Old House Renovation Part 2!!

[jdon]

The fund of knowledge in this forum continually amazes me!

These are the kind of calculations a structural engineer would do (or they would look it up in a table).

Reminds me of the old joke:

A mathematician, a physicist, and an engineer were all given a red rubber
ball and told to find the volume. The mathematician carefully measured
the diameter and evaluated a triple integral. The physicist filled a
beaker with water, put the ball in the water, and measured the total
displacement. The engineer looked up the model and serial numbers in
his red-rubber-ball table.

[jseklund]

JimD - most importantly, where did you learn all that???

Google has changed life in many ways, but the actual calculations for stuff like this is difficult to find. Possibly because the engineers using this stuff don't know them that well anymore either (due to the availability of tables).

The engineer I spoke with was real down and dirty, because he was a builder too, and I think he was just trying to help me achieve the solution in a safe manner without costing me a fortune, nor getting too involved in all of the math unnecessarily. I would have loved to have known those formulas a couple of weeks ago, as it could have helped me to at least have a better understanding of how confident I could have been with the solution we came up with. I am 100% confident that it won't break - mostly due to the somewhat drastic decrease in deflection, but also due to the fact that the beams that were there were shorter and only had very minor signs of cracking after 130 years. I do not think the minor signs actually indicated failure either - I think they were cracked when they were "milled". If 1 6.5" board could hold up for 130 years, I am pretty confident that the same 6.5" still in good shape, sistered 2X8's, some additional 2X8 joists, and 3 rows of blocking should hold up for at least another 20.

It would have been nice, however, to have had a better grasp of how to reduce deflection - it may have sparked ideas (such as the steel plates) that I didn't realize would have made this a B+ or A- job instead of a B job.

[JimD]

I learned that over 30 years ago when I was in school to get my mechanical engineering degree.

I did not have my text books with me so what you got came from googling. I agree it can be confusing - it really helps to have done the calculations before, even if it was a long time ago, so you recognize things when you see them. Trying to read through the messes I saw would take a long time and I'm not sure if it gets you there.

It is also important to understand that the design of buildings is about deflection, not strength. I don't remember what the criteria is but it is a fraction of an inch that is allowed at maximum loading. Maximum loading is 30 lbs per square foot for bedrooms and 40 lbs for the main areas of a house (working from memory which is a risk). That's a lot more load than you would typically get. In a 10 by 10 bedroom, you would be talking about 3000 lbs. Structural failure would be at a totally different point. That is why your grossly undersized floor joists didn't fail. They deflected more than would be OK these days but were not close to structural failure. (there are, of course, equations for that too and they also use the same moment of inertia factor).

Jim

[jseklund]

Haha - I like to think that most people may consider me an extreme DIY'er - and I'm really good at finding answers on the internet - but I couldn't find enough to be comfortable with the span tables 100%.

I hadn't really done the math on the live loads - but calculations indicated that a 2X8 SPF No. 2 with a live load of 40, dead load of 10, and a deflection of L/240 would allow for a span of 14'7" as a maximum span. My thoughts were that I was actually sistering that 2X8 so worst case scenario, I was calculating L/210 roughly, best case scenario of L/300+.

A deflection of L/240 equates to about a drop of 3/4" I figured, which would be a big drop - but doing the same calculations with the 6.5" alone I figured I had an L/90 - L/120 - so I was happy with that 3/4" drop.

I hadn't thought that I'd have to load this 225 sq. ft. room with almost 9,000 pounds to cause that drop.

Of course, that's dead weight, and I know 300-600 pounds of people bouncing up and down probably equates to almost 9,000 pounds with the momentum/inertia factored in...

Long story short, your help makes me feel better that the floor is at least safe, maybe it could be stiffer, but it won't collapse.

[greenacres2]

The fund of knowledge in this forum continually amazes me!

Reminds me of the old joke:

A mathematician, a physicist, and an engineer were all given a red rubber
ball and told to find the volume. The mathematician carefully measured
the diameter and evaluated a triple integral. The physicist filled a
beaker with water, put the ball in the water, and measured the total
displacement. The engineer looked up the model and serial numbers in
his red-rubber-ball table.

The insurance agent/volunteer firefighter/wookworker held the ball up to his ear, heard nothing, and reported a volume of 0.

earl