Structural Core

The structural core and bracing system keep a building upright against sideways forces: wind, earthquake and the small imperfections that would otherwise let a frame sway and buckle. In multi-storey buildings this is usually a concrete core around lifts and stairs, sometimes with additional shear walls; in steel buildings it may be braced bays or rigid moment frames; in low-rise sheds and houses it is bracing in walls and roofs. Because these elements carry little vertical load, their importance is easy to overlook when cutting openings or removing walls.

Seismic design requirements have tightened over time in all three countries, most heavily on the US west coast. Older buildings in seismic regions, including soft-storey buildings with open ground floors over parking, can have lateral systems far weaker than current standards require.

Lateral systems rarely wear out. What shortens their effective life is change:

• Openings and penetrations cut through cores and shear walls for doors, ducts and pipes.
• Bracing removed in fit-outs and alterations, often because it obstructs a new layout.
• Corrosion of steel bracing and connections in exposed or damp areas.
• Added height or mass that increases lateral loads.
• Earthquake damage that weakens the system for the next event.

Openings and penetrations

Core and shear walls are cored and cut for new services, risers and doors throughout a building's life. Each opening removes concrete and may cut reinforcement; several near each other, or near wall ends and corners, can significantly reduce capacity and create weak points. Cores are often more penetrated than any other element.

Bracing removed

In warehouses, bracing bays are removed or relocated for new doors and racking; in offices, braced partitions are removed in fit-outs; in houses, bracing walls are removed to open up living areas. Without bracing, frames sway and can buckle or collapse in wind or earthquake.

Connection corrosion and detailing

Steel bracing connections corrode where water collects; older moment frame connections in seismic regions were found after the 1994 Northridge earthquake to fracture in a brittle way and have since been the subject of retrofit guidance.

Soft and weak storeys

Buildings with open ground floors (for parking or shopfronts) beneath stiffer upper floors concentrate earthquake movement in the ground storey, which can collapse. Many such buildings in California have been required to retrofit.

Construction defects

Cores and walls can have cold joints, honeycombing and poorly compacted concrete, particularly at congested reinforcement, and later reveal these through leaks and cracking.

Cracking and movement

Diagonal cracking in shear walls and cores indicates shear distress; cracking at the base or at openings can indicate overload or settlement; excessive sway in wind shows reduced stiffness.

On site

• Openings, cores and chases in lift and stair core walls; new risers and doors through shear walls.
• Missing bracing in warehouse bays; bracing rods slack, bent or disconnected.
• Diagonal cracking in core and shear walls.
• Occupants noticing sway in wind in taller buildings.
• Open ground floors under residential floors in seismic areas.
• Corrosion at bracing connections and base plates.

In the records

• Structural drawings showing the lateral system.
• Records of coring and openings with engineering.
• Fit-out drawings showing bracing changes.
• Seismic evaluations and retrofit orders.
• Post-earthquake inspections.

• Penetrations in core walls grouped closely together around new risers, or near wall ends and corners, where they matter most.

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 leads; a seismic specialist joins in seismic regions.

• Drawing review to identify the lateral system and compare it with the building as it now is.
• Penetration survey of cores and shear walls, with scanning to locate reinforcement before any new coring.
• Bracing survey in steel and timber buildings for missing, altered or damaged members.
• Condition survey for cracking, corrosion and concrete defects.
• Structural analysis of lateral capacity with the openings and changes accounted for.
• Seismic evaluation to the recognised method for existing buildings in seismic regions.
• Monitoring of sway or crack movement where concerns exist.

The report should state the lateral system's adequacy, the effect of alterations, any urgent measures, and strengthening options.

Where drawings are missing, the lateral system may have to be inferred from the structure itself, which takes opening up and scanning.

• Temporary bracing wherever bracing has been removed, until permanent replacement.
• Reinstated or relocated bracing designed by an engineer to suit the new layout.
• Strengthening around openings with steel frames, fibre-reinforced polymer or added concrete.
• Seismic retrofit: new shear walls or braced frames, strengthened connections, soft-storey strengthening, base isolation for important buildings.
• Connection repair and recoating of corroded bracing.
• Concrete repair of defects and cracks.
• Controls on future penetrations: a permit system and penetration register for cores and shear walls.

• Post-earthquake assessment and repair: rapid safety evaluation, then detailed assessment and repair of damaged elements before reoccupation.
• Documentation: mark core and shear walls on fit-out drawings and on site so trades know not to cut them.

Assessment and analysis are lump sums. Temporary bracing is an immediate cost. Strengthening and seismic retrofit are lump-sum projects, often large, which may require tenants to vacate. Bracing reinstatement in warehouses is per bay. Penetration management is a low-cost procedure that avoids large future costs.

Safety items are immediate; ordinance retrofits carry deadlines; strengthening is a condition or code compliance item; recoating is lifecycle.

Seismic retrofits often attract incentives or are tied to ordinance timetables, which affect funding and programme.

• Australia. AS/NZS 1170.2 sets wind actions and AS 1170.4 earthquake actions; AS 3600 and AS 4100 govern concrete and steel; the National Construction Code requires structural adequacy for alterations; earthquake design has applied nationally since the 1990s.
• United Kingdom. Approved Document A, including requirements to avoid disproportionate collapse, and the Eurocodes govern design; seismic design is rarely required; the Institution of Structural Engineers publishes guidance on appraisal of existing structures.
• United States. ASCE 7 sets wind and seismic loads and ASCE 41 the seismic evaluation and retrofit of existing buildings; the International Existing Building Code governs alterations; Los Angeles and San Francisco require soft-storey and non-ductile concrete retrofits.

Insurance and lending in seismic regions increasingly depend on a building's seismic assessment in all three markets where earthquake risk is material.

• Structural drawings showing the lateral system.
• Records of coring and openings in cores and shear walls, with engineering.
• Fit-out and alteration drawings affecting bracing.
• Seismic evaluations, retrofit orders and completion records.
• Post-earthquake inspection reports.
• Penetration permits or register.

• Earthquake insurance terms and any seismic assessment used for lending.