Thumb Rules and Specifications for Design of Reinforced Concrete Beam

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The design of a reinforced concrete beam is achieved through a trial and error method. However, design codes, certain thumb rules, and past experiences can significantly reduce the lengthy design process.

Initially, certain aspects of design, like geometry and self-weight, should be estimated and then checked during the design process. The applicable codes such as ACI 318-19 provide specifications to help the designer, but some estimations and assumptions are still required to be made.

An experienced designer can come up with very accurate assumptions. In contrast, new designers, who have little or no experience, have to rely on trial calculations or arbitrary rules modified to suit a particular situation.

Design assumptions for reinforced concrete beams may be required for the self-weight of a beam and its geometrical dimensions. Design codes provide specifications regarding bar size selection, spacing, concrete cover, and bar placement.

Assumptions and Specifications for Design of Reinforced Concrete Beam

1. Beam Dimension

The size of a beam is governed by negative moments or shear forces at supports. ACI moment coefficient can be used to calculate moments of a particular span. Alternatively, designers can estimate the depth of a beam at 60 mm to 65 mm per meter of the beam's span.

Requirements related to the width-depth ratio of reinforced concrete beams are not provided by codes. However, as a rule of thumb, it is better to use a depth which is two and a half to three times the beam's width. For long-span beams, it is economical to use deep and narrow sections.

However, architectural considerations may prevent the use of deep concrete beams, and the designer would be obliged to select wide beams. The lesser the number of different beam sizes in a structure, the better it is from an economic standpoint.

2. Selection of Bar Size

After a required reinforcement area is computed, Table-1 can be used to select the number of bars that provide the necessary reinforcement area.

For normal situation, a bar size of No. 32 and smaller is practical to use. It is preferable to use a single bar size in a beam, but it is possible to use two different bar sizes to get the required steel area.

Table-1: Area of Groups of Bars, mm²

Number of bars	1	2	3	4	5	6	7	8	9	10	11	12
Bar Size, IS	-	-	-	-	-	-	-	-	-	-	-	-
10	71	142	213	284	355	426	497	568	639	710	781	852
13	129	258	387	516	645	774	903	1032	1161	1290	1419	1548
16	199	398	597	796	995	1194	1393	1592	1791	1990	2189	2388
19	284	568	852	1136	1420	1704	1988	2272	2556	2840	3124	3408
22	387	774	1161	1548	1935	2322	2709	3096	3483	3870	4257	4644
25	510	1020	1530	2040	2550	3060	3570	4080	4590	5100	5610	6120
29	645	1290	1935	2580	3225	3870	4515	5160	5805	6450	7095	7740
32	819	1638	2457	3276	4095	4914	5733	6552	7371	8190	9009	9828
36	1006	2012	3018	4024	5030	6036	7042	8048	9054	10060	11066	12072
43	1452	2904	4356	5808	7260	8712	10164	11616	13068	14520	15972	17424
57	2581	5162	7743	10324	12905	15486	18087	20648	23229	25810	28391	30972

3. Minimum Beam Width

The minimum beam width required to accommodate multiples of various sizes of bars are given in Table-2.

Table-2: Maximum Number of Bars as a Single Layer in Stems of Beams for Maximum Aggregate Size of 19 mm

Beam Width, mm	200	250	300	350	400	450	500	550	600	650	700	750
Bar Size	-	-	-	-	-	-	-	-	-	-	-	-
16	2	4	5	6	7	9	10	11	12	13	15	16
19	2	3	4	6	7	8	9	10	11	12	14	15
22	2	3	4	5	6	7	8	9	11	12	13	14
25	2	3	4	5	6	7	8	9	10	11	12	13
29	2	3	3	4	5	6	7	8	9	9	10	11
32	1	2	3	4	5	5	6	7	8	8	9	10
36	1	2	3	3	4	5	6	6	7	8	8	9
43	1	2	2	3	3	4	5	5	6	6	7	7
57	1	1	2	2	3	3	3	4	4	5	5	6

Tabl-3: Maximum Number of Bars as a Single Layer in Stems of Beams for Maximum Aggregate Size of 25 mm

Beam Width, mm	200	250	300	350	400	450	500	550	600	650	700	750
Bar Size	-	-	-	-	-	-	-	-	-	-	-	-
16	2	3	4	5	6	7	8	9	10	11	12	13
19	2	3	4	5	6	7	8	9	10	11	11	12
22	2	3	4	5	5	6	7	8	9	10	11	12
25	2	3	3	4	5	6	7	8	8	9	10	11
29	2	2	3	4	5	6	6	7	8	9	10	10
32	1	2	3	4	4	5	6	7	8	8	9	10
36	1	2	3	3	4	5	6	6	7	8	8	9

Note:

Maximum concrete cover assumed to be 40mm to the No. 13 stirrups

4. Concrete Cover

Concrete cover is the distance from the surface of concrete to the stirrup bars in the beam section. The concrete cover requirements in the ACI code are extensive.

For beams that are not exposed to weather or are not in contact with the ground, the minimum clear distance from the bottom of the steel to the concrete surface is 40 mm as per ACI 318-19 section 20.5.1.3, Table 20.5.1.3.1.

The concrete cover is required to protect steel bars from environmental attacks and vandalism and ensure a good bond between reinforcement and concrete.

5. Bar Spacing

The clear spacing between the bars in a single layer should not be less than:

1. 25 mm
2. The diameter of the longitudinal bars
3. 4/3 times the maximum aggregate size         

6. Bar Placement

If the bars are placed in more than one layer, those in the upper layers are required to be placed directly over the bars in the lower layers, and the clear distance between the layers must not be less than 25 mm.

7. Self-weight of Reinforced Concrete Beam

The estimation of a reinforced concrete beam's self-weight is essential for accurate calculation of loads and, consequently, the moment to be resisted. The computation of a dead load of the beam is not straight forward and may involve iterations.

One can assume the dimension of the beam from which the self-weight can be computed. If the estimated self-weight is found to be appreciably less than the weight of the section designed, it would be necessary to resize the beam and recalculate the beam weight and applied moment. Alternatively, Table-4 might be used as a rule of thumb for the estimation of concrete weight.

Table-4: First Estimate of Beam Weight

Design Moment, Mu (KN.m)	Estimated Weight, KN/m
≤271	4.38
>271 but ≤406	5.11
>406 but ≤542	5.84
>542 but ≤678	6.57
>678	7.3

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