Nails

[Armat]

I am puzzled about this.someone has told me that nails and screws, no matter how many you put on a joist, do not weaken its strength. His logic is that nails and screws only part the wood fiber and do not break them. It sounds logical but I wonder if this has any scientific foundation...
I was thinking doing a simple test drive ten nails in the middle of 2x6 10'
say 4 inch space and put some weight on it.Test the same wieght on both boards one nailed and one clean.I am not an engineer but curious about nailing and wood strengh issues.Any inputs?

[mjpliv]

Where the nails/screws are mostly installed in the top edge of the joist which is under compression loads. The bottom edge is under tension and is therefore more sensitive to fastener damage.

[Rich]

Your test in theory would work - but try and get two clean pieces of wood to break at the same time to begin with. Highly unlikely.
If you put enough to make it look like a broom - sure it'll reduce the weight bearing capacity. There are so many other factors to consider - load sharing of adjacent members, knots, wood grain, wane, grade of lumber, loading patterns, etc..etc.

[mjpliv]

If you put in enough nails in it to maike the wood fail you would probably exceed the allowable loading anyway! :lol:

[mjpliv]

If you really sat down and thought about it, the more nails you insert into the top edge of a joist the stiffer it will get!

A floor joist deflects under load because the wood cells on the top compress and the wood cells on the bottom stretch. Every time you pound in a nail, you may tear a bit of wood fiber but you also compress all of the wood cells surrounding the nail. If you insert enough nails you are basically pre-stressing the joist allowing for less compression of the top edge and therefore less deflection.

Thats my story and I'm sticking to it :D

[Armat]

If you really sat down and thought about it, the more nails you insert into the top edge of a joist the stiffer it will get!

A floor joist deflects under load because the wood cells on the top compress and the wood cells on the bottom stretch. Every time you pound in a nail, you may tear a bit of wood fiber but you also compress all of the wood cells surrounding the nail. If you insert enough nails you are basically pre-stressing the joist allowing for less compression of the top edge and therefore less deflection.

Thats my story and I'm sticking to it :D

MJ thanks for your inputs this is fascinating but is your theory scientific or you are just brainstorming. What you are saying makes sense. There is one thing that puzzles me. When a nail goes through the wood I assume it severs the wood fibers, the grain in that particular location. Does as you state fiber cells congregate around the nail hence make the nail sort of part of the grain? Or it disrupts the grain flow including its strength

With this subject in mind. Can anyone help me on this? I want to double up my 2x4 ceiling joints for added strength. The old 1900 joints are slightly wider quarter inch then newer 2x4 ones. Should I screw the new joints in flash with top part or flash with lower part? Does it make a deference? Strength wise. Logically I was thinking flash with top part since it would take some of the load from the old joint when nailing the plywood but thinking about nails and wood being compressed at lower side makes me think perhaps it is better flash with lower side. Any thoughts?

[mjpliv]

The best way to increase the stiffeness of a wood component is to increase the height. Although adding a second ceiling joist to the side of the existing one would increase the carrying capacity, you would be farther ahead attaching a 2x3 on its flat to the top of the existing joists with screws at 6 inch centers. If you use enough screws (and construction adhesive if you really want to beef things up) then the two units work as one piece giving you the same carrying capacity as a 2x5-1/2 joist.

[Rich]

I guess I was looking at it a little different - nails in the long face of a board not in the top / bottom edges. Such as if you were sistering two joists together.

[Armat]

I guess I was looking at it a little different - nails in the long face of a board not in the top / bottom edges. Such as if you were sistering two joists together.

MJ great point about 2x3 on top.Thanks

Rich
Thanks for replying however as MJ already stated nailing or screwing closer to the top edge from the center but not too close to the edge is better then in the center or worse at the bottom edge since bottom edge is more critical.Could you explain your method of mating two joints?Thanks

[Rich]

Like I said - I was looking at a worse case scenario. Not nails in the top edge but in the face or side of a joist. Put enough nails across that face and you will eventually decrease the bearing capacity. Sistering a joist - I simply nail 12d's in a staggered pattern approximately every 16" with adhesive.
But in reply to your original question
no matter how many you put on a joist, do not weaken its strength
in a joist situation it's pretty rare to have anything but sistering.. as adding to the bottom decreases head height and doesn't do much anyway and adding to the top involves removing the flooring above, adding the scab, and then replacing the floor. So IMO that leaves sistering another joist next to it.
In the real world you would never nail so many nails so close together to effect the bearing capacity - but for **its and grins if you wanted to put so many across the face so it "looks like a broom" then I'm pretty certain you would effect the bearing capacity.