Chapter 6
Loads, Structural Behaviour and Building Systems
Bachelors Program in Construction Technology
Fundamentals of Construction Engineering
•
Bachelor
Loads, Structural Behaviour and Building Systems
Introduction
A building is a three-dimensional system that transfers gravity, lateral and environmental actions safely to the ground. Construction professionals must therefore understand not only how individual beams, columns and trusses resist force, but also how components connect into a continuous load path. A sound structural system must have adequate strength, remain stable during construction and service, limit deflection and vibration, and retain reasonable capacity when local damage occurs.
Structural design is normally performed according to national building codes and standards. Such codes specify load values, combinations, material properties, detailing rules, fire requirements and safety factors. The equations in this chapter are educational models for basic determinate systems; they do not replace code-based design or the work of a licensed structural engineer.
Learning Objectives
After studying this chapter, you should be able to:
distinguish dead, live, wind, seismic and construction loads;
trace gravity and lateral load paths through a building;
calculate reactions, shear forces and bending moments for basic determinate beams;
explain axial behaviour in columns and trusses;
distinguish stability, strength, serviceability and robustness;
describe the characteristic behaviour of concrete, steel, timber and masonry systems;
identify appropriate building systems and construction-stage risks; and
apply these concepts to preliminary construction planning and site decisions.
1. Loads and Structural Actions
A load is an external force, pressure or imposed deformation applied to a structure. An action is a broader term that includes loads and effects such as temperature change, shrinkage or support settlement. Loads may be classified by source, duration, direction and degree of uncertainty.
1.1 Dead load
Dead load is the permanent weight of structural and fixed non-structural components. It includes slabs, beams, columns, walls, cladding, ceilings, finishes, fixed services and permanently attached equipment. A preliminary gravity load may be estimated from unit weight:
W = γV
where W is weight (N or kN), γ is unit weight (N/m³ or kN/m³), and V is volume (m³). For a uniform slab, the area load is:
q = γt
where q is load per unit area (kN/m²) and t is thickness (m). Self-weight must be included consistently; omitting it can significantly underestimate demand.
1.2 Live load
Live load is variable imposed load from occupants, furniture, movable partitions, stored materials, vehicles and maintenance activity. Code values depend on occupancy and tributary area. Live load is generally less certain than dead load and may be reduced in some codes for large tributary areas, while storage and assembly areas may require high values.
1.3 Wind load
Wind produces pressure and suction on walls, roofs and exposed equipment. The resulting lateral forces depend on wind speed, terrain, building height, shape, openings, internal pressure and exposure duration. Wind can govern both overall lateral stability and local cladding connections. Uplift at roofs is particularly important because it reverses the usual gravity load direction.
1.4 Seismic load
Earthquake action results primarily from inertial forces caused by ground acceleration. A simplified equivalent lateral force concept is:
V = Cs W
where V is total design base shear, C_s is a code-defined seismic response coefficient, and W is effective seismic weight. This expression is applicable only within the assumptions and limits of the relevant seismic code. The seismic weight may include dead load and specified portions of imposed load. Lateral forces are distributed vertically according to code rules, commonly giving greater force to upper levels because of their larger height and, often, displacement.
Seismic design also depends on ductility, regularity, continuity, connection strength and control of torsion. A heavy but poorly connected building may perform worse than a lighter, well-detailed one.
1.5 Construction loads
Construction loads include workers, temporary storage, wet concrete, formwork, scaffolding, lifting operations, equipment, impact and partially completed structural elements. They can exceed final service loads locally. A young concrete slab, for example, may not have achieved its specified strength when construction materials are stacked above it. Temporary works therefore require their own design, inspection and removal sequence.
1.6 Load combinations
A structure is rarely subjected to one action alone. Codes specify combinations such as permanent load plus imposed load, or permanent load plus wind or earthquake action. Factors reflect uncertainty and the limit state being checked. The exact combinations must come from the governing code; factors should not be transferred between jurisdictions without verification.
2. Load Paths and Structural Systems
A load path is the continuous route by which force travels from its point of application to the foundation and soil. For a typical framed floor, the path is:
occupants and finishes → slab → secondary beams or joists → primary beams → columns or walls → foundations → ground.
A wind path may be:
cladding → diaphragms and collectors → braced frames, moment frames or shear walls → foundations → ground.
A discontinuity, weak connection or unsupported transfer can interrupt this route. Architects, engineers and construction managers must coordinate penetrations, transfer floors, façade anchors, openings and temporary supports.
Building load path from floor loads to the ground
Figure: Building load path from floor loads to the ground
Figure: A simplified gravity load path through a building.
A diaphragm is a floor or roof plane that transfers lateral force to vertical resisting elements. Collectors and drag struts gather and distribute diaphragm forces. Foundations must resist vertical reaction, sliding, overturning and, where relevant, uplift.
3. Equilibrium and Basic Determinate Members
A structure is in static equilibrium when the resultant force and resultant moment are zero. In a plane:
∑Fx = 0, ∑Fy = 0, ∑MO = 0
The symbols represent the sums of horizontal forces, vertical forces and moments about any point O. These equations apply to a body at rest or moving with constant velocity. A determinate structure can be solved using equilibrium alone; an indeterminate structure also requires compatibility and stiffness relationships.
3.1 Supports and reactions
A roller support normally supplies one reaction perpendicular to its surface. A pin supplies two force reactions, usually horizontal and vertical. A fixed support supplies horizontal force, vertical force and moment. Idealizations must match actual restraint closely enough for the analysis purpose.
3.2 Simply supported beam with a central point load
Consider a simply supported beam of span L with a point load P at midspan. By symmetry:
RA = RB = P/2
where R_A and R_B are vertical support reactions. The maximum bending moment occurs at midspan:
Mmax = PL/4
The maximum shear magnitude is P/2. These results assume a straight prismatic beam, ideal simple supports, a concentrated vertical load, small deformation and negligible self-weight or separately included self-weight.
Worked example. A 6 m simply supported beam carries a 20 kN central load. The reactions are 10 kN each. The maximum moment is:
Mmax = (20 kN)(6 m)/4 = 30 kN·m.
The support shear magnitudes are 10 kN. If the beam’s own uniform weight is significant, it must be added as a separate distributed load.
3.3 Uniformly distributed load
For a simply supported beam carrying a uniform load w over the full span:
RA = RB = wL/2
and
Mmax = wL²/8
Here w is force per unit length (kN/m). The maximum moment occurs at midspan. The total load is wL, so the units of wL² are kN·m, as required for moment.
For a simply supported beam with a central point load, the bending moment at a section is found from the external forces on one side. Between the left support and the point load:
M(x) = RAx
where x is the distance from the left support. The shear is constant in that interval. A shear-force diagram changes at concentrated loads, while a bending-moment diagram changes slope where shear changes.
3.4 Strength and bending stress
In elementary elastic beam theory, the normal bending stress is:
σ = My/I
where σ is bending stress (Pa), M is bending moment (N·m), y is distance from the neutral axis (m), and I is the second moment of area (m⁴). The extreme-fibre stress may be written:
σmax = M/Z
where Z = I/y_max is the elastic section modulus (m³). This relationship assumes linear-elastic material behaviour, small deflection, and a beam whose cross-section and loading satisfy elementary beam theory. It does not by itself check shear, local buckling, lateral-torsional buckling, connection capacity or code resistance factors.
3.5 Deflection and serviceability
Strength concerns collapse or unacceptable yielding; serviceability concerns performance in normal use. Excessive deflection can damage partitions, finishes and glazing even when strength is adequate. Vibration can make floors uncomfortable. Cracking, water penetration and appearance are also serviceability concerns.
For a simply supported beam with a central point load, the elastic midspan deflection is:
δmax = PL³/(48EI)
where E is Young’s modulus (Pa) and I is the second moment of area (m⁴). This formula assumes a prismatic, linearly elastic beam, small deflection, simple supports and a point load at midspan. Real design may require consideration of cracking, creep, shrinkage, load duration, composite action and code deflection limits.
4. Columns, Trusses and Stability
4.1 Columns
A column primarily carries axial compression, although real columns commonly experience bending because of eccentricity, frame action, imperfections and lateral loads. Average axial stress is:
σc = P/A
where P is axial force (N) and A is cross-sectional area (m²). This expression is a simple stress estimate and is insufficient where slenderness, buckling, second-order effects or material nonlinearity is important.
A slender compression member may buckle before its material reaches compressive strength. The elastic critical load for an ideal pin-ended column is Euler’s equation:
Pcr = π²EI/L²
where P_cr is critical load, E is elastic modulus, I is the least second moment of area, and L is effective length for the stated pin-ended case. For other end restraints, the effective length is represented by KL:
Pcr = π²EI/(KL)²
where K is the effective-length factor. Euler theory assumes a straight, slender, centrally loaded, elastic member with ideal restraints. It does not represent stocky columns, residual stress, initial crookedness, yielding or connection imperfections. Practical design uses code buckling curves or interaction equations.
4.2 Trusses
An ideal pin-jointed truss has straight two-force members connected at nodes, with external loads applied at joints. Each member carries axial tension or compression. The method of joints applies equilibrium at each node; the method of sections cuts selected members and applies equilibrium to one part.
A truss is stable when its geometry, supports and bracing prevent mechanism formation. A triangular arrangement is inherently stable in its plane, whereas an unbraced rectangular panel can distort into a parallelogram. Real truss joints have stiffness and members may experience bending, so the ideal model is an approximation.
4.3 Global stability
Stability includes resistance to sliding, overturning, sway, racking, uplift and progressive loss of equilibrium. A building may have adequate member strength but still be unstable if lateral bracing is missing. Construction sequencing is critical: a frame that is stable when complete may be unstable before diaphragms, permanent bracing or walls are installed.
5. Material and Building-System Behaviour
5.1 Reinforced concrete
Concrete is strong in compression but weak and brittle in direct tension. Reinforcing steel carries tensile force and controls cracking. Reinforced concrete systems include moment frames, shear walls, flat slabs, beam-and-slab frames and foundations. Their advantages include fire resistance, stiffness, mass and adaptability; limitations include self-weight, curing time, formwork, shrinkage, creep and construction tolerances.
Concrete behaviour changes with age and environment. Creep is time-dependent strain under sustained stress; shrinkage is volume reduction not directly caused by applied load. Both can redistribute forces and increase long-term deflection. Reinforcement anchorage, development length, lap locations, congestion and concrete compaction are major construction-quality issues.
5.2 Structural steel
Steel has high strength-to-weight ratio, predictable properties and rapid prefabrication. Steel frames may resist lateral action through rigid moment connections, concentric or eccentric bracing, or composite systems. Critical failure modes include yielding, local buckling, lateral-torsional buckling, connection fracture and fire-induced strength loss.
Bolted and welded connections must transfer the required forces and accommodate erection tolerances. Temporary erection bracing is essential until the permanent frame is stable. Corrosion protection, fire protection and inspection of welds and bolts influence life-cycle performance.
5.3 Timber
Timber is lightweight, renewable when responsibly sourced and efficient in prefabricated systems. It is anisotropic: strength and stiffness differ parallel and perpendicular to grain. Moisture content affects dimensions, durability and strength. Typical systems include platform framing, heavy timber frames, glulam beams, laminated veneer lumber and mass-timber panels.
Design and construction must address connection slip, splitting, decay, termites, fire charring, vibration and moisture protection. Timber members may be strong along grain but vulnerable to perpendicular-to-grain compression or tension. Keeping timber dry during transport, erection and enclosure is a fundamental quality practice.
5.4 Masonry
Masonry is effective in compression and may form load-bearing walls, infill panels, partitions or reinforced shear walls. Unreinforced masonry has limited tensile and seismic resistance because joints and units can separate. Reinforcement, grout, bond beams, confinement and positive diaphragm connections improve performance.
Masonry construction is sensitive to workmanship: alignment, joint thickness, curing, grout placement and connection details matter. Openings and irregular layouts can create stress concentrations and weak seismic mechanisms.
5.5 System selection
Selection should consider span, height, loads, fire, durability, speed, available skills, local materials, embodied carbon, cost, maintainability and future adaptation. No material is universally superior. A hybrid system may combine concrete cores, steel framing, timber floors and masonry façades, provided interfaces and load paths are explicitly designed.
6. Limit States, Robustness and Risk
The ultimate limit state addresses collapse, rupture, instability, excessive yielding and loss of equilibrium. The serviceability limit state addresses deflection, vibration, cracking, durability, appearance and usability. A structure must satisfy both.
Robustness is the ability to tolerate localized damage without disproportionate collapse. Strategies include continuity, ties, alternate load paths, compartmentation, redundancy, ductile detailing and protection against credible hazards. Robustness is not the same as simply making every member larger; connections and system continuity are often decisive.
Structural safety also depends on management. Design assumptions must be communicated to contractors, temporary works engineers and inspectors. Unauthorized removal of walls, cutting of beams, overloading of slabs and unreviewed changes to equipment can invalidate the original design.
7. Illustrative Case: Temporary Instability During Steel Erection
Background. A three-storey steel-framed office building uses perimeter moment frames and a composite floor diaphragm. Columns and beams are erected floor by floor.
Problem. After the second-floor steel was erected, a high wind forecast was issued. Permanent floor decking and several bays of bracing had not yet been installed. The partially complete frame had adequate member strength under its design gravity loads but inadequate lateral stability as an erection configuration.
Analysis. The construction team reviewed the erection drawings, identified the incomplete diaphragm and missing bracing, checked crane and stored-material loads, and established exclusion zones. The temporary works engineer assessed wind actions on the exposed frame and connections. The load path was incomplete because lateral forces could not yet reach the intended braced bays.
Approach. Work stopped temporarily. Temporary diagonal bracing and guying were installed in accordance with an engineered plan. Materials were redistributed to reduce local floor loading, and decking installation was sequenced to develop diaphragm action progressively. A documented hold point required inspection before work resumed above the second floor.
Outcome. No damage occurred, and erection continued after wind conditions moderated. The event demonstrated that a completed-building analysis cannot be used automatically for an incomplete structure.
Learning points. Construction-stage stability is a design issue, not merely a site housekeeping issue. Temporary load paths, weather forecasts, lifting operations, connection completion and inspection records must be integrated into the method statement. A competent person must have authority to stop unsafe work.
8. Professional Practice, Technology and Contemporary Developments
Building information modelling can represent member geometry, penetrations, connection zones and sequencing, helping teams identify clashes and discontinuous load paths. Four-dimensional models link geometry to time and can expose periods when temporary stability is inadequate. Sensors and structural-health monitoring may measure strain, acceleration, displacement or temperature, but data require calibration, interpretation and secure governance.
AI-assisted tools can support clash detection, image-based progress monitoring, anomaly identification and preliminary quantity checking. They must not be treated as autonomous structural approval systems. Engineers must verify inputs, assumptions, code basis, output units and limitations. Professional accountability remains with qualified persons.
Best practice includes:
establish the governing codes and design criteria;
identify all permanent and construction-stage actions;
draw gravity and lateral load paths;
coordinate structural, architectural and services openings;
check strength, stability, serviceability and robustness;
design and inspect temporary works;
document assumptions, revisions and hold points; and
maintain traceable quality, safety and change-control records.
Chapter Summary
Buildings resist loads through connected structural systems. Dead loads are permanent; live loads vary with use; wind and seismic actions are environmental; and construction loads act during temporary configurations. Equilibrium equations permit analysis of basic determinate beams, while bending, shear, axial stress and buckling concepts explain member behaviour. Load paths must be continuous from applied load to foundation and ground.
Strength addresses resistance to failure, stability addresses equilibrium and buckling, serviceability addresses usable performance, and robustness addresses tolerance of local damage. Concrete, steel, timber and masonry have different strengths, weaknesses, failure modes and construction requirements. The completed building and the structure during erection are different engineering configurations. Safe practice therefore combines calculation, detailing, inspection, temporary works design, code compliance and professional judgement.
Key Terms
Load: An external force, pressure or imposed deformation acting on a structure.
Dead load: Permanent weight of structural and fixed non-structural components.
Live load: Variable load caused by occupancy, movable contents or use.
Load path: Continuous route by which force travels to the ground.
Diaphragm: Floor or roof plane that transfers lateral forces to vertical elements.
Collector: Element that gathers and transfers diaphragm forces.
Reaction: Force or moment developed by a support.
Shear force: Internal force acting parallel to a member cross-section.
Bending moment: Internal moment caused by transverse loads and reactions.
Determinate structure: Structure solvable using equilibrium equations alone.
Serviceability: Ability to perform acceptably in normal use.
Strength: Capacity to resist applied actions without unacceptable failure.
Buckling: Instability involving sudden or progressive lateral deformation.
Ductility: Ability to deform substantially while retaining resistance.
Robustness: Ability to withstand localized damage without disproportionate collapse.
Ultimate limit state: Condition associated with collapse, rupture or loss of stability.
Construction load: Temporary action occurring during building operations.
Effective length: Equivalent buckling length used to represent end restraint.
Tributary area: Area whose load is assigned to a particular member or support.
Temporary works: Supports, bracing, formwork and other provisional construction systems.
Review Questions
Short Answer Questions
Distinguish dead load from live load and give two examples of each.
What is meant by a continuous load path?
State the three plane-equilibrium equations.
Why can a completed-building analysis be unsafe for a partially erected structure?
Distinguish strength from serviceability.
Long Answer Questions
Explain the gravity and lateral load paths in a multi-storey framed building.
Describe the assumptions and limitations of the simply supported beam formulas for a central point load.
Compare reinforced concrete, steel, timber and masonry as structural materials.
Explain column buckling and the significance of effective length.
Discuss how robustness, ductility and redundancy contribute to structural safety.
Analytical / Application Questions
A 6 m simply supported beam carries a 12 kN point load at midspan. Calculate the support reactions and maximum bending moment, stating your assumptions.
A construction team plans to store materials on a young concrete slab before the supporting shores are removed. Identify the relevant loads, failure risks, information required and appropriate controls.
For a regular three-storey building in a high-wind region, propose a lateral-resisting system and explain its load path, construction sequence, stability checks and principal limitations.
Multiple Choice Questions with Answers
Question 1
Which action is normally classified as a dead load?
A. Movable office furniture
B. Occupants
C. Permanent floor finishes
D. Temporary construction workers
Correct Answer: C. Permanent floor finishes
Question 2
For a simply supported beam with a central point load P, the maximum bending moment is:
A. PL/8
B. PL/4
C. P/L
D. PL/2
Correct Answer: B. PL/4
Question 3
Which sequence best represents a typical gravity load path?
A. Foundation → slab → beam → roof
B. Slab → beam → column or wall → foundation
C. Column → slab → soil → beam
D. Cladding → ceiling → occupant → foundation
Correct Answer: B. Slab → beam → column or wall → foundation
Question 4
The primary purpose of a floor diaphragm is to:
A. Increase concrete curing speed
B. Transfer lateral forces to vertical resisting elements
C. Eliminate all dead loads
D. Prevent axial force in columns
Correct Answer: B. Transfer lateral forces to vertical resisting elements