Structural Frame

The structural frame is the skeleton of columns, beams and their connections that carries a building's floors, roof and envelope to the foundations. Frames are reinforced concrete (cast in place or precast), structural steel, composite steel and concrete, timber (sawn, glulam and, increasingly, cross-laminated timber), or combinations. The frame's condition depends on what protects it: concrete cover protecting reinforcement, coatings and fire protection on steel, and dryness for timber.

The era and type of frame tell you where to look. Car parks deserve particular attention: they combine exposed concrete, chlorides carried in on vehicles, water ponding and cycles of wetting and drying, and some older designs are vulnerable to punching shear failure around columns.

What shortens life:

• Low cover to reinforcement, which lets carbonation and chlorides reach the steel early.
• Chlorides: added as accelerators in older concrete, carried in on vehicles from salted roads, or from sea air.
• Water: leaking roofs, failed membranes, ponding car park decks and blocked drainage.
• Alterations: openings cut, columns removed, tendons cut, loads changed.
• Fire, which weakens concrete, steel and timber.

Carbonation-induced corrosion

Concrete protects steel through its alkalinity. Carbon dioxide from the air slowly neutralises it from the surface inward; when this carbonation front reaches the reinforcement and moisture is present, the steel corrodes. The rust expands, cracking and spalling the concrete along the bars. It is slow, widespread and most common where cover is thin.

Chloride-induced corrosion

Chlorides break down the protective layer on steel even in alkaline concrete, producing localised pitting that can remove a large part of a bar's section with little surface sign. It is the main cause of serious deterioration in car parks, marine structures and concrete built with chloride accelerators, and can be structurally significant before spalling appears.

Alkali-silica reaction

Some aggregates react with alkalis in cement to form a gel that swells with moisture, cracking concrete in a map pattern and sometimes causing expansion that distorts elements. It develops over decades and needs moisture.

High alumina cement and other problem concretes

High alumina cement concrete, used in British precast beams in the 1950s to 1970s, loses strength as it converts with age, heat and moisture, and led to collapses in the 1970s. Reinforced autoclaved aerated concrete planks are covered on the roof structure page.

Punching shear

In flat slabs without beams, the slab around each column carries very high shear. Reinforcement corrosion, deterioration and added load (soil and landscaping on podium decks, resurfacing) can lead to a slab punching through around a column, and once one column fails the load can cascade. Older car parks and podiums are the main concern.

Steel corrosion

Steel frames corrode where water reaches them: perimeter columns and beams behind leaking façades, steel encased in masonry, connections and base plates in damp basements, and exposed steel in car parks and industrial buildings.

Alterations

Openings cut through slabs and walls, cores drilled for services, columns removed, beams notched, post-tensioning tendons cut, and heavier loads from new uses all change how the frame carries load. Many are done without engineering.

Fire damage

Fire spalls concrete and reduces the strength of exposed reinforcement, buckles and weakens steel, and chars timber. The structure may stand after a fire with reduced capacity.

On site

• Cracks running along the line of reinforcement, with rust staining; spalled concrete exposing bars.
• Map cracking with gel deposits, suggesting alkali-silica reaction.
• Cracks radiating around columns on the top of car park and podium slabs, and deflection near columns.
• Water ponding on car park decks; failed joints and membranes; staining on soffits.
• Corroded steel at bases, connections and perimeter; rust bleeding through encasement.
• Sagging beams or slabs; cracks in partitions below.
• Openings, cores and cut elements not shown on drawings.

In the records

• Structural drawings and calculations.
• Previous condition surveys, concrete testing and repairs.
• Records of alterations and their engineering.
• Car park inspection reports where required.
• Fire history.

Findings are graded by what they demand: safety (can hurt someone now), failure (failed and damaging the building), condition (distress present and progressing), lifecycle (renewal due on age and exposure), deadline (a regulator or ordinance sets the date), check (investigate before sizing works) and nil (cosmetic or stable).

A structural engineer experienced in existing structures leads, with a concrete testing specialist for durability.

• Desk study: drawings, calculations, construction era and materials, previous surveys and repairs, alterations and their approvals.
• Visual and hammer survey of exposed surfaces, mapping cracks, spalls, rust staining and delamination.
• Cover survey by covermeter or radar, and reinforcement location before any drilling.
• Carbonation depth testing at a sample of locations.
• Chloride profiles from dust samples at increasing depths to the reinforcement.
• Half-cell potential and resistivity mapping to locate active corrosion in chloride-contaminated areas.
• Cores for strength and petrographic examination for alkali-silica reaction, high alumina cement and other problems.
• Opening up at representative locations to measure section loss of bars and steel members.
• Structural assessment of capacity against current loads, including punching shear for flat slabs.
• Monitoring of deflection and crack movement where needed.

The report should state the deterioration mechanisms, their extent and rate, structural capacity, safety issues and a repair and protection strategy with timing.

• Patch repair: break out damaged concrete, clean or replace reinforcement, and reinstate with repair mortars to recognised repair principles. Patches in chloride-contaminated concrete can accelerate corrosion next to them (the incipient anode effect) unless combined with other protection.
• Cathodic protection: impressed current or galvanic anode systems that stop corrosion in chloride-contaminated concrete, widely used in car parks and marine structures.
• Electrochemical treatments: realkalisation of carbonated concrete and chloride extraction.
• Protective coatings and membranes: anti-carbonation coatings on facades and traffic membranes on car park decks to keep water and chlorides out.
• Drainage: fix falls, outlets and joints on car park decks to stop ponding.
• Strengthening: fibre-reinforced polymer wraps and plates, steel plates and brackets, additional columns or beams, punching shear reinforcement.
• Steel repair: blast, repair and recoat; replace badly corroded members; open up and treat encased steel.
• Fire damage repair after assessment.
• Load control: restrict loads, remove surcharge from podiums.
• Demolition and replacement where repair is uneconomic or the structure is unsafe.

Investigation is a lump sum, with testing rates per sample. Patch repair is priced by area or volume of concrete, with access, propping and protection of the area below as major components; repairs in occupied car parks are phased by bay. Cathodic protection is priced by area with monitoring that recurs for its life. Membranes by area, recurring on their cycle. Strengthening is priced per element or as a lump sum.

The usual shape is two-part: investigation first, then a repair programme sized from it, often phased over years. Repairs to stop active deterioration are condition items; membranes and coatings are lifecycle; statutory inspections are deadlines; safety items are immediate.

• Australia. AS 3600 governs concrete structures, AS 4100 steel and AS 1720 timber; repairs follow recognised concrete repair practice; strata buildings carry structural repairs as common property, and New South Wales adds duties for building defects in apartment buildings.
• United Kingdom. The Eurocodes (BS EN 1992 concrete, 1993 steel, 1995 timber) govern design and BS EN 1504 concrete repair and protection; the Institution of Structural Engineers publishes guidance on appraising existing structures; the Building Safety Act 2022 adds structural safety duties for higher-risk buildings; CROSS collects structural safety reports.
• United States. ACI 562 governs assessment and repair of existing concrete structures and the International Existing Building Code alterations; New York City requires periodic inspection of parking structures, and Florida requires milestone structural inspections of condominium buildings of three or more storeys and structural integrity reserve studies.

• Structural drawings and calculations.
• Previous structural surveys, concrete testing and repair records.
• Records of alterations, openings and coring, with engineering.
• Car park condition surveys and statutory inspection reports.
• Cathodic protection monitoring records where installed.
• Fire history and any post-fire assessments.
• For strata buildings, capital works or reserve study entries for structural repair.