Structural Design Pitfalls (The Ones That Actually Cost Money)
Structural design mistakes fall into two categories: dramatic failures that make news, and chronic problems that just cost money and create headaches. The dramatic ones are rare—buildings don’t collapse often if they’ve had any engineering input at all. The chronic problems though? See these constantly.
Most structural issues aren’t about safety per se. They’re about performance—excessive deflection, cracking, vibration, settlement, water intrusion from poor detailing. Building is technically safe but performs poorly and needs expensive repairs. These problems come from inadequate design, poor execution, or both.
Foundation Problems That Show Up Later
Foundation issues are expensive to fix because foundation is literally at bottom of everything. Can’t easily access it, can’t replace it without massive disruption. Getting foundation right initially is critical because correcting mistakes later costs fortune.
Inadequate geotechnical investigation is root cause of many foundation problems. People make assumptions about soil conditions without actual testing, then design foundation based on those assumptions. When assumptions are wrong—and they often are—foundation doesn’t perform as expected.
Soil bearing capacity can vary dramatically even within small site. One corner might have competent soil, another corner weak clay. Foundation designed for uniform conditions fails where soil is weaker. Only way to know is actual borings or test pits with engineering analysis.
Seasonal variation in soil conditions matters here. Soil properties during dry season differ from wet season. Clay especially changes dramatically with moisture content. Foundation designed based on dry season site visit might be inadequate for wet season conditions.
Differential Settlement
Uniform settlement—entire building settling evenly—is less problematic than differential settlement where different parts settle different amounts. This causes structural distress, cracking, doors and windows that bind, tilted floors.
Differential settlement comes from non-uniform soil conditions, varying foundation loads, asymmetric foundation design. Column footings sized for average load when actual loads vary significantly—heavier loaded footings settle more, creating differential movement.
I’ve seen houses where one end settled 50-100mm more than other end. Cracks everywhere, floors noticeably sloped, doors that don’t close. Fixing this means underpinning—jacking up settled portions and installing deeper foundations underneath. Easily 1-2 million baht for moderate-sized house, plus disruption and repairs to damage.
Undersized Structural Members
Using members that are too small is less common than it used to be—computer analysis and codified design procedures prevent most egregious undersizing. But still happens, especially when cost pressure leads to minimizing member sizes.
Beams and slabs that meet strength requirements but deflect excessively under load. Structure is safe but performs poorly—cracked tile, doors that bind, visible sag in ceilings, bouncy floors. Technically compliant with code minimum but inadequate for good performance.
Long-span floors are particularly problematic. People want open spaces without intermediate supports, so they span long distances. But achieving this with minimal beam depth creates deflection and vibration issues. Floor might be strong enough but feels unsafe because it moves perceptibly when walking.
The Vibration Problem
Floor vibration is separate issue from static deflection. Floor can be stiff enough for static loads but still vibrate uncomfortably from walking. This is particularly true for lightweight floor systems—metal deck with thin concrete topping, lightweight concrete, wood framing.
Fixing vibration problems after construction is difficult and expensive. Adding mass helps but means loading structure more. Stiffening means adding beams or posts which disrupts spaces. Prevention through proper design is much better than post-construction remediation.
Inadequate Connection Details
Connections between members often don’t get attention they deserve. Members themselves might be sized adequately but connections fail because they weren’t designed properly or weren’t built as designed.
Concrete structures need adequate reinforcement through connections. Lapping lengths, development lengths, hooks, stirrups in joint regions—these details are critical for force transfer. Insufficient lap length means bars pull out under stress.
Construction errors compound design issues. Reinforcement not placed where shown on drawings, insufficient concrete cover, rebar cutting through joint regions. These mistakes happen regularly on sites without adequate supervision.
Steel connections need proper weld size and quality, or adequate bolt diameter and quantity. Field welding quality varies enormously depending on welder skill and supervision. Poor welds fail at loads well below design capacity.
The Inspection Gap
Critical connections often get concealed by finishes before anyone verifies they were built correctly. Concrete poured over reinforcement before checking placement. Steel connections covered by fireproofing or architectural cladding before weld inspection.
By the time problems manifest—years later when structure is loaded—impossible to verify what actually was built. Might have been done correctly and failure is from other cause, or might have been built wrong and that’s causing problem. Can’t tell without destructive investigation.
Ignoring Thermal Movement
Tropical climate means large temperature differentials—hot sun on exposed surfaces creates temperature differences of 30-40°C between heated and shaded portions. Materials expand and contract significantly with these thermal cycles.
Concrete and masonry expand when heated. Without adequate expansion joints, this movement creates stresses that crack materials. Cracks might be just cosmetic or might compromise waterproofing and durability.
Expansion joint spacing needs to account for expected temperature range, material properties, and restraint conditions. Too far apart and cracking occurs between joints. Joints themselves need proper detailing—sealants, backer rods, flashing—or they leak and defeat their purpose.
Differential Movement
Different materials move differently with temperature. Connecting materials with different thermal expansion coefficients creates interface stresses. Glass and aluminum move differently than concrete. Connections need to accommodate differential movement or something fails.
I’ve seen glass cracking from rigid attachment to concrete frames that moved differently. Metal panels buckling because fastening didn’t allow thermal movement. These problems are predictable and preventable through proper detailing, but details often get shortchanged.
Inadequate Lateral Bracing
Vertical load-carrying capacity gets attention but lateral resistance sometimes overlooked. Wind loads, seismic forces where applicable—structure needs lateral bracing system to resist these without excessive deformation.
Flexible structures rack under lateral loads, creating damage even if structure doesn’t collapse. Non-structural damage to finishes, glazing, partitions, cladding—all from excessive lateral movement. This damage is expensive to repair and recurs with each significant wind event.
Lateral system needs clear load path from where forces are applied through structure to foundation. Shear walls, moment frames, braced frames—whatever system is used needs to be continuous and adequately connected throughout height of building.
Torsional Effects
Lateral resistance that’s not balanced in plan creates torsional response—building twists under lateral loads. This concentrates stresses in certain locations and causes damage there while other areas are understressed.
Symmetric building forms and balanced lateral systems minimize torsion. Asymmetric buildings or lateral resistance concentrated on one side creates torsional problems. Can be addressed through design but needs to be considered—can’t just ignore it.
Water Management Failures
Water intrusion isn’t usually considered structural issue but causes structural damage over time. Water infiltrating concrete corrodes reinforcement, causing spalling and loss of section. Water in wall cavities rots wood framing. Trapped moisture causes deterioration of many materials.
Preventing water entry requires proper flashing, sealants, drainage, and detailing at connections and penetrations. These details are often poorly designed or incorrectly executed, creating chronic water intrusion.
Flat or low-slope roofs are particularly vulnerable. Ponding water finds any weakness in waterproofing and leaks through. Proper slope for drainage, quality waterproofing correctly installed, maintained drainage prevents most roof leaks.
Below-Grade Waterproofing
Basements or below-grade spaces need robust waterproofing because water pressure at depth is significant. Surface-applied coatings might be adequate for above-grade walls but insufficient for below-grade use.
Properly waterproofing below-grade structures requires drainage layer, waterproofing membrane applied to exterior of wall, protection board, and perimeter drainage system. Shortcuts in any of these components leads to water intrusion that’s difficult to fix after backfilling.
Corrosion Of Reinforcement And Steel
Steel corrodes in humid coastal environment. This is slow deterioration that compounds over decades. Coastal exposure accelerates corrosion dramatically compared to inland locations.
Concrete cover over reinforcement is primary protection against corrosion. Specified minimum cover isn’t suggestion—it’s critical dimension that needs to be maintained during construction. Inadequate cover means accelerated corrosion and spalling concrete.
Exposed steel—structural steel members, railings, connections—needs corrosion protection. Galvanizing, painting, stainless steel, or other appropriate protection for expected exposure and desired service life.
The Maintenance Factor
Corrosion protection needs maintenance. Paint systems deteriorate and need recoating periodically. Damaged galvanizing needs repair. Ignoring maintenance means corrosion progresses and eventually causes structural problems.
Budget for ongoing maintenance of corrosion protection is part of building ownership cost. Trying to avoid this maintenance just defers cost until much more expensive repairs or replacement needed.
Soil Conditions That Get Missed
Expansive clays that swell with moisture and shrink when dry create cyclical foundation movement. This is cumulative damage—each cycle causes small amount of distress, adding up over years to significant problems.
Solution is foundations that either go deep enough to reach non-expansive soil, or are designed to accommodate movement without distress. Shallow foundations on expansive clay inevitably have problems.
Organic soils, fill material, loose sand—these all have poor engineering properties and need special foundation design. But sometimes these conditions aren’t discovered until construction begins because geotechnical investigation was inadequate or skipped entirely.
Groundwater Surprises
Groundwater level affects foundation design and below-grade waterproofing requirements. High groundwater creates uplift on below-grade structures—need to design for buoyancy.
Seasonal variation in groundwater is common. Water table during dry season might be deep, but during wet season rises significantly. Design needs to account for highest expected groundwater level, not just conditions at time of site investigation.
Design-Construction Disconnect
Design done without considering constructability leads to details that are difficult or impossible to build properly. Congested reinforcement that can’t be placed, connections that can’t be accessed for welding, members that can’t be lifted into position—these create field problems.
When construction encounters unbuildable details, they modify in field to make it work. These modifications might compromise design intent or structural adequacy if not properly reviewed. Sometimes contractor contacts engineer for solution, sometimes they just improvise.
Having structural engineer available during construction to review field conditions and proposed modifications prevents problems. But this requires engineer being involved beyond just providing drawings—needs construction administration role.
The Substitution Question
Materials or products get substituted during construction for cost or availability reasons. Sometimes substitutions are equivalent, sometimes they compromise design. Engineer needs to review and approve substitutions to ensure they’re acceptable.
Substituting weaker concrete, smaller reinforcement, different steel grade—these affect structural capacity. Can’t just swap materials without engineering review confirming substitution is adequate.
Overconfidence In Standard Details
Standard details and typical sections work for typical situations. But every project has some non-typical aspects that need specific design attention. Using standard details everywhere without considering whether they’re appropriate for specific situation creates problems.
Edge conditions, unusual loads, difficult geometry—these need custom details developed for specific circumstances. Copy-pasting standard details from previous projects and hoping they work is risky approach that sometimes fails.
Cost-Cutting In Wrong Places
Reducing structural costs is legitimate goal but needs to be done intelligently. Value engineering that finds less expensive ways to achieve same performance is good. Just reducing capacity to barely meet minimum code is usually poor economy.
Buildings designed to absolute minimum typically have performance issues and shorter service lives. Spending somewhat more on structural design—better materials, more conservative sizing, robust details—pays back through better performance and lower maintenance costs.
Most expensive structural problems to fix are foundations and major structural members. Skimping on these to save modest costs during construction creates risk of expensive repairs later. Whereas spending on robust initial design is relatively cheap insurance.
Our Approach To Avoiding Pitfalls
At CJ Samui Builders, we’ve seen structural problems from inadequate design and poor construction—these experiences inform our approach to structural work. Start with adequate investigation of site conditions, engage competent structural engineers for design, maintain quality control during construction, and involve engineer in reviewing critical work.
Our structural design services emphasize understanding actual conditions through proper investigation, designing for both strength and serviceability not just minimum code compliance, detailing with constructability in mind, and maintaining engineer involvement through construction phase.
Because structural problems are expensive and difficult to fix after construction. Prevention through proper design and construction is far more effective than trying to remediate problems later. Most structural pitfalls are predictable and preventable—requires appropriate investment in design, quality materials, skilled construction, and adequate oversight. Cutting corners on structural work is false economy that typically costs far more in long run than doing it properly initially would have cost.
