Floor joists
What governs the design
Section titled “What governs the design”Not the capacity
Section titled “Not the capacity”A properly chosen joist passes the strength check comfortably. The limit lies elsewhere.
Deflection
Section titled “Deflection”The serviceability limit state: how far the floor deflects under load. It affects the integrity of the finishes, the operation of doors and windows, and the general feel of the building.
Vibration
Section titled “Vibration”The particularity of a light floor: because the mass is small, the response to footfall is more noticeable. This is not a safety question but a comfort question — and it is what the occupant will notice. Floor vibration.
The two critical local questions
Section titled “The two critical local questions”Bearing at the end of the joist
Section titled “Bearing at the end of the joist”The thin web receives concentrated pressure at the bearing and can crumple locally. This has to be checked separately, and is handled with local reinforcement — a web stiffener or an insert plate.
This is the point at which an otherwise well-sized floor fails.
Service holes in the web
Section titled “Service holes in the web”A hole in the web weakens the joist, in shear above all, which is greatest near the support. So the position and size of the holes is not an installer’s decision. Web holes.
What to know at system level
Section titled “What to know at system level”Joist spacing
Section titled “Joist spacing”Closer spacing allows a smaller joist but means more members and more work. What the board can be fixed to also comes into it.
Blocking between the joists
Section titled “Blocking between the joists”Restraining the joists against rotation — blocking, straps — is a condition of the capacity, not an accessory.
The floor plane as a diaphragm
Section titled “The floor plane as a diaphragm”The floor also plays a part in distributing the horizontal loads. Sheathing diaphragm.
The weight of the build-up
Section titled “The weight of the build-up”The mass of the floor build-up helps the vibration and hurts the load. The choice of build-up is how you balance the two. Floor build-ups.
The point
Section titled “The point”With a floor the question is not whether it carries the load but how it will feel. Deflection and vibration decide whether the occupant rates it good.