This article explores common assumptions made during assessments. Each assumption can mislead even experienced engineers, leading to errors in judgment, design missteps, and, ultimately, unsafe recommendations.
Assessing existing building involves more than visual inspections or scanning through old plans. The process requires rigorous thought, careful judgment, and a deep understanding of construction realities. Yet, many engineers fall into the trap of making assumptions that weaken the reliability of their assessments.
The buildings we assess today were often built decades ago. During that time, materials aged, loads changed, and interventions occurred—sometimes undocumented. However, engineers still assume that what was designed is what exists, and what exists performs as intended. These assumptions quietly shape our engineering decisions.
This article explores common assumptions made during assessments. Each assumption can mislead even experienced engineers, leading to errors in judgment, design missteps, and, ultimately, unsafe recommendations. Understanding them is essential for anyone involved in rehabilitation, retrofitting, or safety evaluations of existing structures.
Assumption 1: Drawings Reflect Reality
Engineers often begin assessments by studying structural drawings. These documents should offer insight into framing, dimensions, and material types. However, running on the assumption that drawings are accurate can be misleading. Construction practices often deviate from design, either due to on-site decisions or constraints oftentimes unknown to the original.
In many cases, reinforcement is omitted, changed, or misplaced. Many buildings even lack complete records altogether. Hence, it is not unusual to find mismatches between drawings and reality, such as missing ties, relocated beams, or altered column sizes. Relying majorly on drawings assumes that construction followed design, which can be rarely guaranteed.
The only way to confirm existing conditions is through validation—using tools like ground-penetrating radar, cover meters, and selective demolition. Without this, engineers may design repairs or retrofits that don’t align with what truly exists.
Assumption 2: Materials Have Maintained Original Properties
Older buildings often contain concrete and steel that have aged well past their design lives. Many engineers assume the concrete has retained its compressive strength or the steel has not suffered section loss. This can lead to inaccurate assessments of capacity and load resistance.
In coastal or humid regions, chloride ingress and carbonation can severely reduce reinforcement life. Concrete may lose strength through leaching or chemical attacks. In fire-damaged buildings, steel may have lost its yield capacity due to elevated temperatures.
Unless samples are taken for testing—such as core testing for concrete or chemical profiling for reinforcement—these assumptions can cause serious overestimation of strength. Material degradation rarely follows uniform patterns, making blanket assumptions risky.
Assumption 3: Previous Modifications Were Properly Designed
Many buildings undergo changes over time—openings added, partitions moved, or slabs cut. These modifications many times often lack proper structural input. Yet, assessors may assume these changes were sound.
One dangerous case involves wall removals in load-bearing masonry structures. Removing a single wall without compensatory framing shifts load paths and may overload surrounding members. Similarly, drilling into slabs to install services may reduce punching shear capacity or weaken reinforcement zones.
Previous work should never be trusted without documentation. Engineers must review any signs of retrofitting, additions, or removals with skepticism. These actions should trigger targeted investigations, including load checks, deflection monitoring, or capacity analysis.
Assumption 4: Load Paths Are Still Intact
Load paths are critical in both analysis and reality. When engineers assess buildings, they often believe the original load paths are still functional. However, numerous interventions can interrupt or weaken these paths.
Partition walls moved or removed can redistribute floor loads. New equipment on roofs may introduce point loads that were never considered. Floor penetrations can sever key reinforcement paths. Misalignment in multistory columns can also result from accumulated construction tolerances or later additions.
These broken or altered load paths are invisible in software unless explicitly modelled to be so. Engineers must trace how each load travels—through slabs, beams, columns, and foundations. When paths become irregular or unsupported, redistribution must be quantified. Assuming the original paths holds can be dangerous.
Assumption 5: Foundations Are Unchanged and Sound
The foundation is often treated as an unchangeable constant. In assessments, engineers assume footing dimensions remain unchanged, settlement is negligible, and bearing pressure remains acceptable. However, decades of soil movement, loading changes, and water table fluctuations say otherwise.
In regions with expansive soils, long-term swelling and shrinkage can cause significant differential settlement. Excavations nearby may have altered lateral support. Foundations under new extensions may not match the originals in design or performance.
In many cases, foundation inspections are skipped altogether. Engineers must instead study crack patterns, monitor settlements, or undertake soil investigations. Blind confidence in unseen foundations may result in improper judgments about safety or suitability for future loads.
Assumption 6: Creep, Shrinkage, and Corrosion Are Negligible
Concrete undergoes long-term deformations—creep and shrinkage—that affect its structural behaviour. In long-span beams or slabs, creep can induce excessive deflections. If these effects weren’t fully accounted for at design, the structure could be nearing serviceability failure even if the strength is adequate.
Shrinkage induces internal tension, which can lead to cracking in restrained elements. These cracks accelerate water ingress and corrosion. Corrosion, in turn, reduces bar diameters and bond, both critical to capacity.
Engineers often assume these effects are minor if cracks aren’t visible. However, damage often occurs internally before it surfaces. Visual inspections alone can miss early deterioration. Measurement of deflection, concrete cover, and bar condition is essential to dispel any assumption that may exist about long-term performance.
Assumption 7: Code Compliance at Time of Construction Equals Safety Today
Many engineers judge a structure’s safety based on the codes it was designed to. They assume that meeting 1980s or 1990s standards still equates to acceptable safety. But codes evolve for a reason—due to lessons learned from failures, advances in research, or revised risk assumptions.
A building that met outdated seismic, wind, or fire codes may now be noncompliant. Older designs may lack ductility, redundancy, or robustness. For example, flat slab buildings with poor punching shear detailing were once common but are now being critically reviewed.
Code compliance at the time of construction does not automatically equal safety now. Engineers must re-evaluate key aspects against current standards—particularly where loads have increased or usage has changed. Structural assessments require modern benchmarks, not outdated checklists.
Assumption 8: What Looks Fine Is Structurally Fine
Perhaps the most dangerous assumption is equating visual soundness with structural safety. A building may show no cracks, no sagging, and no corrosion signs—but that doesn’t confirm integrity.
Many buildings conceal damage beneath finishes or behind cladding. Flat slabs may be overstressed but remain un-cracked. Corroded bars may be hidden beneath painted surfaces. Structural overloading may exist, particularly in repurposed buildings now supporting heavier equipment or denser occupancy.
Visual inspections are only the start. Engineers must dig deeper using measurement, testing, and modelling. Load tests, material sampling, and laser scans reveal what the eye cannot. Relying only on what appears sound can lead to undetected dangers.
How to Overcome These Assumptions
A careful, structured approach can mitigate these misleading assumptions. First, engineers must treat every undocumented detail as a potential deviation. Validation should become the default—through non-destructive testing, intrusive checks, and independent measurement.
Second, documentation from past work must be verified against site conditions. This includes comparing drawings to actual member sizes, materials, and reinforcement layouts. No assumption should stand without evidence.
Third, all assumptions should be listed and challenged during peer review. Engineers should also adopt probabilistic thinking—acknowledging uncertainty and designing with appropriate safety margins. Engaging with geotechnical engineers, testing labs, and experienced contractors can help validate field conditions.
Lastly, current codes should frame evaluations. Even if older buildings remain functional, safety is judged by modern standards. Rehabilitation and retrofitting efforts must meet today’s demands, not yesterday’s minimums.
Conclusion
Assessments of existing buildings requires more than knowledge of structural behavior. It demands vigilance against false confidence. Assumptions, while sometimes necessary, can obscure real problems when left unchallenged.
Every assessment must begin with doubt—not in the structure’s competence but in the assumptions that cloud our judgment. From missing reinforcement to outdated codes, unverified assumptions are the silent root of many failures. Dismantling them leads to clarity and better engineering.
The most responsible engineers do not assume—they investigate, validate, and question. That mindset keeps structures standing and people safe.
Also See: A Background to the Appraisal of Existing Buildings
Sources & Citations
- EN 1992-1-1: Eurocode 2: Design of Concrete Structures – General Rules and Rules for Buildings. CEN, 2004.
Institution of Structural Engineers (2021). Appraisal of Existing Structures. IStructE Publications.
Reynolds, T. & Steedman, J. (2019). Structural Design from First Principles. Red Globe Press.
Building Research Establishment (2014). Condition Assessment of Reinforced Concrete. BRE Digest.
