Question

Why are bearers and joists (like those in a suspended flooring system) always orientated so that the biggest cross section dimension is vertical?

Answer 1

Solution:

• Joists are the horizontal members which supports slab or roof. joists are subjected to vertical and transverse loads which creates bending moments in the members. Due to bending moments, stresses are generated in the joists.

Here,

where,

I = Moment of inertia.

M = Bending moment

• The strength of joists depends upon the geometrical properties of members which is called Moment of Inertia.(MI). as of .
• Moment of inertia is an index to offer resistance to bending stresses.

Here,

where, b = Width of joist

d = Depth of joists

• Now, from above formula it is clear that when we double the b (width of beam), MI merely doubles. But if we doubles the d (depth of beam) , MI increses by around 8 times. So, it is economical approach for high moment of inertia to increse depth of beam.
• Finally, as moment of inertia increses, the strength of the beam also increses to offer great resistance to bending stresses.