Loads, Structural Behaviour and System Selection

Introduction

A foundation and floor system must do more than support visible building elements. It must receive loads, transfer them safely through structural members and soil, control movement, resist environmental actions, and remain serviceable throughout the building’s life. For a civil draftsman, understanding this load path is essential because drawings communicate the assumptions on which structural safety depends.

A drawing that shows an isolated footing, strip footing, slab-on-ground, suspended floor, or pile foundation is not merely a geometric representation. It expresses a response to structural loads, soil conditions, moisture, climate, construction methods, codes, and the intended use of the building. The selected system must therefore be technically adequate, economical, buildable, and coordinated with architectural and services drawings.

This chapter introduces the principal loads acting on low-rise buildings, basic structural behaviour, bearing pressure and settlement concepts, code-related responsibilities, and a practical method for selecting foundation and floor systems.

Learning Objectives

By the end of this chapter, the learner should be able to:

1. Identify dead, live, wind, seismic, water, temperature, and other environmental loads.
2. Explain how loads travel from floor and roof elements to the supporting soil.
3. Distinguish compression, tension, shear, bending, bearing, sliding, overturning, and settlement behaviour.
4. Calculate basic tributary load, average bearing pressure, and simple settlement relationships.
5. Recognize the role of building codes, geotechnical information, and design responsibility.
6. Select preliminary foundation and floor systems using technical and practical criteria.
7. Prepare drawings that clearly communicate levels, supports, joints, reinforcement intent, drainage, and construction interfaces.

1. The Building as a Load-Transfer System

The fundamental structural question is: Where does each load go? A typical load path is:

Figure: A simplified vertical load path from the building to the ground.

In a framed building, floor loads are carried by slabs to beams, beams to columns, columns to footings, and footings to soil. In a load-bearing masonry building, slabs and roofs may transfer loads to walls, which transfer them to strip footings. A slab-on-ground may distribute loads directly to compacted subgrade, although edge beams or thickened zones may still be required.

A load path is not only vertical. Wind and earthquake actions can create horizontal forces. These must travel through diaphragms, walls, frames, bracing, connections, foundations, and finally into the ground. Discontinuous walls, missing ties, weak connections, or unrestrained floor edges can interrupt this path.

2. Types of Load

2.1 Dead load

Dead load is the permanent weight of the building and fixed components. It includes structural members, floor finishes, roof coverings, ceilings, partitions that are permanently located, cladding, fixed equipment, and sometimes soil or water retained by the structure.

For a uniform element, weight is commonly determined by:

where:

• W = weight, in kN;
• γ = unit weight, in kN/m³;
• V = volume, in m³.

For a rectangular concrete member, V = Lbt, where L, b, and t are length, width, and thickness in metres. The equation assumes a known and reasonably uniform material density. Unit weight must not be confused with mass density; structural calculations generally use force units.

Area loads are often useful for floors. If a concrete slab is 0.12 m thick and concrete has a unit weight of 24 kN/m³, its self-weight is:

Finishes, screeds, ceilings and services are then added as separate permanent actions according to the project specification and applicable code.

2.2 Live or imposed load

Live load is variable and arises from people, furniture, movable partitions, stored materials, vehicles, maintenance activity, and movable equipment. Its value depends on occupancy. A library, warehouse, dwelling, classroom, workshop, and roof maintenance area do not have the same imposed-load requirement.

The draftsman must use the occupancy classification and load values specified by the governing building or structural code. A general textbook value must never replace the legally applicable design value. Concentrated loads may also be important: a safe uniform floor load does not automatically guarantee safety beneath a heavy machine, safe, partition, or wheel.

2.3 Wind load

Wind produces pressure and suction on walls, roofs, parapets, signs, and components. It can cause lateral translation, uplift, sliding, overturning, local cladding failure, and vibration. Wind demand depends on wind speed, terrain, height, building shape, openings, shielding, importance, and internal pressure.

Wind must be traced through cladding fixings, wall or roof diaphragms, bracing or moment frames, hold-downs, and foundations. Roof uplift is particularly important for light buildings: the foundation may need enough self-weight and anchorage to prevent separation or overturning.

2.4 Seismic and other dynamic actions

Where seismic design is required, earthquake forces arise from the inertia of building mass. The design response depends on seismic hazard, soil classification, structural system, ductility, regularity, importance, and detailing. Even where a draftsman does not calculate seismic forces, drawings must show continuity of walls, ties, anchors, reinforcement, joints, and connections specified by the engineer.

Dynamic effects may also arise from machinery, traffic, impact, crowd movement, or construction activity. These actions can require vibration control rather than merely checking static strength.

2.5 Environmental loads

Environmental actions include:

• rainwater ponding and hydrostatic pressure;
• groundwater and buoyancy;
• flood forces and debris impact;
• earth pressure against basement walls;
• temperature change and restrained expansion;
• shrinkage, creep, and moisture movement of concrete;
• frost or expansive soil movement where relevant;
• corrosion, aggressive chemicals, salts, and sulfate exposure;
• fire-related temperature effects.

Water is especially significant to foundations. Hydrostatic pressure increases with depth according to:

where p is water pressure in kPa, γ_w is the unit weight of water in kN/m³, and h is water depth in m. Since 1 kN/m² equals 1 kPa, the units are consistent. The formula applies to static fluid pressure and does not include flow, impact, or wave effects.

3. Structural Actions and Behaviour

3.1 Compression and tension

Compression shortens or squeezes a member; tension stretches it. Concrete is strong in compression but relatively weak in direct tension, while reinforcing steel is effective in tension. Masonry walls generally perform well under compression but require careful detailing for tension, bending, and lateral loads.

3.2 Shear and bending

Shear tends to make adjacent portions of a member slide past each other. Bending produces curvature, with compression on one side and tension on the other. Floor slabs, beams, lintels, and footings commonly experience bending and shear.

A simply supported beam carrying a central point load has maximum bending moment:

where P is the point load in kN and L is span in m, giving M_max in kN·m. This is a special case applicable only to a simply supported beam with one central point load and idealised supports. It should not be used for continuous beams, fixed supports, distributed loads, or slabs without adapting the analysis.

For a simply supported beam with a uniformly distributed load w over the full span:

Here w is in kN/m. These relationships are useful for understanding behaviour, but member design also requires checks for shear, deflection, reinforcement, anchorage, buckling, durability, and code load combinations.

3.3 Bearing, sliding and overturning

A footing transmits compression to soil over a contact area. If horizontal forces act, the system must resist sliding through friction, passive resistance, shear keys, piles, or other approved mechanisms. If a lateral force acts above the base, it creates an overturning moment. The foundation must maintain adequate contact and stability.

A simple preliminary eccentricity relationship is:

where e is eccentricity in m, M is overturning moment in kN·m, and V is vertical resultant in kN. The expression assumes a single resultant and is used to locate the line of action. More complete design must consider contact limits, soil response, code factors, and possible uplift.

4. Tributary Area and Load Transfer

A tributary area is the portion of a floor or roof whose load is assigned to a particular beam, column, wall, or footing for preliminary analysis. For regularly spaced supports, it is often bounded by lines halfway to adjacent supports. Tributary methods are approximate and are most reliable for regular layouts and broadly uniform loading.

Suppose a column supports a tributary floor area of 16 m². The characteristic permanent floor load is 3.5 kN/m² and the characteristic imposed load is 2.0 kN/m². Ignoring self-weight of the column and footing for this preliminary calculation:

The characteristic vertical load is therefore 88 kN. The symbols G_k and Q_k denote characteristic permanent and variable actions. A final design load must use the load combinations and factors required by the applicable code; characteristic loads must not be confused with factored design actions.

5. Bearing Pressure and Foundation Size

For a centrally loaded rectangular footing, a first estimate of average contact pressure is:

where N is the service vertical load in kN and A is footing area in m². The result q_avg is in kPa, because kN/m² = kPa.

If the 88 kN service load above is supported by a 2.0 m × 2.0 m footing, then:

This value is only an average. It does not prove adequacy. The allowable or design soil pressure must come from a geotechnical report, code provision, or competent engineering assessment. The footing must also be checked for self-weight, eccentricity, punching shear, one-way shear, bending, durability, minimum dimensions, drainage, construction tolerances, and settlement.

For a rectangular footing under a vertical load and moments about both axes, a preliminary linear pressure model may be written:

where M_x and M_y are moments about the two plan axes and Z_x, Z_y are the corresponding area section moduli of the contact area. The signs produce maximum and minimum pressures at different corners. This simplified expression assumes a rigid footing, linear soil pressure distribution, full contact, and small enough eccentricity for the assumed contact area. If uplift occurs, the full-contact equation is no longer valid and a reduced-contact analysis is required.

6. Settlement and Serviceability

A foundation can have adequate ultimate strength and still perform poorly because of excessive settlement. Settlement may be:

• Immediate settlement, caused by rapid deformation of soil under loading;
• Consolidation settlement, caused by gradual expulsion of water from saturated compressible soil;
• Secondary compression, caused by time-dependent soil deformation;
• Differential settlement, where adjacent supports move by different amounts.

Differential settlement is often more damaging than uniform settlement because it produces cracking, distortion, sticking doors, sloping floors, service-pipe damage, and stress redistribution.

A simplified elastic settlement relationship is sometimes expressed as:

where s is estimated settlement, q is net foundation pressure, B is a representative footing width, ν is soil Poisson’s ratio, I_s is a dimensionless influence factor, and E_s is soil elastic modulus. This is an idealised estimate for appropriate soil models and loading conditions; it is not a substitute for a geotechnical assessment. Soil layering, groundwater, footing shape, embedment, drainage, time effects, and construction sequence may substantially alter the result.

Drafting implications include showing founding levels, stepped footings, movement joints, slab isolation joints, compacted fill zones, moisture barriers, drainage paths, and transitions between different foundation types.

7. Foundation Systems

7.1 Strip footings

Strip footings support continuous load-bearing walls or closely spaced columns. They are economical where soil is reasonably uniform and near-surface bearing capacity is adequate. The drawing should identify footing width, depth, reinforcement, wall alignment, steps, corners, laps, concrete specification, and relation to finished ground level.

7.2 Isolated pad footings

Pad footings support individual columns or posts. They are suitable where column loads are separated and soil conditions are relatively consistent. They may become inefficient where columns are close together, loads are high, or property boundaries prevent centring.

7.3 Combined and strap footings

A combined footing supports two or more columns, often where an exterior column is close to a boundary. A strap beam can connect an edge footing to an interior footing and help balance eccentricity. These systems require accurate setting-out and careful coordination with reinforcement and services.

7.4 Raft or mat foundations

A raft supports much or all of a building footprint. It can reduce average contact pressure and help bridge local weak zones, but it is not a universal solution for poor soil. Rafts require control of differential movement, punching, bending, water ingress, and construction joints.

7.5 Pile and pier foundations

Piles or piers transfer load through weak upper soils to deeper competent strata or develop resistance along their shaft. They may be selected for high loads, restricted sites, expansive or loose near-surface soils, scour, or settlement control. Design depends heavily on geotechnical investigation, installation method, group effects, negative skin friction, lateral loads, and testing.

8. Floor Systems and Their Behaviour

8.1 Slab-on-ground

A slab-on-ground transfers much of its load directly to prepared subgrade. Its performance depends on subgrade uniformity, compaction, moisture control, drainage, joint layout, slab thickness, reinforcement, edge restraint, and construction quality. Reinforcement may control cracking rather than eliminate it.

Typical details include a compacted subbase, vapour barrier where required, perimeter insulation or thickening, construction and contraction joints, penetrations, and a finished-floor level related to damp-proofing and external drainage.

8.2 Suspended concrete floors

Suspended slabs span between beams, walls, or columns and transfer loads through those supports. They are useful where ground is weak, flood risk exists, services need access, or ground movement makes direct support undesirable. They are heavier and may require more formwork, reinforcement, and temporary works.

8.3 Timber and steel floor systems

Timber joists and steel beams provide relatively light suspended construction. Their design must address span, vibration, fire, moisture, corrosion, acoustic performance, connections, and service penetrations. A draftsman must not cut or drill structural members unless the engineer’s details explicitly permit it.

9. Codes, Documents and Professional Responsibility

The governing building code establishes minimum requirements for safety, health, fire, accessibility, energy, and durability. Structural standards provide rules for actions, concrete, steel, timber, masonry, geotechnical design, wind, seismic actions, and foundations. Local requirements may also control flood levels, termite protection, drainage, energy performance, and materials.

A competent workflow is:

1. Confirm project location, occupancy, importance, hazard classification, and applicable code edition.
2. Obtain architectural geometry, levels, spans, materials, and intended use.
3. Review the geotechnical report for soil profile, bearing parameters, groundwater, aggressive conditions, and settlement risks.
4. Confirm design actions and combinations with the responsible engineer.
5. Coordinate structural, architectural, civil, hydraulic, electrical, and mechanical drawings.
6. Issue drawings with revision control, notes, dimensions, levels, schedules, details, and specifications.
7. Record assumptions and identify unresolved information rather than silently guessing.

The draftsman supports design communication but does not independently approve engineering design outside their competence or authorization. Unclear or conflicting drawings must be referred to the engineer. Safety-related omissions, hidden changes, and unrecorded site decisions create professional and legal risk.

10. Selecting a Foundation and Floor System

System selection should combine engineering evidence with construction and lifecycle considerations. A useful decision matrix can be scored after unsuitable options have first been eliminated by code or geotechnical constraints.

A simple weighted score may be calculated as:

where S is the total score, w_i is the weight assigned to criterion i, and r_i is the rating of an option for that criterion. Weights should sum to 1 if a normalized score is desired. The matrix supports judgement; it does not replace structural calculations, code checks, or geotechnical advice.

For example, on firm uniform soil with low-rise masonry walls and no flood exposure, strip footings with a slab-on-ground may be efficient. On weak compressible soil with a lightweight building, a raft or piled solution may be investigated. On a flood-prone site, a suspended floor with elevated foundations may be preferable. These are preliminary directions, not automatic design decisions.

11. Illustrative Case: Small Community Workshop

Background

A single-storey community workshop is proposed on a gently sloping site. The building contains a small office, an open workshop, storage racks, and a covered loading area. The proposed floor level is close to existing ground. Preliminary investigation identifies variable sandy fill over firmer natural soil, with seasonal groundwater approximately 1.8 m below ground.

Problem

The client requests the lowest-cost floor and foundation system. However, the workshop includes movable equipment, storage loads, vehicle access, and a large opening on one side. The site also experiences intense seasonal rain.

Analysis

A simple residential slab assumption would be unreliable because loading is non-uniform and the fill is variable. The draftsman identifies the following risks:

• differential settlement between filled and natural areas;
• cracking at the large loading opening;
• concentrated rack and equipment loads;
• surface water entering at the loading apron;
• possible uplift or moisture problems if groundwater rises;
• lateral wind action on the open-sided canopy.

Approach and solution

The design team requests additional geotechnical information and defines equipment and rack loads. A stiffened slab or raft-like slab is considered for the main workshop, with local thickening or designed pads beneath concentrated loads. The loading apron is separated by a joint, and finished-floor levels are coordinated with external falls and drainage. The canopy receives independent checks for uplift and lateral stability. The drawings identify subgrade preparation, fill replacement limits, joints, damp-proofing, reinforcement zones, and construction sequencing.

Outcome

The selected system costs more than a basic slab but reduces uncertainty and provides a clearer load path. The decision matrix favours it because settlement risk, operational loading, drainage, and durability receive high weights. The final design remains subject to the engineer’s calculations and code approval.

Learning points

Cost comparison must include risk, remedial work, programme, maintenance, and operational performance. A floor system cannot be selected independently of soil, drainage, openings, equipment, and foundation behaviour.

12. Best Practices, Limitations and Contemporary Practice

Best practice begins with reliable information. Do not infer soil capacity from surface appearance, use outdated code tables, or copy a footing detail between sites without checking conditions. Keep structural grids consistent, dimension from reliable datums, distinguish existing from proposed work, and show interfaces clearly.

Building information modelling can improve clash detection, quantity coordination, level control, and revision management. Digital models, laser surveys, drones, and geotechnical databases can support coordination, but software output remains dependent on input quality and professional review. Artificial intelligence may assist with drawing checking, classification, and preliminary option comparison; it must not be treated as an approving engineer or as a substitute for code interpretation, site investigation, or accountable judgement.

Important limitations include uncertainty in soil properties, variability in construction quality, changing groundwater, incomplete load information, simplified preliminary equations, and differences between national codes. Drawings should communicate uncertainty through notes, assumptions, hold points, and references to approved engineering documents.

Chapter Summary

Buildings are load-transfer systems. Dead, live, wind, seismic, water, temperature, soil, and other environmental actions must travel through a continuous structural path to the ground. Structural members may experience compression, tension, bending, shear, bearing, sliding, overturning, and movement.

Tributary area methods provide useful preliminary load estimates. Average bearing pressure can be estimated by q_avg = N/A, but final foundation design requires soil parameters, load combinations, settlement assessment, structural strength checks, and code compliance. Settlement, particularly differential settlement, is a serviceability issue as well as a geotechnical issue.

Strip, pad, combined, raft, pile, slab-on-ground, suspended concrete, timber, and steel systems each have appropriate applications and limitations. Selection should consider soil, groundwater, loads, movement, climate, constructability, cost, durability, coordination, and regulatory requirements. Decision matrices can organize professional judgement but cannot replace engineering design. Accurate, coordinated, revision-controlled drawings are essential to convert the design into safe construction.

Key Terms

Dead load: Permanent weight of structural and fixed building components.

Live load: Variable load caused by occupants, furniture, storage, equipment, or use.

Wind load: Pressure, suction, or frictional action caused by wind.

Environmental load: Action caused by water, temperature, flood, seismic effects, soil movement, corrosion, or similar conditions.

Load path: Continuous route by which forces travel through a structure to the ground.

Tributary area: Area whose load is assigned to a particular supporting member.

Bearing pressure: Contact pressure transmitted from a foundation to soil.

Settlement: Downward or deformational movement of soil and foundation.

Differential settlement: Unequal settlement between parts of a building.

Eccentricity: Distance between a resultant force line and the centroid of a foundation area.

Overturning: Rotational instability caused by an applied moment.

Sliding: Horizontal movement caused by lateral force exceeding resistance.

Strip footing: Continuous footing supporting a wall or line of closely spaced supports.

Pad footing: Isolated footing supporting a column or post.

Raft foundation: Broad foundation supporting much or all of a building footprint.

Pile foundation: Deep foundation transferring load to or through deeper soil.

Slab-on-ground: Floor slab supported directly on prepared ground.

Suspended floor: Floor spanning between supports rather than resting directly on ground.

Serviceability: Performance related to deflection, cracking, vibration, movement, and usability.

Decision matrix: Weighted comparison of alternatives against defined criteria.

Review Questions

5 Short Answer Questions

1. What is meant by a building’s load path?
2. Distinguish dead load from live load and give two examples of each.
3. State two reasons why average bearing pressure does not prove that a footing is adequate.
4. What is differential settlement, and why can it be harmful?
5. Identify two documents or sources that should be reviewed before selecting a foundation system.

5 Long Answer Questions

1. Explain how gravity and lateral loads travel through a low-rise framed building to the soil.
2. Compare strip, pad, raft, and pile foundations in terms of application, advantages, and limitations.
3. Explain the main structural and construction considerations for a slab-on-ground floor.
4. Discuss how groundwater, flood, temperature, and expansive soil movement can affect foundation and floor selection.
5. Describe a professional workflow for producing coordinated foundation and floor drawings.

3 Analytical / Application Questions

1. A column has a 20 m² tributary area. The characteristic permanent floor load is 3.0 kN/m² and the characteristic imposed load is 2.5 kN/m². Calculate the two characteristic load components and the total before adding column and footing self-weight. Explain why the result is not yet a final design action.
2. A centrally loaded service force of 120 kN is supported by a 2.5 m × 2.0 m footing. Calculate the preliminary average bearing pressure and list four additional checks required before approval.
3. A proposed building occupies variable fill beside a flood-prone drainage channel and includes heavy storage racks. Recommend two foundation/floor options for investigation, identify the information required to compare them, and justify the criteria in a decision matrix.

Multiple Choice Questions with Answers

Question 1

Which action is normally classified as a dead load?

A. Occupants moving through a room

B. A permanently fixed ceiling system

C. A temporary stack of materials

D. Wind pressure on a wall

Correct Answer: B. A permanently fixed ceiling system

Question 2

A floor has a tributary area of 12 m² and a uniform load of 4 kN/m². What load is assigned to the supporting member using the tributary-area method?

A. 3 kN

B. 16 kN

C. 48 kN

D. 96 kN

Correct Answer: C. 48 kN

Question 3

What is the preliminary average bearing pressure for a 100 kN service load on a 2 m² footing?

A. 20 kPa

B. 50 kPa

C. 100 kPa

D. 200 kPa

Correct Answer: B. 50 kPa

Question 4

Which condition most directly describes differential settlement?

A. Uniform downward movement of the whole building

B. Unequal movement between different building supports

C. Horizontal movement caused by wind

D. Temporary vibration from machinery

Correct Answer: B. Unequal movement between different building supports

Question 5

Which foundation is generally continuous beneath a load-bearing wall?

A. Pad footing

B. Pile cap only

C. Strip footing

D. Isolated pier footing

Correct Answer: C. Strip footing

Question 6

For static water pressure, which relationship is used?

A. p = γ_w h

B. p = γ_w/h

C. p = h/γ_w

D. p = γ_w + h

Correct Answer: A. p = γ_w h

Question 7

A footing subjected to an overturning moment must particularly be checked for which issue?

A. Paint colour

B. Eccentricity and possible loss of contact

C. Ceiling acoustic absorption only

D. Window ventilation rate

Correct Answer: B. Eccentricity and possible loss of contact

Question 8

Which floor system transfers much of its load directly to prepared subgrade?

A. Slab-on-ground

B. Suspended beam floor

C. Pile-supported deck

D. Roof truss floor

Correct Answer: A. Slab-on-ground

Question 9

What is the main purpose of a geotechnical investigation in foundation selection?

A. To determine interior paint colours

B. To establish soil and groundwater conditions relevant to support and movement

C. To calculate electrical demand

D. To select furniture layouts

Correct Answer: B. To establish soil and groundwater conditions relevant to support and movement

Question 10

A decision matrix is most appropriately used to:

A. Replace all structural calculations

B. Approve a foundation without a site investigation

C. Organize comparison of feasible alternatives against weighted criteria

D. Determine concrete strength without testing

Correct Answer: C. Organize comparison of feasible alternatives against weighted criteria