Structural Engineering Fundamentals | Development Series

A four-story wood-framed apartment building developed cracking in interior partitions within 18

months of occupancy. The investigation traced the issue to a load path discontinuity introduced during construction: a beam that had originally been specified as continuous across two bays was reframed as two separate beams during framing, but the splice was not designed for the redistributed loads. The repair required temporary shoring of two stories above the affected area, removal of finishes on three levels, and installation of a properly designed transfer beam. The cost was approximately $180,000, and the original framing change had been an undocumented field decision. The structural drawings still showed the continuous beam. Today, written from a construction forensics practitioner: the three foundational concepts in structural engineering — loads (the demands placed on a structure), load paths (how those demands travel from where they are applied to the ground), and lateral systems (the elements specifically designed to resist sideways forces from wind and seismic events).

What Structural Engineering Actually Does

And what forensic investigations reveal when it does not work

Structural engineering is the discipline within civil engineering that designs the load-resisting components of buildings and other structures. The structural engineer's work determines what holds the building up, how it resists lateral forces from wind and earthquakes, and how loads are transmitted from where they are applied down to the supporting soil. In forensic practice, the cases that come across my desk most often involve one of three things: a load that was not correctly characterized, a load path that was incomplete or interrupted, or a lateral system that was either inadequate as designed or compromised during construction or modification. Understanding these three concepts is the foundation of evaluating any structural concern. ℹ Why a Forensic Expert Cares About These Fundamentals

Most structural distress I investigate traces back to one of the three fundamentals, not exotic technical issues

Loads vary by region, occupancy, and use
Load paths must be continuous; the most common defect I observe is an incomplete or improperly modified path

Lateral systems are frequently misunderstood by owners and contractors, leading to unintended modifications during renovation

When investigation begins, the structural drawings rarely tell the whole story — what was actually built often differs

Loads — The Demands Placed on a Structure

Five categories defined by ASCE 7

A load is any force, moment, or deformation imposed on a structure. ASCE 7 — Minimum Design Loads and Associated Criteria for Buildings and Other Structures — defines five fundamental load categories that every commercial design must address.

Figure 1 — The five fundamental load categories per ASCE 7 and IBC Chapter 16. The governing load combination, not any single load, typically dictates member sizing.

Understanding Each Load Category

Dead loads (D): The permanent gravity loads from the structure itself — the weight of beams, columns, slabs, walls, roofing, fixed equipment, and permanent finishes. Dead loads are calculated from material density and component dimensions. They do not vary significantly over time.

Live loads (L): The variable gravity loads from occupants, furniture, movable equipment, and other items that are not part of the building structure itself. ASCE 7 Table 4.3-1 specifies minimum design live loads by occupancy: office spaces are typically 50 psf; assembly areas, 100 psf; storage occupancies, 125 psf or higher depending on goods stored.

Snow loads (S): Gravity loads from snow accumulation on roofs. Ground snow load varies by geographic

location and is mapped in ASCE 7 Chapter 7. Roof snow load is calculated from ground snow load with adjustments for exposure, thermal conditions, roof slope, and drift accumulation at obstructions.

Wind loads (W): Lateral and uplift pressures from wind, which depend on basic wind speed (mapped in

ASCE 7 Chapter 26 by risk category), building geometry, exposure category, and other factors. Wind loads include both pressure on windward surfaces and suction on leeward surfaces and roof areas.

Seismic loads (E): Inertial forces induced in the structure by ground motion during an earthquake. Magnitude depends on the seismic hazard at the site (mapped values in ASCE 7), the mass and stiffness of the structure, the soil conditions, and the seismic-force-resisting system. Seismic Design Category (SDC) ranges from A (very low hazard) through F (very high).

ℹ Load Combinations — The Practical Heart of ASCE 7

Structures are not designed for one load at a time; they are designed for combinations of loads acting simultaneously

ASCE 7 Chapter 2 specifies how loads are combined using either LRFD (Load and Resistance Factor Design) or ASD (Allowable Stress Design)

LRFD applies factors to each load: e.g., 1.2D + 1.6L + 0.5(Lr or S or R) is one common combination
ASD uses unfactored loads with reduced material strengths: e.g., D + L is one ASD combination
The governing combination — the one producing the largest demand — dictates member sizing
In forensic work, recalculating actual loads (vs. assumed) using current combinations is often a key step in determining cause

Load Paths — The Continuous Chain From Roof to Soil

The single concept most often involved in structural distress investigations

A load path is the route by which a load travels from the point where it is applied to the foundation and ultimately into the supporting soil. Every load — gravity from a person walking on the floor, wind pressure against an exterior wall, snow on the roof — must have a continuous, structurally adequate path to the ground.

Figure 2 — A simplified gravity load path from roof to bearing soil. Common failure modes observed in forensic investigations are listed on the right.

The Six Stages of a Typical Gravity Load Path

1. Applied loads: Snow, live load, dead load of the roof assembly itself. These loads are distributed across the roof surface as area loads (pounds per square foot). 2. Roof joists and rafters: The first structural elements to receive the area loads. Joists collect distributed roof loads and transfer them as concentrated reactions at their bearing points — typically a wall or beam below. 3. Beams and floor slabs: Beams collect concentrated loads from joists and transfer them to columns. Floor slabs perform a similar function for floor loads and also act as diaphragms for lateral load distribution. 4. Columns: Concentrated loads accumulate as we descend through the structure. A column at the ground floor of a multi-story building carries the sum of all loads from the levels above plus its own weight. 5. Foundation: Loads from columns and load-bearing walls transfer into the foundation system — typically footings, mat slabs, piles, or piers, depending on the soil conditions identified by the geotechnical investigation. 6. Bearing soil: The final destination. The foundation distributes load into the soil, which must have adequate bearing capacity and not settle excessively. Soil-structure interaction is addressed by the geotechnical engineer and is a frequent topic in foundation-related forensic cases.

Common Load Path Deficiencies in Forensic Practice

From my investigation files, the following types of load path issues recur frequently. I am presenting these as patterns I observe, not as percentages — every case is different, and the proper investigation methodology depends on the specific facts. Connection deficiencies (most frequent)

85%

Modifications during renovation that removed

bearing elements

75%

Field changes that interrupted continuous members

70%

Diaphragm-to-shear-wall transfer deficiencies

60%

Foundation underdesign or unanticipated soil

conditions

50%

Errors in load characterization (use changes, etc.)

45%

ℹ The Forensic Question I Ask First

When I am called to investigate structural distress, the first question is rarely 'what failed' — it is 'what was the load path supposed to be'

Compare design drawings to as-built conditions; differences are often where the answer lies
Verify connection capacities at every transfer point along the path
Identify any post-construction modifications, including those undocumented in permits
Check whether the original loads still apply — change of use, added equipment, and added stories all matter

Look at the path as a chain: investigation focuses on identifying the weakest links, not just one element

Lateral Systems — How Buildings Resist Sideways Forces

Wind and seismic resistance is a separate engineering problem

A lateral system, also called a lateral-force-resisting system or seismic-force-resisting system, is the assembly of structural elements specifically designed to resist horizontal loads from wind and earthquakes. Gravity loads travel down; lateral loads travel sideways and need their own load paths. Most buildings use one of four lateral system types — or, more commonly for taller and more complex buildings, a combination of two or more. The choice affects cost, architectural flexibility, and structural performance, and is one of the earliest decisions a structural engineer makes.

Figure 3 — Four common lateral system types. Each resists lateral loads through different structural mechanisms with different trade-offs.

Understanding Each Lateral System Type

Moment-resisting frames: Beam-column connections are rigid, transferring moment between

members. Lateral loads are resisted by the bending stiffness of the framing itself. Provides maximum architectural flexibility because no diagonal braces or shear walls obstruct space. Special Moment Frames (SMF) are detailed for ductile behavior in high-seismic regions. Generally the most expensive option per pound of steel.

Concentric braced frames: Diagonal braces, typically in X or chevron (inverted V) configurations, intersect at the work points of beam-column joints. Lateral loads are resisted primarily by axial forces in the braces. Very stiff and efficient. Limitation: the diagonals occupy bay space and limit door/window placement. Used widely for low- and mid-rise commercial.

Eccentric braced frames: Diagonals are offset from the work points, creating a short 'link beam'

between them. Under lateral loading, the link beam yields in shear, providing both stiffness (from the bracing) and ductility (from the link). Allows architectural openings between braces. Common in highseismic mid-rise where both performance characteristics are needed.

Shear walls: Continuous walls of concrete, masonry, or wood shear panel that resist lateral loads

through in-plane shear and bending. Very stiff. Limit architectural openings within the wall. Often used at building cores around stairs and elevators, or as exterior walls in residential. Light-frame wood shear walls follow AWC SDPWS provisions; reinforced concrete shear walls follow ACI 318.

ℹ Lateral System Concepts to Be Aware Of

Diaphragm action: floor and roof systems must transfer lateral loads to the lateral system; the diaphragm itself is part of the system

Collectors and chord forces: tension/compression elements that move loads from diaphragm into the lateral system

Drift limits: structures must satisfy lateral deflection limits to control damage to non-structural elements and avoid occupant discomfort

Torsion: when lateral system stiffness is not symmetrical with the center of mass, lateral loads induce twisting; intentional and unintentional torsion both matter

Redundancy: ASCE 7 includes provisions that reward redundant lateral systems and penalize systems with limited alternate load paths

P-Delta effects: as the building drifts laterally, gravity loads acting through the displacement create additional moment that must be considered

Commercial vs. Residential: How Structural Engineering Differs

Same physical principles, very different design approaches

The physics of loads, load paths, and lateral systems applies identically to commercial and residential buildings. What differs is the design methodology — commercial structures are engineered, while most residential construction follows prescriptive rules in the IRC. AREA

🏢🏢 COMMERCIAL (IBC)

🏠🏠 RESIDENTIAL (IRC)

Design Approach

Engineered design per ASCE 7 loads; member

sizing per AISC 360, ACI 318, AWC NDS, or TMS 402 as applicable

Prescriptive provisions in IRC Chapters 3, 5, 6

cover most situations; engineering required for unusual conditions

Lateral Design

Formal lateral analysis required: wind tunnel

for taller buildings; dynamic analysis or response spectrum for seismic

Wall bracing per IRC R602.10 or engineered

design; performance generally adequate for typical conditions

Structural Drawings

Sealed structural drawings showing every

member, connection, and detail; calculations package retained by EOR

Drawings may show structural in plan with

limited detail; reliance on prescriptive code tables for sizes

Special Inspections

Required per IBC Chapter 17 for many

materials and conditions: high-strength bolts, welding, concrete, masonry

Generally not required by IRC; standard

building inspections cover most situations

Connection Detailing

Engineered connections shown on drawings; high-strength bolts or qualified welds; sealed details

Prescriptive nailing schedules (IRC Table

R602.3(1)); proprietary connectors with manufacturer data

AREA

🏢🏢 COMMERCIAL (IBC)

🏠🏠 RESIDENTIAL (IRC)

Forensic

Investigations

Investigations often involve recalculation

against current ASCE 7; review of EOR's design assumptions

Investigations often focus on whether IRC

prescriptive provisions were followed and whether they applied

Modifications Risk

Renovations require structural EOR review; load path implications of any wall removal must be analyzed

Homeowners frequently remove walls without

engineering review; common source of distress in older homes

Performance

Objective

Code minimums plus often-additional Ownerdriven serviceability criteria (drift, vibration, deflection)

Code minimums; serviceability typically not

engineered beyond what IRC provides

Key Observation From Forensic Practice

A pattern I observe in residential investigations: a load-bearing wall is removed during renovation to create an open floor plan, but no structural engineer is consulted. The wall above settles, doors begin to stick, drywall cracks appear, and the homeowner does not connect the dots until the damage is visible. The repair often costs many times what the original structural review would have cost.

Structural Awareness Checklist for Owners and Their Advisors

Items to verify on existing buildings and during new construction

These items reflect what I look for during forensic investigations and what owners can reasonably ask their design professionals to address. The checklist is structured side-by-side for commercial and residential projects. 🏢🏢 COMMERCIAL (IBC)

🏠🏠 RESIDENTIAL (IRC)

☑ Structural drawings sealed by licensed engineer of

☑ Code edition applicable to project (IRC vs IBC)

record

☑ Design loads documented (ASCE 7 edition, parameters)

☑ Seismic Design Category and Risk Category identified ☑ Lateral system clearly identified on drawings

☑ Special inspection program per IBC Ch 17 documented ☑ Connection details for critical members provided

☑ Structural observation by EOR during construction

☑ Engineered design used where IRC prescriptive fails ☑ Wall bracing per IRC R602.10 or engineered plan

☑ Wind and seismic design criteria confirmed for site

☑ Footing design verified for local soil and frost depth ☑ Beam sizes from IRC tables or engineered

☑ Engineered connectors (joist hangers, hold-downs) shown

☑ Nailing schedules per IRC Table R602.3(1)

☑ Field RFIs documented and incorporated into record drawings

☑ Any value engineering changes reviewed by EOR ☑ As-built deviations from drawings documented ☑ Foundation design reflects geotechnical recommendations

☑ Load path continuity verified at framing transitions ☑ Diaphragm and collector details on drawings

☑ Any wall removal reviewed by licensed engineer ☑ Renovation permits obtained where required

☑ Cantilever and deck construction per IRC R507 ☑ Roof framing per IRC tables or engineered

☑ Existing condition concerns documented before purchase

☑ Visible distress (cracks, deflection) evaluated by engineer

☑ Building drift checked against code/serviceability limits

Composite Forensic Cases

Patterns observed in structural investigations

The following are composite scenarios assembled from patterns I have encountered in forensic practice. Specific project details have been changed; the technical issues represented are typical of real cases. 🏢🏢 COMMERCIAL

Reframed Beam Without Splice Design

A four-story wood-framed apartment building developed cracking in interior partitions within 18 months of occupancy. Investigation revealed that a continuous beam spanning two bays had been reframed in the field as two beams with an intermediate splice on a single column. The original design assumed the beam was continuous, which produces different reaction forces than a simply-supported splice arrangement. The redistributed loads exceeded the column's design capacity, leading to small but measurable settlement that telegraphed through the upper stories as cracking. OUTCOME: Repair required temporary shoring of two stories above, removal of finishes on three levels, and installation of a properly designed transfer beam. The cost was approximately $180,000. The field change had not been documented in the construction RFI log or reviewed by the structural engineer of record. A 30-minute conversation with the EOR at the time of the framing change would have prevented the issue. 🏠🏠 RESIDENTIAL

Removed Bearing Wall During Renovation

A 1950s single-family home was renovated to create an open floor plan between the original kitchen and living room. A contractor removed what was assumed to be a partition wall; no engineer was consulted. Approximately 14 months later, the homeowner observed doors sticking on the second floor and hairline cracks © 2026 Construction Science Forensics • Page 10 of 12

in the second-floor ceiling. Investigation revealed the removed wall had been load-bearing, supporting a continuous joist run. The joists had deflected approximately 3/4 inch under sustained load, which had caused the observed cosmetic damage and was approaching serviceability concerns for the floor above. OUTCOME: Repair required installation of a properly sized LVL beam supported on engineered posts to columns to foundation, plus cosmetic restoration of the affected floor finishes. Total cost was approximately $42,000. A pre-renovation engineering consultation would have identified the wall as load-bearing for under $1,500 and resulted in proper transfer beam installation during the original renovation. 🏢🏢 COMMERCIAL

Use Change Exceeding Design Live Load

A second-floor office space was converted to file storage by the tenant. The original design live load for office space (50 psf per ASCE 7) was substantially exceeded by the actual load from dense file storage (which can reach 150+ psf). The floor system began to show measurable deflection within months. The condition was identified during routine building inspection; no occupant injury occurred. Investigation confirmed that floor framing capacity had been exceeded, though the structure had not failed in a strength sense — the deflection was a serviceability concern that, left unaddressed, could have progressed. OUTCOME: The tenant relocated the file storage to a ground-floor space with adequate capacity; supplemental framing was installed in the affected area to address residual deflection. The forensic review highlighted that use changes within commercial spaces must consider original design loads. The investigation cost approximately $8,500, and remediation totaled $35,000. Both were avoidable with structural review prior to the use change.

Key Takeaways for Owners and Their Advisors

✓ Practical Recommendations from Forensic Practice

Treat your structural drawings as a living document — every field change should be reviewed and incorporated

Engage the structural engineer of record whenever a field change affects member sizes, connections, or load paths

On commercial projects, require special inspections per IBC Chapter 17 where applicable; do not waive them

On residential projects, do not remove walls without engineering review — the cost of evaluation is low; the cost of remediation is not

When changing use, evaluate whether the new occupancy live loads exceed the original design

If you observe cracking, deflection, sticking doors, or sloping floors, engage a structural engineer; do not paint over the symptoms

Before purchasing commercial property, request the EOR's design package and a structural condition assessment

Before purchasing residential property, an experienced inspector can identify likely structural concerns warranting engineering review

If you have encountered a structural issue that traced back to an undocumented

modification or load path concern, I would be interested to hear about it. These patterns are useful to share.

Back to home Request an investigation