Reinforced Concrete Stirrup Spacing Calculator

Jonathan Ochshorn

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Directions: This calculator designs stirrup spacing for uniformly-loaded reinforced concrete beams, based on simplified procedures outlined in ACI-318 (2008 or 2011), and assuming either simply-supported spans or typical interior spans meeting the conditions specified in the ACI Code for design shear values of wuln / 2 at the face of supports. The S.I. option simply converts the Imperial numbers to metric ones, but uses the same underlying Code logic. Choose units (Imperial or S.I.); then enter values for concrete and steel properties, beam dimensions, stirrup (rebar) size, clear span between supports, and maximum design shear force, Vu.

Press "update" button. The stirrup spacing to resist diagonal tension ("shear") is computed, based on one of the nine stirrup distribution scenarios illustrated in Figure 1 (click on the figure for more information).

The value for maximum shear force must be taken at the face of the support—the calculator automatically reduces this value to the force measured at a distance, d, from the face of the support. If necessary, use the drop-down menus to choose a lightweight concrete, or to indicate that the beam is small enough so that the requirement for minimum steel area is exempted. The calculator also computes a constant spacing for the value of maximum shear force that has been entered; this can be used to design stirrup spacing where the shear force is constant (e.g., for girders with concentrated loads). To find the maximum shear force at the face of supports, if it is not already known, scroll down to the bottom of the calculator for some tools: the first computes the design shear, Vu, at the face of supports when the "unfactored" dead and live loads, clear span, and beam spacing are known. The second, finds Vu at the face of supports when the design shear force at the support centerline is known.

Stirrups are deployed in segments of constant spacing; the spacing changes at key points along the length of the beam, e.g., at those points where maximum spacing governs, or where no stirrups are needed. Nine possible scenarios for stirrup spacing (shown as "A" through "I" in Figure 1) can be identified: see this paper for more information, or click on the figure. The particular spacing scenario that applies for the parameters entered is displayed below in red font.

More detailed explanations and examples can be found in my text; a detailed explanation of the nine stirrup distribution scenarios is here.

Fig. 1. Nine possible stirrup spacing scenarios (labeled "A" - "I") depend on the relative values of maximum shear force, concrete and steel capacity, and the points at which maximum spacing governs the design.

 Units stirrup size Imperial (U.S.) S.I. No.3 (10) No.4 (13) No.5 (16) Uniform load "" Yes No Stirrup spacing for uniform load: Data normal weight sand-lightweight all-lightweight Capacity of concrete to resist shear, Vc 0.5Vc (upper boundary for Zone I: when shear force divided by φ is below this value, no stirrups are needed) 3Vc (this value of shear force divided by φ is boundary between Zones III and IV -- "reduced" max. spacing governs in Zone IV; "regular" in Zone III) 5Vc (upper boundary for Zone IV, above which shear force divided by φ is too great and beams are not permitted) Maximum shear force divided by φ at distance, d, from face of support Shear force divided by φ corresponding to "regular" maximum spacing Shear force divided by φ corresponding to "reduced" maximum spacing Maximum spacing, unrounded (when Vs <= 2Vc) Maximum spacing, reduced and unrounded (when Vs > 2Vc) Warnings Concrete cylinder strength f'c must be greater Steel yield stress must be greater Beam width, b, must be greater Beam effective depth, d, must be greater Shear force Vu must be greater than zero Span must be greater than zero Span must be at least 4 times the effective depth Stirrup spacing too small; try using larger stirrup size or larger beam Criteria for small total beam depth