Shear and Stirrup Spacing Calculator for Concrete Beams

Construction
All tools
Report

Heading to the field? Install this site from your browser menu and this tool works with no signal. See how

Shear and Stirrup Spacing Designer

Concrete shear capacity, required stirrup spacing and a practical stirrup schedule along the span, to ACI 318. Enter a uniformly loaded beam and the tool works out the shear at every section, or enter a shear force directly to check one location. Forces in kN, everything else in millimetres and MPa.

Calculation, step by step

    Beam under uniform load with closely spaced stirrups near the supports, diagonal cracks shown near the ends, a shear force diagram with the concrete capacity and half capacity marked, and the resulting stirrup spacing zones along the span
    Shear peaks at the supports and passes through zero at midspan, so a sensible stirrup layout is tight at the ends and opens out toward the middle. The dashed lines are the concrete capacity and half of it, the two thresholds that set where stirrups are needed at all.

    How to use this shear designer

    The beam mode is the one you want for a normal floor or roof beam. Enter the section, the materials, the clear span and the factored uniform load. The tool works out the shear at every section, applies the concrete and steel capacities, and returns a stirrup schedule you can write straight onto a drawing.

    The single section mode is for checking one location, for example at a point load, at a change of section, or when your shear came from a frame analysis rather than a simple uniform load. Enter the shear directly and the tool returns the spacing that location needs.

    The schedule line is the practical output. It reads like a drawing note, for example twelve stirrups at 100 millimetres then eight at 175 and the rest at 250. Spacings are rounded down to 25 millimetre multiples because nobody sets out steel at 163 millimetre centres.

    Why shear failure is the one to worry about

    A beam that runs out of bending strength gives you plenty of notice. The steel yields, the beam sags, cracks open up on the tension face and anyone walking underneath can see something is wrong. A beam that runs out of shear strength does not do any of that. A diagonal crack opens near the support and the end of the beam simply drops off, often with almost no deflection first.

    That is why the strength reduction factor for shear is 0.75 rather than the 0.9 used for bending. The code deliberately builds in a larger margin because the consequences of getting it wrong are worse and the behaviour is less predictable.

    It is also why the sensible design philosophy is to make sure shear never governs. If a beam is going to reach its limit somewhere, you want it to be in bending.

    Where the numbers come from

    Concrete carries some shear on its own, through the uncracked compression zone, through aggregate interlocking across the crack faces and through dowel action in the main bars. Rather than model those separately, the code lumps them into a single term proportional to the square root of the concrete strength and to the web area.

    Stirrups pick up the rest. The mental model is a truss: the concrete forms diagonal compression struts running at roughly forty five degrees between the cracks, and the stirrups are the vertical ties holding those struts together. A diagonal crack at forty five degrees crosses a length of beam roughly equal to the effective depth, so the number of stirrups crossing it is d divided by the spacing. Each one can pull its full yield force, which gives the familiar expression for the steel contribution.

    Halve the spacing and you double the steel contribution, which is why the answer comes out as a spacing rather than a bar size.

    The design shear is taken at a distance d from the face of the support rather than at the support itself. The reason is physical: a diagonal crack starting closer than d to the support runs straight into the support and the load is carried directly by compression, so it never really loads the stirrups.

    The limits matter as much as the strength

    There is an upper cap on how much the stirrups can contribute. Past that point the diagonal concrete struts crush before the stirrups can yield, and adding more steel achieves nothing at all. When the tool says the section is too small, that is the check that failed, and the only fixes are a wider web, a deeper section or stronger concrete.

    There is also a cap on spacing, independent of strength. A diagonal crack has to be crossed by at least one stirrup to be any use, so the spacing cannot exceed half the effective depth. When the steel is working hard that cap tightens to a quarter of the depth, because the cracks are more numerous and steeper.

    Finally there is a minimum area rule. Even where the concrete alone is sufficient, most beams carry light stirrups, because the concrete shear capacity is a statistical average and an unlucky beam can fall below it. The exception is shallow members like slabs, where a shear failure would be local rather than sudden.

    A worked check you can repeat on paper

    Take a 300 by 550 beam, 40 mm cover, 10 mm stirrups, 20 mm main bars, 28 MPa concrete and 420 MPa stirrup steel.

    Effective depth is 550 minus 40 minus 10 minus 10, which is 490 mm. Concrete shear is 0.17 times the square root of 28 times 300 times 490, which is 132.2 kN, or 99.1 kN after the 0.75 factor.

    Suppose the design shear at the critical section is 250 kN. The stirrups must carry 250 divided by 0.75 minus 132.2, which is 200.4 kN. Two legs of 10 mm give 157.1 square millimetres, so the spacing is 157.1 times 420 times 490 divided by 200,400, or 161 mm.

    Check the caps. The threshold at which spacing tightens is 0.33 times the square root of 28 times 300 times 490, which is 256.7 kN, and the required 200.4 kN is below it, so the cap is half the effective depth, or 245 mm. The calculated 161 mm governs, and rounding down gives 150 mm centres.

    Related Calculators and Reference