This document provides an example of using a design aid table to help size reinforced concrete beams more efficiently than through trial and error. It gives two examples - one solving for the usable moment capacity of a given beam using the table, and another designing a beam to meet given criteria. The table allows looking up the usable moment capacity directly from values of moment, concrete strength, steel grades, and beam dimensions. This streamlines the iterative process of beam design.

1. Beam Design Aid Calculation
Posted by Ackerley Acton May 30, 2012 - 588 views - Filed in Civil Engineering
It is still difficult to directly design a reinforced concrete beam even if dimensions and material
properties are known. The use of design aids are commonly used to streamline the design process
instead of laboriously using a trial-and-error approach. The design aid shown below is used for
design or analysis. Values of Mu/ bd 2
are in units of PSI. It can be used to directly solve for act
knowing factored actual moment Mu, f’c, fy, b and d.

2. Example 1
GIVEN: A rectangular concrete beam with dimensions is shown below (stirrup bars

3. not shown). Use concrete f’c = 4000 PSI and grade 60 bars.
REQUIRED:
1) Determine the usable moment capacity Mu of the beam using formula.
2) Determine the usable moment capacity Mu of the beam using Table 2.
Step 1 – Determine usable moment capacity Mu of the beam using formula:

4. Step 2 - Determine the usable moment capacity Mu of the beam using Table 2:
From Table 2 above:
Example 2
GIVEN: The concrete beam below. Use the following:
 Concrete f’c = 4000 PSI
 Steel grade 60
 Concrete cover = ¾”
 #8 bars are to be used for main tension bars
 #3 stirrups
REQUIRED: Design the rectangular beam such that h =1.5b and pact = ½ (pmax).
wu = 2 KLF (includes anticipated beam weight = 300 PLF)

5. Step 1 – Determine maximum factored moment, Mmax:
Step 2 – Select values from Table 2:
Step 3 – Solve for “b” and “d” by substituting Mmax for Mu in above equation:

6. Step 4 – Select beam dimensions:
From above, use b = 12” and d = 18”
Step 5 – Determine required area of main tension bars:

7. From above, pact = 0.0107 = As/bd
Step 6 – Determine number of #8 main tension bars:
Step 7 – Check beam height with “Minimum Thickness of Beams” Table:
From Table:
h = 21” which is approximately = 20” as designed
Step 8 – Check originally assumed beam weight:

8. It was assumed that the beam weight = 300 PLF (See Step 1).
Actual beam dimensions:
Recall:
Beam Weight = 250 PLF < 300 PLF (assumed) OK
Step 9 – Draw “Summary Sketch” labeling all information necessary to build it:

9. Source: faculty.delhi.edu
Prepared by: zone4info.com