Preliminary Design of Beams
A. Beams in Intermediate Moment Resisting Frame (IMRF)
a) Minimum Height
If a structure is classified under an Intermediate Moment Resisting Frame (IMRF) system and uses non-prestressed concrete, the minimum beam depth (hmin) is determined as follows:
| Support Condition | Minimum h |
|---|---|
| Simply supported | l / 16 |
| One end continuous | l / 18.5 |
| Both ends continuous | l / 21 |
| Cantilever | l / 8 |
Where:
l = beam span length
This table is based on Table 9.3.1.1 from ACI 318-14, which assumes normal-weight concrete and reinforcement yield strength of 60,000 psi.
- For reinforcement strengths above 60,000 psi, refer to Clause 9.3.1.1.1.
- For lightweight concrete, refer to Clause 9.3.1.1.2.
Definitions:
- Simply supported: a beam with two supports only.
- Continuous: the beam is connected to adjacent spans on one or both sides.
- Cantilever: a beam supported only at one end.
In practice, for beams other than cantilevers, a common simplification is to use:
hmin = l / 10
b) Minimum Width
There is no specific requirement in ACI 318-14 for the minimum width of beams during preliminary design for IMRF. However, practical field applications commonly use:
- bmin = 1/2 h, or
- bmin = 2/3 h
B. Beams in Special Moment Resisting Frame (SMRF)
For beams in a Special Moment Resisting Frame (SMRF) system, additional dimensional requirements apply:
- The clear span (ln) shall be at least 4d
- The beam width (bw) shall be at least the leseer of 0.3h and 10 in
- The projection of the beam width beyond the width of the supporting column on each side of the column shall not exceed the lesser of c2 and 0.75c1 .
Where:
- d = effective depth of the beam
- c1 = length of the column
- c2 = width of the column
Conclusion
Preliminary design is an essential step in structural design. It includes estimating initial dimensions based on:
- The selected structural system, as each moment frame type (OMRF, IMRF, SMRF) uses different design assumptions.
- The material strengths of concrete and reinforcing steel. For example, beams with 60,000 psi reinforcement require specific formulas outlined in the code.
By conducting a thorough and code-compliant preliminary design, engineers lay the foundation for accurate, safe, and cost-effective structural solutions.