Errors in Load Assumptions and Their Impact on Structural Design

Date of last update: 04.09.2025

A structural design results from three critical components: accurately assumed loads, a well-constructed analytical model, and carefully refined execution details. A mistake in any of these areas can undermine even the best architectural concept. Below—in our METIB team—we present a practical overview of the most frequent pitfalls we observe during third-party project reviews and design supervision.

Table of contents:

1. Why erroneous load assumptions are critical
2. Common sources of errors in loads
3. METIB’s Operational Approach to Avoiding Load Errors
4. Practical Examples: Typical “Surprises” in Practice
5. Investor / General Contractor Checklist
6. Red Flags (Act Immediately)

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Why erroneous load assumptions are critical

Incorrectly defined loads "skew" the entire calculation model from the outset, simultaneously compromising safety, usability, constructability, and cost. That’s why METIB emphasizes the load matrix—because a single initial mistake can multiply risks at every subsequent stage of design and execution.

Safety and compliance with standards
Underestimated loads (e.g. snow, wind, live loads) lead to under-design of sections, connections, and foundations. This increases the risk of local failures, buckling, excessive deflections, and—in extreme cases—progressive collapse. Overestimated loads, on the other hand, result in over-dimensioning and unnecessary costs.

Serviceability (SLS)
Even when ultimate limit state (ULS) checks pass, incorrect loads often manifest in serviceability: excessive deflections, vibrations (affecting occupant comfort or equipment), cracking in reinforced concrete, or roof leaks (e.g., due to ponding phenomena).

Interdisciplinary coordination and construction stage
If erection loads or temporary supports are not accounted for, an element may meet the standards post-installation—but fail during installation. This often leads to claims and delays on site.Lifecycle considerations and change of function
With too narrowly defined load assumptions, the structure lacks reserves for future loads (for example, tenant changes from office to warehouse), complicating later expansions or fit‑outs.

Common sources of errors in loads

Errors in load assumptions rarely stem from a single oversight—they typically accumulate from minor simplifications, data gaps, and shortcut decisions. These issues appear at the intersection of changing functional requirements, local climate conditions, technological needs, and construction realities—the very point where disciplines and project stages converge.

Poor classification of live loads
Treating a space classified as "office" when it will be used as an archive or warehouse (which requires different characteristic values).

Ignoring concentrated loads from equipment (e.g., battery racks, high-storage shelving, VRF units, IT racks).

Outdated or improperly interpreted climate data and exposure
Selecting snow/wind zones based on rough maps without checking local topography, elevation, and exposure (open vs. built-up areas).
Not accounting for wind pressures and drifts at parapets, skylights, dormers, or diff erential roof heights.
Neglecting water loading due to clogged drains or ponding on waterproof membranes.

Omitted exceptional loads and long-term effects
Thermal effects (shrinkage, creep, temperature gradients in spans and facades).
Seismic loads (even low) relevant for non-structural elements, shelving systems, tanks, installations.
Accidental loads (e.g., impacts in dock zones).
Settlements or landslides (due to groundwater changes, excavation dewatering).

Incomplete data from technology suppliers
Masses and dynamic parameters of devices (e.g., fans, compressors, pumping units, overhead cranes).
Loads from cable trays and ducts—often added at the end and frequently conflicting with beams or ribs.

Errors in load combinations and factors
Mixing ULS and SLS combinations; confusing partial vs. combination factors (γ, ψ0/ψ1/ψ2).
Automatic FEM settings without manual review—models may automatically downplay secondary influences.

Lack of staged (construction-phase) thinking
Loads during transport, storage, and erection (e.g., asymmetrically lifted prestressed concrete slabs, steel frames without temporary bracing).
Non-simultaneous loading (e.g., roofs loaded with ballast before ring beams or braces have cured).Scope drift and mismatches
Tenant changes, layout or installation updates after design completion without updating the load matrix.
Premature "freezing" of load values when the functional program is still evolving.

METIB’s Operational Approach to Avoiding Load Errors

Avoiding load errors is not about luck—it’s about disciplined process, from day one. Below is a set of practical measures we implement at every stage (from load matrix to independent review) to minimize risk and maintain control over changes.

• Load Matrix from Day One
  Create a "Load Matrix" (spreadsheet + BIM model): documenting who defines what load where, with assigned responsibility (architect, services, supplier, structural).
  Use ranges (min–max) for sensitive inputs and perform sensitivity tests: “How much does the section size increase if load is +20%?”
  
• Respect local and climatic conditions—no shortcuts
  Check snow/wind zones and exposure, considering height and topography. Analyze critical roof areas (local drifts, edge zones).
  Always include a scenario of drain blockage (water ponding) in flat roof design.
  In tall, slender buildings—verify dynamic wind-induced vibrations (SLS), not only ULS forces.
  
• User requirements and technology validated with tenant/supplier
  Request device datasheets (mass, operating frequencies, dynamic coefficients, support points).
  For warehouse zones—design rack systems (type, loads per column, spacing) before sizing floor slabs or columns.
  If program is uncertain—integrate local structural reserves (e.g., strengthened slabs in modular zones).
  
• Separate ULS and SLS combinations and calibrate the model
  Clearly list ψ-factors.
  Perform manual sanity checks of key sections before trusting FEM outputs.
  Include second-order (P‑Δ) analysis for slender systems and verify global stability.
  Avoid simplifying connections to fully rigid or pinned unless justified.
  
• Stage-based planning and temporary states
  Define erecting loads and temporary bracing schemes (dedicated chapter in documentation).
  For precast elements—include lift and transport scenarios: lifting hook locations, moment growth in elements.
  
• Change control and version tracking
  Every change in function/tenant = recompile load matrix and auto-update BIM load schedules.
  Weekly "Loads & Open Issues" sync (15 minutes) with architect and MEP teams: to‑do list with task owner.
  

Independent review (four-eyes principle)
A second structural engineer verifies load assumptions and combinations—not just section results.
For atypical projects (e.g., wind girders, cranes, heavy mechanical equipment), commission a techno‑dynamic review from supplier.

Practical Examples: Typical “Surprises” in Practice

Even with well-defined loads, the real world can introduce surprises—especially when tenant changes, erection details, or site realities come into play. Below are common case studies we encounter in METIB reviews and supervision, with lessons reflecting quick corrective actions rather than costly rework.

• Converting offices into archive storage
  Assumed office loads proved insufficient for shelving systems and point loads after tenant change—resulted in post–structural-work reinforcement.
  Lesson: Always inquire about potential fit‑out scenarios—leave modular reserves.
  
• Snow drift at parapet + clogged drain
  Roof design neglected local drifting and ponding. After thaw, membrane sagged and leaked.
  Lesson: Mandatory scenarios: ventilation/drainage checks, drift zones, emergency drainage design.
  

Overhead crane with updated dynamic parameters
Supplier at tender stage changed crane parameters, increasing accelerations. No update in model caused under-design of connection joints.
Lesson: Every vendor proposal or parameter change should trigger load matrix update and recheck of critical connections.

Investor / General Contractor Checklist

• Functional program with future-use scenarios (reserves).
  
• Current geotechnical data (groundwater, settlements).
  
• Confirmed climate maps and exposure by designer.
  
• Equipment catalogue (masses, dynamics, support points, operating cycles).
  
• Roof design guidelines: drainage, local drifts, maintenance.
  
• Erection plan: work sequence, temporary bracing, equipment constraints.
  
• Change regime: who approves and when the load matrix is updated.
  
• Red flags: items that require immediate reaction (see below).

Red Flags (Act Immediately)

These raise alarm that the model may calculate the wrong outputs—or that costs and schedules may spiral:

• “Let’s do what we did in the previous hall—it worked.” (Different location, different wind/snow/exposure!)
  
• Absence of ψ‑table and combination list in technical documentation.
  
• Missing erection loads.
  
• “Device TBD”: no load ranges or support points defined.
  
• Single generalized roof load without zoning or drift analysis.