24-Bld-A7 Building Envelope Design · December 2017

Question 6 of 6

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Question 6 (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part A — identifying the thermal bridge. It shows a wall built up of an inner insulation layer and an outer masonry/veneer layer, through which a reinforced-concrete floor slab projects continuously to form an exterior balcony — the slab is a single, uninterrupted pour from the heated interior floor all the way out past the exterior cladding line, with no visible break in the insulation layer where the slab crosses it. The thermal bridge is exactly that continuous slab: reinforced concrete has a thermal conductivity roughly two orders of magnitude higher than the insulation it interrupts, so the balcony slab provides a direct, low-resistance conductive path from the heated interior floor straight through the building's insulation layer to the outdoor balcony — heat flows out (and cold flows in) along the ENTIRE cross-section of the slab, not just at a point. Two secondary consequences follow directly from this primary bridge: (i) the interior floor and ceiling surface immediately around the slab's penetration runs measurably colder than the rest of the room in winter, a comfort complaint and a localized condensation/mould risk at that surface if the interior humidity is not tightly controlled; and (ii) the balcony slab itself, being thermally connected to the (warmer, humidity-buffered) interior, sits above the true outdoor temperature, subtly altering its own freeze–thaw and de-icing-salt exposure pattern versus a fully-isolated exterior element.

New design to eliminate the bridge. The slab cannot simply be cut, since it is also the primary structural connection carrying the balcony's gravity load and resisting the horizontal/uplift loads on it; the fix is a STRUCTURAL thermal-break module (e.g. a Schöck Isokorb-type unit) inserted at the building line, replacing the continuous concrete pour with: compression bearing elements (small, high-strength concrete or stainless bearing pads) that carry the downward load across a narrow insulation gap; stainless-steel shear/tension reinforcing bars cast into both the interior slab and the exterior balcony slab, running through (and only through) a thin, high-performance insulation module rather than through solid concrete; and a continuous layer of insulation surrounding the module so the wall's insulation layer is restored to being continuous everywhere except the small residual bridge of the reinforcing bars themselves (a far smaller, more manageable bridge than the full slab cross-section). Where the balcony load and geometry allow it, an even more complete fix is to de-couple the balcony structurally altogether — support it on its own independent columns or steel brackets bearing on the foundation/facade rather than cantilevering from the interior slab at all — which removes the thermal connection entirely rather than merely reducing it. Either approach keeps the insulation layer continuous across the building line, which is the design intent the as-found cross-section fails to achieve.

Part B(1) — the "Crumbling Concrete" failure mechanism. The case study (David H. Nicastro, P.E.) describes a 20-storey building whose precast concrete window sills began spalling and cracking roughly seven years after completion. The panels were poorly fabricated — internal voids, honeycombing, and weak structural details at the narrow reinforcing line, with plaster-type patching material and even wood substituted in some spots during original fabrication — and, critically, the panels were never waterproofed to reduce or stop further deterioration once exposed. Once a crack initiated (from the fabrication defects and normal thermal/moisture movement), rainwater was carried into it; when that water froze, hydraulic pressure wedged the crack open further; wind-driven rain and de-icing chemicals splashed up onto the sill and were carried into the same cracks, and each freeze–thaw cycle progressively enlarged the damage until sections of concrete began spalling and falling. The case study's own "Sharing the Blame" section attributes the failure to a fabrication-quality gap that was never verified before or after installation — nobody caught that the panels were built with internal defects and shipped without any surface protection to compensate.

Proper precast window-sill design to prevent recurrence. (1) Positive drainage slope — the sill's top surface must be sloped a minimum of roughly 6% (about 1:16) away from the window frame toward the exterior, so water cannot pond against the frame or on the sill itself. (2) A drip edge (kerf) at the outer, underside edge of the sill, so water reaching the underside cannot travel by surface tension back along the soffit and re-enter the wall below — it is broken off cleanly instead. (3) Waterproofing the panel — a penetrating sealer or, better, a liquid-applied membrane on the sill's top face and returns (and on the face of the panel above/below at the joint) BEFORE installation, so incidental cracking does not give water a direct path into the concrete's pore structure in the first place; this is the step the as-built failure skipped entirely. (4) Backer rod and sealant at every panel-to-panel joint, sized and detailed per ASTM C1472 exactly as in Q4, so the joint itself does not become the crack-initiation point. (5) Adequate concrete cover over reinforcement (verified during fabrication, not assumed from the shop drawings) — the case study's fabrication defects (voids, honeycombing, substituted materials) are exactly the kind of quality gap that a documented pre-pour and post-strip inspection regime, with the structural engineer or a qualified inspector actually verifying cover and consolidation, would have caught before the panels ever left the plant.