Skip to reading
    Book overview
    THE DEMOLITION SUPERVISOR’S FIELDBOOK / CHAPTER 43
    Part VI · Plans, structures and engineering interface

    Structural behaviour: compression, tension, bending and combined actions

    Stress, strain, bending, shear and torsion describe different aspects of structural response. Material behaviour, connections and restraint determine whether a member can resist the actions placed on it.

    Force, stress and strain describe different things

    Force is a push or pull and has direction. Structural force is commonly expressed in newtons or kilonewtons; one kilonewton is one thousand newtons. Stress describes force distributed over an area. Strain describes deformation relative to an original dimension. Neither stress nor strain is simply another word for load.

    In an ideal axial calculation, a force of twelve kilonewtons acts uniformly through a cross-sectional area of six hundred square millimetres. Twelve kilonewtons is twelve thousand newtons. Dividing twelve thousand by six hundred gives twenty newtons per square millimetre, equivalent to twenty megapascals. In the same ideal calculation, suppose the effective area is halved to three hundred square millimetres while force stays the same. The average stress then doubles to forty megapascals.

    At unchanged force, loss of effective section increases average stress. Average stress alone does not establish the capacity of a corroded member. Real stress can be uneven because of holes, notches, eccentricity, bending and connections, and capacity depends on the material, stability and design criteria.

    For a strain calculation, a two-thousand-millimetre specimen lengthens by zero point six millimetres. Dividing zero point six by two thousand gives zero point zero zero zero three, or zero point zero three percent. Strain has no length unit because the two lengths are divided. The calculation says how much relative change occurred; it does not say whether that change is acceptable or whether the material will recover after unloading.

    Technical terms

    Hazard — A source or situation with potential to cause harm.

    Risk — The possibility of harm, considered with its likelihood and consequence in context.

    Exposure — The opportunity for a person or receptor to come into contact with a hazard.

    Consequence — The harm or loss that could result.

    Control — A measure that eliminates or reduces risk.

    Remaining risk — Risk considered after the specified controls have actually been applied.

    Elimination — Removing the hazard or hazardous exposure from the work.

    Substitution — Replacing a hazard with a less hazardous alternative.

    Isolation — Separating people from a hazard.

    Engineering control — A physical or designed measure that acts on a hazard or exposure pathway.

    Administrative control — An arrangement such as procedures, scheduling, information or supervision.

    PPE — Personal protective equipment.

    RPE — Respiratory protective equipment.

    SWMS — Safe work method statement; a document for relevant high-risk construction work.

    DWP — Demolition work plan.

    Permit — A bounded authorisation within a defined control system, not universal proof of safety.

    Induction — Introduction to relevant site conditions, responsibilities and arrangements.

    Pre-start — A check or briefing before work or equipment use; distinct from scheduled servicing.

    Verification — Checking evidence that the required condition or control exists and works as intended.

    Consultation — Sharing relevant information, hearing affected workers' views and considering them in decisions.

    Load path — The connected route by which forces pass through a structure to its supports.

    Compression — A pushing action within a material or member.

    Tension — A pulling action within a material or member.

    Bending — A response to loading that tends to curve a member.

    Shear — Action tending to make parts move past one another.

    Temporary works — Engineered or other temporary arrangements supporting construction or demolition needs; specialist design and control may be required.

    Respirable dust — Particles small enough to reach deep into the lungs.

    LEL — Lower explosive limit; the lowest flammable gas or vapour concentration in air at which flame can propagate under the relevant conditions. Percentage of LEL is not percentage gas concentration or a safe-breathing decision.

    Receptor — The person, property or environmental feature potentially affected.

    Change control — The process of recognising a change, reviewing affected assumptions and controls, and communicating the revised arrangement.

    Air monitoring — Competently planned measurement of airborne contaminants to assess exposure and control effectiveness.

    Health monitoring — Medical monitoring for health effects, carried out or supervised by an appropriately experienced doctor when required.

    Compression and tension interact with material behaviour

    Compression tends to shorten a member along the direction of the force. Tension tends to lengthen it. A member carrying an axial force is an idealisation in which the force acts along its axis. If the force acts away from that axis, it can also create bending. Real connections and geometry determine whether an apparently simple load is actually eccentric.

    Materials respond differently. Concrete is comparatively effective in compression but has limited tensile resistance, so reinforced-concrete design uses reinforcement to carry specified tensile actions. Steel can carry tension and compression. But a slender steel component may become unstable in compression before its material reaches the strength someone expects from a sample. Timber properties depend on direction relative to the grain, moisture, defects and condition. Masonry's behaviour depends on units, mortar, geometry, connections and restraint.

    Elastic deformation is recoverable when the load is removed within the relevant range. Plastic deformation is permanent. Ductility describes an ability to undergo significant deformation before failure; brittle behaviour involves relatively little such deformation. These concepts do not promise a visible warning. A connection can fail differently from the main material, and deterioration can change the behaviour of the assembly.

    An important confusion is to treat stiffness and strength as identical. Stiffness describes resistance to deformation; strength concerns resistance to failure under the relevant action. A member can be relatively stiff yet fail in a brittle manner, or be strong but deflect beyond an acceptable condition. Structural assessment considers both, together with stability and the requirements of the whole system.

    Case visual LA-17: Left: inward end forces illustrate compression and shortening

    LEARNING AID A generic learning aid explains a principle. It does not establish a project fact, hidden condition, capacity, approval or work release.

    Compression member with inward arrows; gently sagging beam with a downward central arrow and upward support arrows; tension member with outward arrows. Dashed outlines show reference shapes.Open full-size illustration

    Left: inward end forces illustrate compression and shortening. Centre: a downward load and upward support reactions illustrate bending. Right: outward end forces illustrate tension and elongation. Dashed outlines are reference shapes; changes are exaggerated and not to scale. No capacity or failure is predicted.

    A generic learning aid explains a principle. It does not establish a project fact, hidden condition, capacity, approval or work release.

    This visual is draft case-study teaching material. Reading it is not an inspection, a measurement or evidence of performance.

    Bending combines force and distance

    Bending occurs when actions tend to curve a member. In the ideal elastic model, consider a simple beam under downward loading. One region can be in compression while another is in tension, with a neutral region between them. The orientation of those regions depends on the support and loading arrangement. It is not always correct to assume the same face is in tension everywhere.

    A bending moment expresses a turning effect and is measured in force multiplied by distance, such as kilonewton-metres. It is not a mass. For example, a straight beam is supported at each end, six metres apart, and carries one downward point load of six kilonewtons at the middle. Assume ideal simple supports, static loading and no self-weight or other actions. By symmetry and vertical balance, each support reaction is three kilonewtons.

    At midspan, the three-kilonewton reaction acts over three metres on either half of this ideal beam. The bending moment there is nine kilonewton-metres. This is the calculated bending action, not the beam's resistance. Establishing capacity also requires the member's section, material, connections and stability conditions.

    Moving a load, changing a support or changing the span changes the action pattern. A continuous beam over several supports behaves differently from the simple model. A cantilever has another arrangement, with its support needing to resist the relevant turning effect. Changing continuity or restraint during demolition can therefore change bending behaviour without an obvious change in the remaining beam's appearance.

    A uniformly distributed load is a different model from the central point load. Consider an ideal simply supported beam carrying a uniform downward load across its entire span, with no other actions. Let w mean load per metre and L mean span. The total load is w multiplied by L. Each vertical reaction is half that total, and the maximum bending moment is w multiplied by L squared, divided by eight.

    In the generic learning example, w is four kilonewtons per metre and L is three metres. The total load is twelve kilonewtons, each reaction is six kilonewtons, and the maximum moment is four point five kilonewton-metres. These are the relationships shown in the bounded calculation aid LA-04. Keep the model name and units beside the calculation.

    03 / LOAD & RESPONSE · CHAPTER 43

    Same load. Different demand.

    Move the load. Watch the reactions and bending moment change together.

    CPCCDE4005
    IDEAL SIMPLY SUPPORTED BEAMStatic model
    6.0 kN point load↑ 3.00 kN↑ 3.00 kNSpan 6.0 mBENDING MOMENT / POSITIVE SAGGING SHOWN BELOW BASELINEDiagram height auto-scales. Compare the numbers, not heights between settings.
    Total applied load6.00 kN
    Maximum moment9.00 kN·m
    Moment at cursor9.00 kN·m
    Enlarged diagram · swipe sideways if needed
    OBSERVE

    At the default settings, the same 6 kN total produces 9 kN·m maximum moment as a central point load, or 4.5 kN·m when spread uniformly.

    INTERPRET

    Load magnitude, position, distribution and span all affect demand. A smaller total load does not by itself establish a safe condition.

    DO NOT INFER

    This is demand, not capacity. Material, section, connections, condition, lateral restraint and dynamic effects are not assessed. No deflection is calculated.

    Try next

    Move a point load towards one support. Compare both reactions and the peak moment. Explain why the reactions still sum to the applied load.

    Source, assumptions & limits

    Ideal static pin-and-roller beam; vertical loading only. Self-weight is excluded. Point load: Rₗ = P(L − a)/L; Rᵣ = Pa/L; M(x) = Rₗx − P max(0, x − a). Uniform load: Rₗ = Rᵣ = wL/2; M(x) = wx(L − x)/2. Positive moment denotes sagging. The bending-moment diagram is not the displaced shape of the beam. Chapter examples can be explored with P = 6 kN, L = 6 m, a = 3 m; or w = 4 kN/m, L = 3 m.

    Connected teaching: Chapter 43

    Original generic learning model. It establishes no condition, capacity, approved method or release for a real project. Qualified structural review is pending.

    The result describes demand in that ideal arrangement. It does not establish resistance, restraint or connection adequacy in a real member. A continuous beam, cantilever, uneven load or changed support needs a model that represents those conditions.

    Gravity loads and lateral stabilityA concrete slab rests on beams supported by columns and pad footings in the ground. Blue arrows show downward load transfer. A separate inset identifies an orange diagonal brace providing lateral restraint. No capacities or removal sequence are specified.Open full-size illustration

    Roof and floor loads pass through supporting beams or walls, then through columns or walls, into foundations and the ground. Each connection in the vertical chain represents a dependency: the receiving element must transfer the relevant action to the next support.

    Lateral actions involve a different but connected set of relationships. Bracing, connections and restraints limit unwanted movement. An element that carries little gravity load can still provide essential lateral stability. Removing it may change the behaviour of the structure that remains.

    Demolition alters loads, connections and restraints as work progresses. The partly dismantled structure must therefore be considered at each stage, using the current engineering information. Hidden construction, deterioration, ground conditions and temporary works may affect the assessment.

    The schematic identifies load-transfer relationships, not member capacities or a removal sequence. Stage-specific structural decisions require information about the actual elements, their condition and their connections. An unresolved load path requires technical review.

    Case visual LA-04: Three exact model diagrams for pressure, a simply supported beam and an axial…

    LEARNING AID A teaching aid may illustrate a principle but does not establish a project fact, capacity, condition, authority approval or work release.

    Learning aid: Three exact model diagrams for pressure, a simply supported beam and an axial memberOpen full-size illustration

    Three exact model diagrams for pressure, a simply supported beam and an axial member

    A teaching aid may illustrate a principle but does not establish a project fact, capacity, condition, authority approval or work release.

    This visual is draft case-study teaching material. Reading it is not an inspection, a measurement or evidence of performance.

    Shear and torsion describe other action patterns

    Shear tends to make adjacent parts of a material slide relative to one another. In a beam, shear is connected to the transfer of transverse loads towards supports. Connections can also carry shear through bolts, welds, fasteners or other details. Shear failure is not simply a member bending too far, and the relevant resistance may be concentrated in a small connection region.

    Punching shear is a local action around a concentrated support or load in a slab. It helps explain why a broad floor area and a small contact area raise different questions. A slab can require consideration of local failure as well as overall bending. Its appearance from above does not establish its resistance to either.

    Torsion is twisting about a member's axis. An off-centre force can create a twisting effect as well as other actions. A connection or cross-section that resists one direction effectively may behave differently in torsion. The load's location relative to the member is therefore part of the structural question, not a minor geometric detail.

    Combined actions are normal in real structures. A column can carry compression with bending. A beam can experience bending, shear and torsion together. Wind, gravity, applied plant forces and temporary restraints can interact. It is unsafe to check each action informally against a remembered isolated value and assume the combined condition is acceptable. The structural design process considers their interaction, relevant combinations and failure modes.

    Buckling is a stability problem

    Buckling is a loss of stability in which a member or part of a member deforms into another shape under load. A slender compression member can move sideways; a thin plate element can buckle locally; a beam can experience a coupled sideways and twisting instability. These are related ideas, not one universal failure mechanism.

    Unsupported length, cross-sectional shape, end restraint, intermediate restraint, material stiffness and imperfections all influence behaviour. A member that is restrained at intervals can behave very differently after those restraints are lost. A connection assumed to prevent rotation or translation must actually provide that function. The structural system, not just the member catalogue, determines the relevant condition.

    04 / COLUMN INTERPRETATION

    Same column. Different bending direction.

    Identify the steel that is present before interpreting what a change might mean.

    Concept study
    LOOKING DOWN AN UPRIGHT I-SECTION
    FlangeWebFlangeBending about the major axisDashed line = bending axis · Blue arrow = lateral action in plan
    INTACT SECTION / NO CUTTING DETAILSection axes are not compass bearings.
    Enlarged diagram · swipe sideways if needed
    PRE-WEAKENING: WHAT CHANGES?

    An alteration can affect continuity, stiffness, resistance and local stability at the same time. The original intact-section properties no longer establish the altered member’s behaviour.

    ORIENTATION IN THE WAREHOUSE

    First relate the section to the actual plan and bracing. A north–south action is not automatically major-axis or minor-axis bending: the installed column orientation must be established.

    VERIFY THE STRUCTURAL DETAIL

    Check which member and face are shown, the drawing’s viewing direction, the specified condition and the related restraint assumptions. Use a checked, applicable structural detail. An isolated or incorrect sketch cannot establish the behaviour of the whole frame.

    Try next

    Switch between axes. Identify the bending axis separately from the action direction. Then identify what information is still missing before relating either to the warehouse.

    Source, assumptions & limits

    Original intact-section teaching diagram. The supplied 27 Frank Street student report drawings were identified as incorrect by the resource owner. They are excluded as authority for section orientation, bracing, weakening details and movement. This diagram is independently constructed and does not represent a verified column at that site. No cut geometry, sequence, residual capacity or predicted collapse is provided.

    General member-stability reference: Steel Construction Institute — Stability of steel beams and columns. This supports the conceptual distinction between section behaviour and member restraint; it is not the Australian project design basis.

    Connected teaching: Chapter 42 Chapter 43 Chapter 44

    Original generic learning model. It establishes no condition, capacity, approved method or release for a real project. Qualified structural review is pending.

    Two otherwise similar slender compression members can have different unsupported lengths. The longer unrestrained condition is generally more vulnerable to instability, all else being equal. Capacity for a particular member requires the actual geometry, restraints, loads, imperfections and design rules.

    A common confusion is to equate the absence of crushing with adequate compression capacity. Buckling can occur while the material has not reached a simple crushing or yielding limit. Another is to assume that a member which carried yesterday's loads will carry today's loads after adjoining construction is removed. The restraint conditions may have changed. Recognising this mechanism explains why seemingly secondary ties and bracing must remain within the stage-specific structural plan.

    Structural distress and the limits of observation

    Structural distress can appear as unexpected movement, cracking, distortion, separation at connections, falling fragments or a change in alignment. These observations can be important, but their cause and significance depend on the material, system, history and current work. A crack pattern is not a universal diagnosis, and the absence of visible distress does not demonstrate adequate capacity.

    A report that a member has moved sideways records an observation. A statement that it has buckled interprets the mechanism and may require technical confirmation. A connection has visibly separated is different from guessing its original capacity. Useful communication records the location, time, relevant activity and observed change without exposing anyone to obtain a closer view.

    Where movement, damaged supports or a changed structural assumption creates uncertainty, maintain the applicable stop and exclusion arrangements and seek the required competent structural response. Do not test the structure by adding a load, removing another piece or approaching it to see whether it moves again.

    The structural assessment needs to establish the forces and where they act, the material and connections resisting them, and the restraints preventing instability. A changed assumption in any of these can affect the conditions for the demolition stage.

    Responding to changed conditionsA five-stage clockwise loop shows recognising change, protecting people and stopping affected work, responsible review, verification and authorisation of revised controls, and communication and monitoring. The centre states that affected work remains stopped while a condition is unresolved.Open full-size illustration

    A changed condition can invalidate the assumptions supporting a work stage. Protecting people and stopping affected work limits further exposure while the responsible people review the change. Revised controls require the appropriate technical input, verification and authorisation before affected work resumes.

    The revised arrangement must reach everyone whose work depends on it. Monitoring then establishes whether it remains effective. An unresolved condition is not cleared by the continued existence of an earlier plan. Emergency response follows the site's emergency arrangements and the authority responsible for the incident.