iwire said:
...but I would not be calling an engineer for a directions on how to hang a 400 lb item unless the existing structure was weak.
I partially agree...
In wood framing construction, ceilings and floors make some design assumptions based on the type of lumber used and the span. In these assumptions are two types of loads considered: live loads and dead loads. There are two other types—snow and wind loads—but they are not considered for interior framing. The dead load is the weight of the building materials alone. The live load is the additional weights due to occupancy, i.e. people, furnishings, and stored materials. Span tables have been developed for this very purpose.
Since we know there is a room above where this fixture will be supported, necessitating that the framing members be rated as floor joists, the information I have at hand says such a floor is typically designed for 40 psf live load and 10 psf dead load. However, if the space above is a sleeping room or attic, the live load rating can be decreased to 30 psf.
At this point, we must discuss what I believe to be a common misconception: interpretation of the load ratings especially that of the live load. Using the example of 40 psf, this does not mean you cannot exceed 40# of "live" weight on any one square foot of floor space. The live and dead load ratings have more to do with weight distribution and the structural members needed to support the floor or what have you. Say you have a 10' x 10' room with load bearing walls under it and around the perimeter. With a total live/dead loading of 50 psf on 100 sq ft of floor space and 40' of load bearing wall, the wall must be capable of supporting it's own weight plus 50 psf x 100 sq ft ? 40 ft... and that is an additional 125# per ft of wall and the floor is capable of supporting 4,000# of fairly distributed live weight. This is the basic concept of load design.
Getting back to floor design, if we have a 15' x 20' room and the joists are spaced 16 oc across the short dimension, each joist is capable of supporting a total weight of 15' x 1⅓' x 50 psf = 1,000#, which would be 800# live weight and 200# dead weight. Adding a 100# fixture to this one joist is not something that can be compensated for by conservative dead weight assumptions. However, if we support the fixture by distributing the weight to two joists (and this means supporting it dead center in the cavity if the supporting member can flex at either end, or structurally tying the two joists together), that would be 50# per joist. I would again say this is not an amount which would be covered by conservative dead weight assumptions. We would therefore have to go to three or four joists, or determine if as-built limits permit heavier loading (which is where a knowledgeable person comes into play)... or we can cheat and lower the live load rating for the joists involved :grin:
