7.1 Beams, Joists & Flexural Reinforcement Detailing (Stirrups, Spacing, Torsion)

7.1 Beam Flexural, Shear & Torsional Reinforcement Detailing

The beam mark takes you from the framing plan to the approved section, reinforcement schedule and support details. Read the sections at both ends and near midspan. Table 1705.3 of the model IBC lists reinforcement placement as periodic special inspection. Check the installed cage before placement and report discrepancies through the project procedure.

Flexure, cutoffs and integrity

In a typical continuous beam under downward gravity loading, negative bending puts the top near an interior support in tension and positive bending puts the bottom near midspan in tension. A simply supported beam under downward gravity loading typically has positive bending between supports. Actual moment reversals, cantilevers and lateral loads can change the tension face. Match each top and bottom bar callout to the relevant span and support, including number, grade, location, cutoffs and anchorage.

Under ACI 318-19 Section 9.7.3.3, a flexural bar generally extends beyond the theoretical point where it ceases to be required by at least the greater of $d$ and $12d_b$. The exceptions are at supports of simply supported spans and at free cantilever ends. If the design identifies such a theoretical cutoff for a No. 8 bar in a beam with $d=22$ inches, compare 22 and 12 inches. The extension criterion is 22 inches. Continuing bars have a separate development requirement under Section 9.7.3.4. Terminating bars in a tension zone also calls for one of the conditions in Section 9.7.3.5. At the cutoff, the design can use $V_u\le(2/3)\phi V_n$, or, for No. 11 and smaller bars, continuing reinforcement of at least twice the required flexural area with $V_u\le(3/4)\phi V_n$. The third route provides additional stirrup or hoop area for a distance $3d/4$ from the cutoff with the required area and spacing. Each route is a design check. Use the approved cutoffs and ask for design disposition if bars were shortened.

At a simple support, Section 9.7.3.8.1 requires at least one-third of maximum positive moment reinforcement to extend along the bottom into the support at least 6 inches, with a precast bearing-center exception. At other supports, Section 9.7.3.8.2 uses at least one-fourth and the 6-inch extension. The latter case requires anchorage to develop $f_y$ at the support face if the beam is part of the primary lateral-load-resisting system. For negative reinforcement at a support, Section 9.7.3.8.4 requires at least one-third to extend past the point of inflection by at least the greatest of $d$, $12d_b$ and the clear span divided by 16.

Under Section 9.7.7, perimeter beams have continuous positive reinforcement of at least one-quarter of the maximum and continuous support negative reinforcement of at least one-sixth, with a minimum of two bars or strands for each. Check the specified enclosing stirrups or hoops, continuation through the region bounded by the column's longitudinal reinforcement, and anchorage to develop yield at a noncontinuous support face. The section also gives integrity alternatives for other beams. Where continuous integrity reinforcement is spliced, check positive steel at or near supports and negative steel at or near midspan. The permitted splice methods include a qualifying mechanical or welded splice or Class B tension lap.

Shear stirrups

Stirrup legs cross inclined cracks and transfer shear after cracking. Verify the leg count and orientation, hook anchorage and all changes of spacing along the beam. A wider beam can require multiple legs across its width.

The required stirrup contribution $V_s$ comes from the design. ACI 318-19 Table 9.7.6.2.2 uses $4\sqrt{f'_c}b_wd$ in inch-pound units as the boundary between two spacing rows. The table distinguishes spacing along the member from spacing between legs across its width.

For an ordinary nonprestressed beam with $f'_c=4000$ psi, $b_w=12$ inches and $d=24$ inches, the boundary is about 72.86 kips. If the required $V_s$ is 80 kips, the table limits longitudinal spacing to 6 inches and leg spacing across the width to 12 inches. If the drawing specifies 5 inches along the beam, verify the 5-inch layout. Use the approved detail for the first stirrup offset. For a special moment frame beam, Section 18.6.4.4 separately limits the first hoop to 2 inches from the supporting column face.

Torsion and skin steel

Where torsional reinforcement is required, Sections 9.7.5 and 9.7.6.3 call for longitudinal perimeter reinforcement and transverse closed stirrups complying with Section 25.7.1.6 or hoops. Under Section 25.7.1.6 the usual stirrup detail has 135-degree standard hooks around longitudinal bars. The code permits specified alternative anchorage when a flange, slab or similar member restrains the surrounding concrete against spalling. The two-piece U-stirrup and crosstie option in Section 25.7.1.6.1 also has a restraint condition on the 90-degree end. Check the actual closure and restraint before accepting the cage.

For torsion, check a longitudinal bar at each corner and bar spacing along the stirrup perimeter of at most 12 inches. The bar diameter is at least 0.042 times transverse torsional reinforcement spacing and at least No. 3. Where transverse torsion reinforcement is required, its spacing is limited by the lesser of $p_h/8$ and 12 inches, with $p_h$ measured around the centerline of the outermost closed transverse torsional reinforcement. For $p_h=72$ inches, the spacing ceiling is 9 inches. Check longitudinal development at both support faces and the detailed extension of the transverse steel beyond the zone required by analysis.

For nonprestressed and Class C prestressed beams with overall depth $h$ exceeding 36 inches, Section 9.7.2.3 calls for uniform longitudinal skin reinforcement on both side faces for $h/2$ measured from the tension face. At a section of a 40-inch beam with bottom tension, the zone extends 20 inches up each side. For deformed bars use the lesser of $15(40{,}000/f_s)-2.5c_c$ and $12(40{,}000/f_s)$ under Table 24.3.2. Here $f_s$ is the applicable service stress in psi and $c_c$ is the clear cover in inches to the side face. For $f_s=40{,}000$ psi and $c_c=2$ inches, the two results are 10 and 12 inches. A 10-inch ceiling governs this example. Check the approved detail, since the tension face and spacing can change along the beam.

One-way joists

Section 9.8 covers nonprestressed one-way joist construction. Check at least 4 inches rib width, overall rib depth at most 3.5 times the minimum width, and at most 30 inches clear between ribs. With removable forms or other fillers under Section 9.8.3.1, the slab thickness is at least the greater of one-twelfth of the clear distance and 2 inches. The qualified structural-filler case of Section 9.8.2.1.1 uses the same span fraction and a 1.5-inch floor. For removable forms with a clear 27-inch gap, the span fraction yields 2.25 inches and governs. Section 9.8.1.5 allows the design to use 1.1 times the Chapter 22 concrete contribution $V_c$ in qualifying construction. Verify the reinforcement actually shown and check two-way joist systems under the separate Chapter 8 provisions.

Before concrete covers the work, trace changes from the support through each reinforcement zone. Measure cover to the outermost applicable steel and check that aggregate can pass through the cage. Record the member mark, detail and revision, actual dimensions and resolution if reinforcement differs from the approved detail.