Rebar in Concrete Beams: Placement Basics

In a concrete beam, the main rebar sits near the bottom face, where bending pulls the concrete apart, with more bars along the top over supports and in continuous spans. Stirrups wrap around those bars along the length, and every bar sits inside a specified layer of concrete cover. Getting a beam right on site comes down to four things: the right bars in the right faces, the cover to the stirrups, enough gap between bars for concrete to flow, and a cage that stays put while the concrete goes in.

This post covers how beam steel is arranged and placed. How the bars are sized and detailed belongs to the engineer and detailer; our posts on choosing rebar sizes for beams and columns and rebar detailing for beams and columns go into that side.

Where the bars go in a beam

Picture a cross-section of a simple beam carrying a floor. Under load it sags, so the bottom stretches and the top squeezes. Concrete handles the squeeze well and the stretch badly, so the steel goes where the stretch is.

The arrangement flips at supports. Where a beam runs continuously over a column or wall, the top of the beam is in tension over the support, which is why top bars there are heavier and longer than the bars at midspan. Placing extra top bars at midspan or short bottom bars past the supports is a classic site error that an inspector will catch, so place each bar mark exactly where the placing drawing shows it.

From the outside of the beam in, the order is always the same: the formwork face, the concrete cover, the stirrup, then the main bars. That order matters for every dimension that follows.

Concrete cover in beams

Cover protects the steel from corrosion and fire and gives the bars enough concrete around them to bond. CSA A23.1 sets the minimum cover by exposure class. The common values for beams:

Two points trip people up:

• Cover is measured to the nearest bar, which is usually the stirrup. The main bars sit further in by the stirrup diameter. With 30 mm cover and 10M stirrups, the bottom bars sit roughly 41 mm above the soffit to their underside.
• The drawings govern. Fire ratings, exposure class and the engineer's notes can all call for more than the minimum. Our concrete cover guide explains the exposure classes, and rebar placement tolerances covers how much deviation is allowed.

Spacing between bars and between layers

Concrete has to flow between the bars and fully surround them, or the bars cannot bond and voids form where nobody can see them. CSA A23.1 sets the minimum clear distance between parallel bars at the largest of:

• 1.4 times the bar diameter,
• 1.4 times the nominal maximum size of the coarse aggregate, and
• 30 mm.

The same clear distance applies between a contact lap splice and the bars or splices next to it, which is why lap locations matter in a crowded beam.

When the bars needed will not fit in one row, the designer puts them in two layers. The upper layer must sit directly above the lower layer, not staggered, so concrete and vibrators can get down between the columns of bars. The gap between layers is held during placing with spacers or short separator bars, as the drawings or placing drawings show. Our upcoming post on minimum clear spacing between rebar in beams works through how many bars fit in common beam widths.

Stirrups: placing them on site

Stirrups do two jobs in a beam: they resist shear, and they hold the main bars at the right height and position. A few placing basics:

• Spacing zones. Shear is highest near supports, so stirrups are usually closest there and spread out toward midspan. The beam schedule gives each zone. Our post on stirrup spacing in beams and columns explains the limits.
• The first stirrup. RSIC standard practice places the first and last stirrup no more than 50 mm from the face of the support, unless the drawings say otherwise.
• Tolerance. RSIC practice permits a variation of 25 mm either way in stirrup placing. Small shifts to clear a conflict are normal; dropping stirrups or opening up a zone is not.
• Open or closed. Open U-stirrups are common where a slab is cast on top. Closed stirrups are used where the beam resists torsion or the design calls for them, and RSIC notes that stirrups shown in edge and spandrel beams must be closed, with a bar in each corner.
• Size. RSIC practice details stirrups 80 mm less than the beam width and depth, out to out, unless the drawings say otherwise. That allowance is what leaves room for cover on each face.

If you are unsure of the difference between a beam stirrup and a column tie, see rebar stirrups vs ties.

Holding the cage in the form

A beam cage that is perfect when tied can still end up wrong if it moves during the pour. The steel has to be supported at its cover height and held against floating, sagging and drifting toward one side of the form.

• Bottom support. Bottom bars and stirrups sit on beam bolsters or individual chairs at the correct cover height, or on precast blocks where the beam is cast on soil. Wood, broken brick and stones are not supports.
• Side cover. Wheel or clip-on spacers on the stirrups keep the cage off the side forms.
• Tying. Bars are tied at enough intersections to keep the cage rigid. RSIC practice uses 16-gauge wire unless the engineer specifies otherwise.
• Building in place or dropping in. RSIC notes the best way to assemble closed stirrups around beam bars is to preassemble the whole cage and drop it into the form. Where the beam bars have to thread through tight column steel, crews sometimes spring closed stirrups open to feed bars in, which can crack the bar, so two-piece closed stirrups (a U with a cap tie) are often detailed instead.

Do not cut, bend or relocate bars to clear an obstruction without the engineer's approval. Cutting a bar to fit a sleeve weakens the beam at exactly that spot.

Laps, column joints and congestion

Beam bars rarely run one piece end to end. Where they lap matters: bottom bars are usually spliced near supports and top bars near midspan, the opposite of where each set is working hardest. Our guide to lap length and splice zones covers both the lengths and the locations.

Where a beam frames into a column, the beam's top and bottom bars pass through or anchor inside the column core, alongside the column's vertical bars and ties. That joint is the most congested spot in most concrete frames. Placing drawings show the sequence and which bars go inside which, and following that sequence saves a lot of pulling bars back out.

Tie beams and grade beams

A tie beam connects footings, pile caps or columns so they move together. When it sits at or below grade it is usually called a grade beam, and RSIC uses the abbreviations TB and GB on placing drawings. The rebar looks like any other beam: continuous top and bottom bars with stirrups or ties, sized by the engineer.

What changes is the detailing at the ends. In buildings designed for seismic loads, CSA A23.3 (the 2004 edition we checked) requires grade beams acting as horizontal ties to have continuous longitudinal bars that are developed into or past the supporting column, or anchored in the pile cap or footing, at every discontinuity. For grade beams that are not connected to a slab, it also sets a minimum depth and requires closed ties at a spacing of not more than half the smallest cross-section dimension or 300 mm, whichever is smaller. Confirm the edition and clauses your engineer is designing to.

Because grade beams are cast against earth, the 75 mm cover for concrete cast against soil applies on those faces, and supports have to bear on the ground without sinking.

A pre-pour check for beam steel

Before the inspector arrives, walk each beam with the placing drawing:

1. Right bar marks in the right faces, with extra top bars over the correct supports.
2. Cover correct on the bottom and both sides, measured to the stirrups.
3. Clear gaps between bars and layers, with the upper layer directly above the lower.
4. Stirrup count and spacing zones match the schedule, first stirrup near the support face.
5. Laps in the right zones and at full length.
6. Cage tied, supported and unable to float or shift.
7. Bars clean of mud, oil and loose scale.

Getting beam steel right from the start

Most beam problems on site trace back to the bar list: stirrups bent to the wrong size, top bars cut short, laps landing in the wrong zone. Send us your structural drawings through our quote request form and we will prepare a free take-off and quote, then detail, cut and bend the bars from 10M to 55M, with stirrups and ties bent to size, tagged by bar mark and delivered across the GTA and Ontario. We also supply and install rebar. See our rebar detailing, cut and bend services for the full scope.