Replace part of the joist

[justusandersson]

Nogging pieces have some impact on the deflection by distributing parts of a point load to adjacent joists. The total load-bearing capacity of the floor structure is not affected at all by nogging pieces. A 4.5-meter-long beam with a dimension of 70x160 mm will have a deflection of 7 mm if a 100 kg person stands in the middle. If nogging pieces are used to distribute 25 kg to adjacent joists, the deflection will instead be 5.3 mm. This is all under the condition that C 24 graded timber or equivalent is used. This should be compared with 1.5 mm, which is the current deflection requirement. It is admittedly a fairly new rule, but it is research-based and has some justification.

 

[johan_p]

The thread you're referring to is mostly about cross-bracing.
It's important to keep the concepts separate. Kortling is NOT the same as cross-bracing.

Stickan

No, not the same thing, but the gist of the thread seems to be that they are directly comparable in terms of what they achieve.

 

[johan_p]

Blocking has some impact on the deflection by distributing parts of a point load to adjacent joists. The total load-bearing capacity of the floor is not affected at all by blocking. A 4.5-meter long joist with the dimension of 70x160 mm will have a deflection of 7 mm if a 100 kg person stands in the middle. If blocking can distribute 25 kg to adjacent joists, the deflection will instead be 5.3 mm. This is all under the condition that it involves C 24 classified wood or equivalent. This should be compared to 1.5 mm, which is the current deflection requirement. Although it is a relatively new rule, it is research-based and has some justification.

Good answer, thanks!
You seem knowledgeable on the subject, may I ask what happens if you replace say 1m of these 4.5m with 45x160. Will the deflection be as if the entire joist consists of 45x160, or will some additional support be obtained since the weakening with the slimmer joist occurs at one outer end?
I guess the latter, but I might be thinking a bit incorrectly here.

 

[justusandersson]

A beam that rests freely on two walls and is subjected to a uniformly distributed load is exposed to two types of forces. One is a vertical load at each end that is half of the total load on the beam (shear force), and the other is a downward bending moment that is greatest at the center of the beam. If you plot a curve describing the size of the bending moment, it becomes arc-shaped. To withstand the shear forces, a certain minimum amount of material is required at the ends of the beam. To withstand the bending moment, a certain construction height of the beam is primarily required, most in the middle and decreasing towards the sides. In materials other than ordinary construction timber, such as steel and glulam, it is common to make the beam arc-shaped to save material. The problem with ordinary wood is its fiber structure, which makes it prone to cracking when cut at an angle to the fibers.

A long explanation to clarify that, theoretically, less material can be used towards the ends of the beam, but it is difficult to make it hold.