Deflection
- In addition to checking bending and shear stresses, a designer should also check beams
for excess deflection potential.
- According to 1997 UBC 2106.2.9,
elements that support masonry shall be designed so that Dtotal
£ L / 600.
- Since our masonry beams will most likely support masonry walls above, this deflection criteria is used here.
- Deflection is a function of load, span length, modulus of elasticity, and moment of inertia.
- For example, the maximum deflection for a simply supported, uniformly loaded beam
calculated at midspan is:
- For example, the maximum deflection for a simply supported, uniformly loaded beam
calculated at midspan is:
- The moment of inertia for masonry beams is a function of whether or not the beam is cracked or uncracked.
- To determine if the beam is cracked or not, calculate Mcr and compare against the design M.
- If M > Mcr, use Icr in deflection calculations.
- To determine if the beam is cracked or not, calculate Mcr and compare against the design M.
- Cracked I, Icr:
- k is defined as before for a cracked cross-section.
- Uncracked I, Ig:
- But, you need to find k for the uncracked section before you can use this uncracked
I equation. To do so, find the centroid of the transformed section with tension steel only
as follows:
- But, you need to find k for the uncracked section before you can use this uncracked
I equation. To do so, find the centroid of the transformed section with tension steel only
as follows:
