Assessing the Structure of an Older Building Before You Buy or Extend

What an engineer actually does to establish whether an existing frame can carry more, and the order in which an owner should buy that information.

Establishing what an existing structure can carry is a staged investigation, not a single opinion. It begins with whatever records exist, moves to measurement of what is physically present, then to testing of material strength and reinforcement, and only then to analysis. Buying the stages in the wrong order is how owners pay for a full investigation of a building that a records search would have ruled out in a week.

Before stage one

Define the question before engaging anyone. Whether a structure is safe for its current use, whether it can carry an additional floor, whether a wall can be removed, and whether an older frame meets current seismic and loading expectations are four separate questions needing different depths of work. An engineer asked to assess the structure in general will either overspend or produce a report that answers none of them.

• Write the structural question as a sentence stating the intended use or the intended change.
• Search for original drawings, calculations and any permit records held by the owner or the authority.
• Establish the approximate construction date, since it indicates the code generation in use.
• Identify any alterations made since construction, particularly removed walls or added loads.
• Confirm access for investigation, including whether finishes may be opened and made good.

Stage 1: Desk study of available records

Collect original drawings, structural calculations, soil investigation reports, permit documents and any previous survey. Where drawings exist, they establish the structural system, member sizes and the design assumptions, which can shorten the investigation dramatically. Where they do not, the assessment becomes a measurement exercise and the budget rises accordingly.

• Deliverable: A record pack with a statement of what is available and what is missing.
• Owner: Owner, with the engineer advising what to look for.
• Approval gate: A decision on whether records allow a desk based opinion or whether survey is required.

Stage 2: Visual inspection and structural layout survey

An engineer walks the building to establish the structural system, identify the load path from roof to foundation, locate columns, beams and load bearing walls, and record visible distress, deflection, cracking and corrosion. This also identifies alterations: a wall removed in a previous fit out, a slab opening formed for a stair, or an added mezzanine that nobody declared.

• Deliverable: A measured structural layout with visible defects recorded and photographed.
• Owner: Structural engineer, with safe access provided by the owner.
• Approval gate: The structural system is understood well enough to plan the testing.

Stage 3: Non destructive investigation

Cover meter and reinforcement scanning locates bars and measures cover. Rebound hammer and ultrasonic methods give an indication of concrete uniformity. Half cell potential mapping indicates corrosion activity. These techniques are quick and non damaging, and they tell you where to concentrate the destructive testing, which is the expensive part.

• Deliverable: Scanning results mapped onto the structural layout with areas of concern marked.
• Owner: Specialist testing contractor under engineer direction.
• Approval gate: Targeted locations identified for sampling rather than random testing.

Stage 4: Destructive sampling and material testing

Cores are taken and tested for compressive strength, reinforcement is exposed at selected points to confirm diameter and condition, and chloride and carbonation profiles are measured where corrosion is suspected. Sampling is limited, located to avoid weakening the structure, and made good afterwards. The results convert assumptions into values that can be used in analysis.

• Deliverable: A laboratory report with strength results and durability measurements.
• Owner: Testing laboratory, with results issued directly to the engineer.
• Approval gate: Enough results to characterise the structure rather than one sample from one element.

Stage 5: Foundation and ground investigation

Where additional load is proposed, the foundations and the ground become the governing question. Trial pits expose foundation type and depth, and boreholes with testing establish the ground conditions and bearing capacity. An older foundation designed for three floors may be the limit on adding a fourth, regardless of how sound the frame above is.

• Deliverable: A geotechnical report with foundation details and allowable bearing pressures.
• Owner: Geotechnical specialist, instructed through the structural engineer.
• Approval gate: Foundation capacity established before any scheme for additional load is designed.

Stage 6: Analysis against current requirements

The engineer models the structure using the measured geometry and tested material properties and checks it against the loading and seismic provisions of the applicable code for the intended use. An older building designed under an earlier code generation may be adequate for its current use and inadequate for a change of use that increases imposed load or occupancy.

• Deliverable: A structural assessment report stating capacity, deficiencies and the code basis used.
• Owner: Structural engineer, signing the assessment.
• Approval gate: A clear statement of what the structure can and cannot do, in writing.

Stage 7: Strengthening options and cost

Where capacity falls short, options range from local strengthening with steel or fibre reinforced polymer to adding new elements, enlarging foundations or reducing the proposed loading. Each should be costed and assessed for disruption, because a technically elegant solution requiring the building to be emptied may be worse commercially than a heavier one that can be built in phases.

• Deliverable: Costed strengthening options with disruption and programme implications stated.
• Owner: Engineer with a contractor providing indicative pricing.
• Approval gate: The owner can compare strengthening cost against the value of the intended change.

Handovers between stages

The joint where money is wasted is between stages one and four, when an owner authorises full testing before the records search and the visual inspection have narrowed the question. The joint where risk is created is between the assessment and the design of the new work, because an assessment prepared for one scheme does not transfer to a different one. If the proposal changes, the assessment has to be revisited rather than reused.

How you know it is finished

The assessment is complete when it answers the question you posed, states the code basis and the material values used, declares its own limitations and the areas not investigated, and either confirms capacity or prices the strengthening needed. Code provisions, including loading and seismic requirements, are updated over time, so confirm the current applicable version with the reviewing authority before designing any change, and take advice from a licensed adviser where the findings affect a purchase contract or a permit application.

Domira works on buildings that already exist: surveying what is installed, specifying the systems that have to be replaced, and running maintenance and refurbishment work around the people using the property. Tell Domira which asset you are responsible for and we will say what we would inspect first.