generally I agree with your suggestions. RE: Capacity of existing wood joist MiketheEngineer (Structural) 12 Apr 11 10:23 If it is a house, is the joist span and loading something you would expect to accomplish with sawn lumber. Some of this may also boil down to engineering judgment. Without knowing the history of loads that the members have seen, it is harder to determine the true capacity. ![]() I also recommend you look at ASTM D245:Practice for Establishing Structural Grades and Related Allowable Properties for Visually Graded Lumber.Īnd 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. I know that the 1944 NDS is on the American Wood Council Website as a viewable pdf file. I would look for a Wood Handbook or look at an NDS from the 1940's, which is an era where data is published. The historical capacities do not have to be from 1900. Based on that, I would review current capacities and then compare to historical capacities to determine my comfort level for an allowable stress. IMO the grade and species is the most important part because it should be field determined. I would try to estimate the grade and species of material used and then determine an allowable capacity based on published data. I have little experience in this area, but I will offer my penny. RE: Capacity of existing wood joist woodman88 (Structural) 12 Apr 11 08:26 The bottom line, IMHO - Don't count on 19th century lumber to be superior. Engineers then were aware of variation in strength depending on the quality of tree used for lumber. There are vague suggestions on what safety factor to use (1/4 to 1/8). For starters, ultimate strengths (of questionable accuracy) are given. You'll quickly notice that things were done differently back then. The data presented in the attachment is from 1880. Publication began in 1872, I have the 1937 (21st Edition. "Trautwine, The Civil Engineer's Reference-Book". To answer your question about references for timber strength in the 19th & early 20th centuries. 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.
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