Diagonal Crack Above a Window Lintel
A four-year-old two-storey load-bearing brick building has a 1.5 m wide ground-floor window in its 230 mm external wall. The RCC lintel over the opening is 1.7 m long, giving only 100 mm bearing at each end, and no concrete sill was provided below the window. A diagonal crack now runs from the top corner of the opening upwards at roughly 45 degrees to the roof slab. It is 2 to 3 mm wide at the top and hairline at the bottom. There is no crack in the plinth, in the footing, or on the inner face away from the opening, and the crack has not widened noticeably during three months of tell-tale monitoring. The owner wants the crack simply filled and plastered before repainting, and has asked whether the family can keep living on the first floor while the work is done. The junior engineer must advise on the cause, on safety, and on what repair is technically justified.
1. Establish the most probable cause of the crack and state whether the building can be occupied while repairs are planned. Justify both answers from the evidence given.
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Cause. The lintel transfers its load to masonry over a very short bearing - 100 mm at each end. Masonry spreads that load downwards and outwards at roughly 45 degrees, so a short bearing concentrates stress in a small triangle of brickwork directly under the lintel ends. Combined with the absence of a sill, which normally ties the masonry below the opening, the stressed triangle above the bearing splits along its natural 45-degree line. That produces exactly the observed pattern: a diagonal crack from the top corner of the opening running up and away.
Why not the alternatives.
- Foundation settlement would show a crack continuing into the plinth and typically mirrored on the inner face; there is none, and the crack is local to the opening.
- Thermal movement produces horizontal cracks near the roof slab and at building corners, not a 45-degree crack stopping at an opening.
- Plaster shrinkage gives hairline map cracking, not a 2 to 3 mm crack that follows a structural line.
Occupancy. The evidence points to a stable serviceability defect, not imminent collapse: the load path still exists, there is no distress elsewhere, and tell-tales show no movement over three months. Occupation can continue with monitoring, provided the lintel soffit shows no crack or visible deflection and the masonry under the bearings does not sound hollow. If the crack grows, or the lintel deflects or cracks, prop the opening and vacate above it immediately.
2. Set out the repair you would specify, and say why simple filling and plastering is not acceptable.
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Why filling alone fails. A 2 to 3 mm crack is a live path for rainwater. Water reaching the lintel bearing corrodes the reinforcement, the products of corrosion expand, and the bearing spalls. Plaster over an untreated crack also re-cracks within one monsoon because the movement has not been restrained and the crack edges are not keyed.
Repair sequence:
- Open the crack into a V-joint, clean out dust and loose mortar, and wet the surface.
- Stitch across the crack with 8 mm bars about 300 to 450 mm long, set in drilled and grouted holes at 45 degrees to the crack, at 300 mm centres, or use a proprietary helical/GFRP stitch. This restores tensile continuity across the failure line.
- Fill and finish with a non-shrink grout, then apply a 300 mm wide galvanised mesh or fibre mesh strip bedded in mortar over the repaired line before plastering.
- Improve the bearing: cast a small RCC corbel or pilaster under each lintel end, or fix a steel angle bracket, so the load is spread over at least 150 mm; alternatively replace the lintel with a longer one if it is cracked or deflected.
- Provide a concrete sill band below the window on the same principle for the future, and check the other openings in the wall for the same defect.
Specify monitoring tell-tales for three months after the repair, and do not pay for plastering until the crack has stayed closed.
3. State the bearing and detailing rule you would apply to lintels on the remaining works, with the reasoning behind it.
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Rule. Give every lintel a clear bearing of not less than 150 mm on solid masonry, and never less than the minimum permitted by the project specification or IS 1905, whichever is more onerous. Bear on the full wall thickness. Where a 150 mm bearing cannot be achieved, provide a designed bearing pad or corbel rather than trimming the lintel.
Reasoning. The bearing is not a point support; masonry under it acts as a spreading medium. A longer bearing engages more of that spreading triangle and reduces the edge stress that opened the crack. It also gives the lintel room to rotate slightly under load without crushing the brick arris.
Detailing points that matter as much as the dimension:
- Keep the bearing clear of chases, conduits and hollow blocks; a chase running through the bearing line removes the very material carrying the load.
- Provide a concrete bearing pad or a minimum of two courses of solid brickwork under each end where the masonry is of low strength.
- Ensure the lintel is long enough that both bearings sit beyond the 45-degree line projected from the reveal.
- Add a sill band below large openings in external walls to tie the masonry beneath.
- Show the bearing dimension on the structural drawing and check it at the shuttering stage, not after the masonry above is built.