Bridge topics · Substructure

Hammerhead Pier Cap: Geometry, Strut-and-Tie Design and Detailing

One column, a wide cantilevered head and every girder reaction carried out to the tips: the hammerhead pier is the workhorse of highway viaducts, and its cap is a classic strut-and-tie problem.

What a hammerhead pier is

A hammerhead pier is a single column, or a single wall, topped by a cap that cantilevers out on both sides to pick up the girders. Seen from the road below, it looks like a hammer: a narrow shaft and a wide head. It is the default pier for highway overpasses and urban viaducts because it needs only one column footprint at ground level, leaving room for the roads, railways or rivers underneath, while still giving the deck a wide row of bearings at the top.

The cap is the interesting part. It carries every girder reaction out to its tip and brings it back into the column, so it works as a pair of short, deep cantilevers. That short, deep shape is exactly where ordinary beam theory stops being reliable.

Geometry and proportions

The geometry is set by the deck above and the clearances below:

• Cap length follows the deck width and the outer girder lines, usually reaching to just beyond the exterior bearings.
• Column width in the transverse direction follows the clearance and the moment from unbalanced live load; on a typical two-lane to four-lane highway deck it is often 1.5 m to 3 m.
• Cantilever length from the column face to the outer bearing therefore ends up in the range of a few metres, and the cap depth at the column face is commonly of the same order as the cantilever, so the ratio of shear span to depth sits around 1 or below.
• Depth at the tip is usually reduced to about half the root depth, giving the tapered underside that defines the look of the pier.

These are indicative ranges from common highway practice, not limits. Long caps for wide decks, railway decks and caps that must stay shallow for clearance all move outside them, and that is when post-tensioning becomes attractive.

Why the cap is a D-region

Beam theory assumes plane sections remain plane, which holds well away from supports and concentrated loads. Near them, the strain distribution is disturbed. AASHTO LRFD calls these zones disturbed regions, or D-regions, and takes them to extend about one member depth either side of the discontinuity. In a hammerhead cap the bearings are concentrated loads, the column is a support, and the whole cap is only a depth or two long. Practically the entire cap, and the top of the column, is a D-region.

For such regions, codes direct the designer to the strut-and-tie method (STM). Instead of a bending and shear check, the cap is idealised as a truss:

• a horizontal tie along the top of the cap, formed by the main top reinforcement, carries the tension from bearing to bearing over the column;
• inclined concrete struts run from each bearing down into the column, carrying compression;
• nodes under the bearings and at the column are checked for concrete crushing, and the tie bars must be anchored beyond the outer node, which is why bars are hooked, looped or anchored with plates at the tip.

The tie force follows from simple trigonometry: for a bearing reaction P at a horizontal distance a from where the strut meets the column, and a lever arm z between tie and the strut node, the tie force is about P·a/z. Summing all bearings on one side gives the design tension for the top steel. The ASPIRE guidance on STM for piers shows that a statically determinate model of the cap alone, with the column replaced by equivalent nodal loads, is quick and accurate enough for design.

Laboratory work backs this up. University of Texas tests on reinforced concrete pier caps under concentrated bearing loads, reported for TxDOT and FHWA in 1995, found that the caps carried load as a tied arch, and that the strut-and-tie models matched the tests best while remaining conservative. The older corbel and deep-beam provisions underestimated capacity by a factor of about three to four because they counted only the concrete shear capacity.

Crack control and detailing

A cap designed by STM for strength still needs distributed reinforcement to control cracking. AASHTO requires a minimum orthogonal grid of crack control reinforcement in D-regions designed by STM, placed near each face. The common problem areas are:

• Re-entrant corners where the underside of the cap meets the column face. Stress concentrates in the corner, and diagonal cracks appear first there if the corner is not reinforced with diagonal or well-anchored orthogonal bars.
• Top tie anchorage at the tip. Straight bars with short anchorage beyond the outer bearing are a known weak point. In the Texas tests, a continuous perimeter loop of top steel improved both confinement and capacity, and caps with straight, short top bars showed bond distress.
• Bearing seats close to the edge, which need confinement steel and enough edge distance for the strut to spread.
• Congestion at the cap and column joint, where the column bars, the top tie and the stirrups all meet. Bar layout should be drawn in three dimensions early.

Prestressed caps

When the cantilever grows, the deck is wide, or the cap depth is restricted by clearance, the top tie becomes heavy and the crack widths hard to control. Post-tensioning the cap replaces most of the tie with tendons anchored at the tips. The prestress puts the top of the cap in compression under permanent load, which closes cracks, reduces the reinforcement and allows a shallower, lighter cap. It also introduces anchorage zones at the tips, which need their own bursting and spalling reinforcement, and a stressing sequence that suits the erection of the girders above.

Construction

Hammerhead caps are usually cast in place on falsework or on brackets fixed to the completed column, so that no props are needed down to the ground, which matters over live traffic. The column is cast first in lifts, the cap soffit form is supported on the brackets or on a steel shoring frame, and the cap is cast in one or two pours depending on its depth. On accelerated projects the cap is precast in one or more pieces and connected to the column with grouted bars or ducts, which shortens the time over the road below. Bearing plinths are cast last, to the exact levels needed for the girders.

Checklist for the designer

• model the cap with STM and check the tie, the struts and every node;
• anchor the top tie fully beyond the outer bearing;
• provide crack control reinforcement on both faces and at the re-entrant corners;
• check unbalanced live load and construction stages with girders erected on one side only;
• consider post-tensioning when the cap must be shallow or the cantilever is long;
• check that bars, ducts and bearings fit together in three dimensions before issue.

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Questions engineers ask

What is a hammerhead pier?

A bridge pier made of a single column or wall with a cap that cantilevers out on both sides to support the girders. It keeps a small footprint at ground level while giving the deck a wide row of bearings.

Why are hammerhead caps designed with strut-and-tie models?

The cap is short and deep and carries concentrated bearing loads near a support, so plane sections do not remain plane. AASHTO treats such disturbed regions with the strut-and-tie method, which represents the cap as compression struts and a tension tie.

Where do hammerhead caps crack?

Most often at the re-entrant corners where the underside of the cap meets the column, at the tips where the top tie is poorly anchored, and near bearing seats close to the edge.

When should a hammerhead cap be post-tensioned?

When the cantilevers are long, the deck is wide or the cap must stay shallow for clearance. Post-tensioning reduces the top reinforcement, controls cracking and allows a lighter cap.

How is a hammerhead cap built over traffic?

Usually cast in place on brackets or a shoring frame fixed to the completed column, so no props are needed to the ground, or precast and connected to the column with grouted bars on accelerated projects.

Sources

• ASPIRE, Fall 2020: Strut-and-Tie Modeling (Concrete Bridge Technology)
• Denio, Yura and Kreger (1995), Behavior of Reinforced Concrete Pier Caps Under Concentrated Bearing Loads, CTR Report 1302-1
• Tuchscherer, Brown and Bayrak (2010), Bridge Pier: Hammerhead Bent Cap, ACI SP-273 (TRID record)

Updated 2026-10-25 · written by a practising bridge engineer