Upper Floor Slabs & Structures

Upper floors carry occupants, furniture, partitions, storage and equipment and transfer them to beams, walls and columns. They are reinforced concrete flat slabs or beam-and-slab floors, post-tensioned slabs, precast hollowcore planks, composite slabs on steel decking, timber joists and boards, and, more recently, cross-laminated timber panels. Each has characteristic weaknesses: deflection and cracking in concrete, tendons in post-tensioned slabs, bearings in precast, corrosion of decking, and notching, decay and bounce in timber.

Floors are designed for imposed loads that depend on use: offices, homes, assembly areas, storage, libraries and plant rooms all differ. A change of use or tenant can put loads on a floor that were never designed for it, particularly compactus and mobile shelving, archives, server racks, gym equipment and water tanks.

What shortens life:

• Overload from changed uses, storage and heavy equipment.
• Alterations: openings cut, cores drilled, joists notched for services.
• Water from leaking wet areas, roofs and plant.
• Long-term creep in concrete, which keeps deflections growing for years.

Floors rarely reach the end of their structural life; their limits come from the loads placed on them and the holes cut through them.

Deflection and creep

Concrete floors continue to deflect for years after construction as concrete creeps. Long spans and thin slabs sag; partitions below crack, doors bind, and on balconies and podiums water ponds. Deflection is usually a serviceability problem, but large or increasing deflection can signal loss of capacity.

Overload from changed use

Mobile shelving, archives, plant, water features and heavy equipment can exceed design loads by a wide margin. Overloaded floors crack and deflect, and in extreme cases fail.

Post-tensioned tendon damage

Drilling and coring into post-tensioned slabs without scanning can cut tendons, releasing them violently and removing part of the slab's capacity. Water reaching unbonded tendon anchorages at slab edges corrodes them.

Hollowcore and precast issues

Precast planks need adequate bearing on supports; short bearings and planks cut near their ends for services are vulnerable. Water trapped in hollow cores freezes or corrodes strands.

Composite decking corrosion

Steel decking under composite slabs corrodes where water leaks through from wet areas, car park decks or roofs, losing its role as reinforcement.

Timber floor defects

Joists are notched and drilled by plumbers and electricians, cutting through critical zones; joist ends decay where they bear on damp walls; floors are springy or bouncy where joists are undersized or have lost strutting; insects attack timber.

Vibration

Light, long-span floors in timber, steel and post-tensioned concrete can be lively under footfall, a comfort problem that matters for sensitive uses such as laboratories and hospitals.

On site

• Floors noticeably sloping or sagging; ponding on balconies and podiums.
• Cracks in partitions and ceilings below; doors binding.
• Cracks on slab soffits; rust staining from decking or reinforcement.
• Heavy storage, mobile shelving, plant or water features on upper floors.
• Cored holes and cut openings in slabs.
• Notched and drilled joists; springy floors; decay at joist ends.

In the records

• Structural drawings with design loads.
• Post-tensioning drawings.
• Records of coring and openings.
• Change of use approvals.
• Floor loading notices.

• Load notices on floors and racking, or their absence where heavy storage is present.
• Patched or filled holes in slabs whose history is unknown.

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, with scanning specialists for coring and concrete testing as needed.

• Load review: current and proposed uses against design loads, including specific heavy items.
• Level survey of deflections, repeated over time where growth is suspected.
• Scanning of slabs to locate tendons and reinforcement.
• Condition survey of soffits, decking, bearings and timber.
• Opening up of timber floors to inspect joists and bearings.
• Vibration measurement for sensitive uses.
• Capacity assessment for the intended use.

The report should state floor capacity against use, condition, any overload or damage, and strengthening options.

• Load testing in special cases where calculations cannot confirm capacity, carried out under engineering control.
• Cores and concrete testing to confirm strength and reinforcement where drawings are missing.

Investigations are best timed before a lease commits to a use, so capacity limits are known at negotiation.

• Load control: relocate heavy items over beams and columns, limit storage, post load notices.
• Strengthening: fibre-reinforced polymer on concrete, added steel beams, additional supports.
• Tendon repair by specialists where tendons have been cut or anchorages corroded.
• Decking repair and stopping leaks from above.
• Timber floor repair: sister or replace joists, restore strutting, repair bearings, replace decayed timber.
• Vibration control: added stiffness or mass, tuned mass dampers for special cases.
• Penetration controls: scanning and engineering for every new hole.

• Tenant controls: lease clauses setting floor load limits and requiring engineering approval for heavy equipment and storage.
• Screed and topping replacement where cracked or debonded toppings add load without strength.

Assessment is a lump sum. Strengthening is priced per element or as a lump sum. Timber repairs per joist or per area. Load relocation is an operational cost. Scanning per area or per location.

Strengthening for a new use sits with the change of use; repairs are condition items; safety items are immediate.

The cost of a heavy-use tenancy often includes structural strengthening that should be recovered through the lease rather than absorbed by the owner. Scanning before every coring job is inexpensive compared with repairing a cut tendon.

• Australia. AS/NZS 1170.1 sets imposed floor loads by use; AS 3600 governs concrete and AS 1684 and AS 1720 timber; a change of use needs building approval showing the structure is adequate.
• United Kingdom. BS EN 1991-1-1 sets imposed loads; Approved Document A applies to material changes of use; the Concrete Society publishes guidance on post-tensioned floors.
• United States. ASCE 7 sets live loads; the International Existing Building Code governs changes of occupancy; ACI 318 governs concrete design.

In all three jurisdictions, leases and building management rules commonly set floor loading limits, and strata or condominium by-laws restrict alterations that cut slabs. Changes of use (for example, office to residential, or office to laboratory) trigger structural review under each country's rules for existing buildings, and may require strengthening before approval. Post-tensioned slabs need specialist contractors for any repair or alteration.

• Structural drawings with design loads.
• Post-tensioning drawings.
• Coring and opening records.
• Change of use approvals.
• Floor load notices and tenant equipment schedules.

• Lease clauses on floor loads.
• Engineering approvals for heavy tenant equipment.
• Deflection monitoring records where available.
• Timber pest and decay reports for timber floors.