Structural Loads, Foundation Engineering, and Seismic Design
1. Types of structural loads
a structural load is a force or action acting on a building that produces stress, strain, deformation or displacement. Proper assessment of loads is essential because structural failure can occur when the load-carrying capacity of a structure is lower than the actual load acting on it. Structural loads may produce compression, tension and shear. Tos v unit 01 part i 2026
1. Dead load
dead load is a permanent or stationary load acting throughout the life of the structure. It mainly consists of the self-weight of structural members and permanently fixed components.
it includes:
- self-weight of slabs, beams, columns and walls.
- permanent floor and roof finishes.
- fixed equipment and permanent furniture.
- other permanently attached components.
the unit weight of materials is used for calculating dead load. For example, the notes give brick masonry as 18.8 kn/m³, timber as 5.8 kn/m³, pcc as 24 kn/m³ and rcc as 24 kn/m³.
2. Imposed or live load
live load is produced by the occupancy and use of a building. It is not permanently fixed and may vary with time.
it includes:
- people.
- movable furniture.
- equipment.
- stored materials.
- impact and vibration associated with occupancy.
live load is generally considered as a uniformly distributed load or, where applicable, a concentrated/point load. Tos v unit 01 part i 2026
3. Wind load
wind load is mainly a horizontal load produced by the movement of air against the building. It depends on wind speed, building height, shape, surrounding terrain and geographical location.
4. Snow load
snow load acts mainly on roofs in regions where snow accumulation occurs. It depends on:
- roof geometry.
- shape and size of structure.
- insulation.
- wind conditions.
- duration of snow.
- geographical location.
5. Earthquake load
earthquake or seismic load is caused by ground motion during an earthquake. It produces dynamic forces in the building and can act in different horizontal directions.
important principles for earthquake-resistant buildings include:
- lightness.
- simple and symmetrical configuration.
- strength in both directions.
- ductility.
- proper connection of non-structural components. Tos v unit 01 part i 2026
6. Longitudinal load
longitudinal loads include tractive and braking forces, particularly relevant to structures associated with moving loads.
7. Accidental loads
these arise from unusual human-related events and are generally short-duration loads. Examples include:
- collision/impact.
- fire.
- explosion. Tos v unit 01 part i 2026
8. Other loads
other possible actions include:
- foundation movement.
- vibration.
soil pressure.- hydrostatic pressure.
- elastic axial shortening.
- fatigue.
- stress concentration.
- impact reactions.
sketch: draw a building frame and show downward arrows for dead/live loads, horizontal arrows for wind, lateral arrows for earthquake, and soil/hydrostatic pressure at the foundation.
2. One-way slab and two-way slab — 5/10 marks
one-way slab
a one-way slab is a slab in which the load is primarily transferred in one direction, generally toward the two opposite supporting sides.
- it generally occurs when the slab is supported on two opposite sides.
- it can also occur when the ratio of longer span to shorter span is sufficiently large.
- main bending occurs along the shorter span.
- main reinforcement is therefore provided along the shorter span.
- distribution reinforcement is provided perpendicular to the main reinforcement.
- load travels primarily from slab → supporting beams/walls → columns.
two-way slab
a two-way slab transfers load in two perpendicular directions.
- it is generally supported on all four sides.
- load is distributed toward all four supporting edges.
- bending occurs in both directions.
- reinforcement is therefore provided in both directions.
- it is commonly used when the slab panel is relatively square.
- corner and edge behaviour must also be considered in design.
main difference:
one-way slab → load/bending mainly in one direction → main reinforcement in one direction.
two-way slab → load/bending in two directions → reinforcement in both directions.
sketch: draw a rectangular slab with arrows moving toward two opposite sides for a one-way slab and arrows moving toward all four sides for a two-way slab.
3. Seismic zones of india
india is divided into seismic zones according to the expected level of seismic activity. The seismic zoning map is used to understand the earthquake risk of different regions and to design buildings appropriately.
the four commonly identified zones are:
zone ii – low damage risk zone
- relatively lower seismic intensity.
- buildings still require appropriate structural design.
zone iii – moderate damage risk zone
- moderate seismic risk.
- earthquake-resistant design becomes increasingly important.
zone iv – high damage risk zone
- high seismic risk.
- buildings require stronger earthquake-resistant provisions.
zone v – very high damage risk zone
- highest seismic risk category.
- special attention to structural configuration, ductility and earthquake-resistant design is required.
sketch: draw a simplified map of india and mark zone ii, zone iii, zone iv and zone v with different hatchings/colours.
4. Magnitude of earthquake — 5/10 marks
magnitude represents the size or energy released by an earthquake at its source.
- it is a measure of the earthquake itself rather than its effect at a particular location.
- magnitude is determined from recorded seismic waves.
- it is expressed using a numerical scale.
- a larger magnitude indicates greater energy release.
- magnitude is generally represented as a single value for a particular earthquake.
- modern seismology commonly uses the moment magnitude scale (mw) for significant earthquakes.
- magnitude should not be confused with intensity.
- a single earthquake has one overall magnitude, although different locations experience different intensities.
example: the 2001 bhuj earthquake had a magnitude of approximately mw 7.7.
memory:
magnitude = energy/size of earthquake at source.
5. Intensity of earthquake — 5/10 marks
intensity describes the observed effects of an earthquake at a particular location.
- it is based on the actual effects experienced by people, buildings and the ground.
- intensity varies from place to place during the same earthquake.
- it depends on distance from the epicentre.
- it is influenced by local soil conditions.
- it also depends on the construction quality of buildings.
- ground conditions can amplify or reduce earthquake effects.
- intensity is commonly expressed using the modified mercalli intensity (mmi) scale.
- the scale ranges from i to xii.
- lower intensities may be felt only by a few people.
- higher intensities may produce severe structural damage and ground effects.
example: during a major earthquake, a location close to the epicentre may experience mmi viii, while a distant location may experience a much lower intensity.
difference:
magnitude = energy released at source.
intensity = observed effect at a particular place.
6. Causes of earthquake — 10 marks
covers: assignment-2 q4.
an earthquake is the sudden release of accumulated energy within the earth’s crust, producing seismic waves and ground motion.
1. Tectonic movement
the most important cause is movement of tectonic plates. Plates continuously move because of forces within the earth.
2. Fault movement
when stress accumulated along a geological fault exceeds the strength of the surrounding rock, sudden movement occurs.
this releases stored strain energy as seismic waves.
3. Elastic rebound
rocks deform due to accumulated stress. When the stress exceeds the rock’s capacity, the rock suddenly slips back toward a less strained condition. This sudden release produces an earthquake.
4. Volcanic activity
movement of magma and volcanic activity can generate earthquakes around volcanic regions.
5. Landslides and rock failures
large landslides and sudden underground rock movements can produce local seismic vibrations.
6. Human-induced activity
certain human activities can induce seismic events, including:
- large reservoir loading.
- mining.
- fluid injection.
- extraction of underground resources.
effects
earthquakes may cause:
- ground shaking.
- structural damage.
- foundation failure.
- landslides.
- liquefaction.
- surface rupture.
- tsunami in underwater earthquakes.
7. World’s major earthquakes — 5/10 marks
major earthquakes demonstrate the destructive potential of seismic events and the importance of earthquake-resistant design.
1. 2001 bhuj earthquake, india
- occurred in gujarat.
- major destruction occurred in bhuj, ahmedabad and surrounding areas.
- it demonstrated the importance of seismic-resistant construction.
2. 2004 indian ocean earthquake
- a major undersea earthquake near sumatra.
- generated a devastating tsunami.
- affected several countries around the indian ocean.
3. 2015 nepal earthquake
- caused extensive damage in nepal.
- kathmandu valley was severely affected.
- historic buildings and infrastructure were damaged.
4. 2011 tÅhoku earthquake, japan
- a very large offshore earthquake.
- generated a major tsunami.
- demonstrated the importance of advanced earthquake-resistant engineering.
5. 2010 haiti earthquake
- caused severe destruction around port-au-prince.
- poor structural vulnerability significantly increased losses.
architectural lesson: major earthquakes demonstrate the importance of simple building configuration, ductility, adequate strength, proper detailing and appropriate foundation design.
8. Ultimate bearing capacity and safe bearing capacity — 10 marks
covers: mid-term q1(e) + assignment-1 q4.
ultimate bearing capacity
ultimate bearing capacity is the maximum pressure at which the soil begins to fail under a foundation.
it represents the absolute maximum pressure that the soil can sustain before shear failure. Doc-20260827-wa0002_260908_0011…
safe bearing capacity
safe bearing capacity is the maximum permissible pressure that soil can safely carry without shear failure and without exceeding permissible settlement. Doc-20260827-wa0002_260908_0011…
the unit is generally kn/m².
relationship
the notes state that safe bearing capacity is obtained from ultimate bearing capacity using a factor of safety.
sbc = ultimate bearing capacity / factor of safety
the notes indicate that a factor of safety of 3 is generally considered for soil. Doc-20260827-wa0002_260908_0011…
methods of determination
1. Plate load test
- a test pit is prepared.
- a standard plate is placed on the soil.
- load is applied gradually.
- settlement corresponding to each load increment is recorded.
- the load-settlement relationship is studied.
- ultimate bearing capacity and settlement behaviour are determined.
the plate load test is used to determine both bearing capacity and settlement. Doc-20260827-wa0002_260908_0011…
2. Standard penetration test
- a standard tube is driven into the soil.
- the number of blows required for penetration is recorded.
- this penetration resistance is used to assess soil strength and bearing capacity. Doc-20260827-wa0002_260908_0011…
other tests include direct shear, triaxial shear and unconfined compression tests.
9. Soil investigation — 5/10 marks
covers: mid-term q1(f).
soil investigation is the systematic study of the soil and geological conditions of a site before construction.
objectives
- determine the type and characteristics of soil.
- determine safe bearing capacity.
- study settlement characteristics.
- identify groundwater conditions.
- determine depth and nature of suitable founding strata.
- select an appropriate foundation system.
- identify problematic soils.
- assess whether the site can safely support the proposed building.
a soil report is generally prepared by geotechnical/civil engineers after site investigation and indicates whether the ground conditions can provide the required support and stability. Doc-20260827-wa0002_260908_0011…
typical investigation process
site survey → boreholes/test pits → soil sampling → laboratory/field tests → soil classification → sbc assessment → groundwater assessment → foundation recommendation
importance
without adequate investigation:
- foundation settlement may occur.
- differential settlement may develop.
- shear failure may occur.
- expansive soil may damage foundations.
- foundation selection may be inappropriate.
10. Classification of soil for engineering purpose — 10 marks
covers: mid-term q3 + assignment-1 q3.
soil classification is the arrangement of soils into groups having similar engineering behaviour under given physical conditions. The two major properties used are grain-size distribution and plasticity. Doc-20260827-wa0002_260908_0011…
1. Unified soil classification system (uscs)
developed originally by a. Casagrande.
- coarse-grained soils are classified mainly by grain-size distribution.
- fine-grained soils are classified mainly by plasticity.
- gravel is represented by g.
- sand is represented by s.
- silt is represented by m.
- clay is represented by c.
- organic soil is represented by o.
- peat is represented by pt.
coarse-grained soil has 50% or more material retained on the 0.075 mm sieve. Gravel has 50% or more of the coarse fraction retained on the 4.75 mm sieve; otherwise it is classified as sand. Doc-20260827-wa0002_260908_0011…
2. Aashto classification
the aashto system divides soil into groups a-1 to a-7, with an additional a-8 for peat.
it uses a group index (gi) to indicate the suitability of soil for highway construction.
- gi = 0 → generally good subgrade.
- higher gi → poorer subgrade.
- gi of 20 or more indicates a very poor subgrade in the notes. Doc-20260827-wa0002_260908_0011…
3. Indian standard soil classification system (isscs)
isscs is broadly similar to uscs.
the major distinction is that fine-grained soils are subdivided according to low, medium and high compressibility.
coarse-grained soils are classified according to grain-size distribution and fine-grained soils according to plasticity.
11. Three-phase diagram of soil — 10 marks
covers: assignment-1 q2 + mid-term q3.
soil is generally considered a three-phase system consisting of:
- solid phase – soil particles.
- water phase – water occupying some of the voids.
- air phase – air occupying the remaining voids.
three components
soil solids + water + air = total soil mass/volume system
the voids between soil particles may contain water, air, or both.
important terms
volume relationships:
- total volume = volume of solids + volume of voids.
- volume of voids = volume of water + volume of air.
therefore:
v = vs + vv
and
vv = vw + va
water content
water content represents the amount of water present in soil in relation to the weight of soil solids.
degree of saturation
it indicates the proportion of voids filled with water.
- 0% saturation → completely dry soil.
- 100% saturation → all voids filled with water.
importance
the three-phase system helps understand:
- soil density.
- water content.
- void ratio.
- porosity.
- degree of saturation.
- compaction.
- settlement.
- bearing behaviour.
12. Foundation types — shallow and deep — 10 marks
covers: assignment-1 q5.
foundations transfer structural loads safely to the supporting soil.
they are broadly classified into:
a. Shallow foundations
b. Deep foundations
a. Shallow foundations
1. Isolated/individual footing
used for a single column.
- usually square or rectangular.
- suitable when soil has adequate bearing capacity at shallow depth.
- size depends on column load and sbc.
- rectangular footing may be used when moments/eccentricity occur. Doc-20260827-wa0002_260908_0011…
2. Combined footing
used when two or more columns are close enough that their individual footings would overlap.
- usually rectangular.
- supports more than one column.
3. Strip/wall footing
used below load-bearing walls or closely spaced supports.
- spreads load over a larger area.
- suitable when adequate bearing soil exists close to ground level.
4. Raft/mat foundation
a large foundation slab covering a major part or the entire building footprint.
used when:
- loads are high.
- individual footings would occupy a large area.
- differential settlement needs to be controlled.
- soil has relatively low bearing capacity. Doc-20260827-wa0002_260908_0011…
b. Deep foundations
5. Pile foundation
used to transfer heavy loads to deeper competent strata.
it resists load through:
- skin friction.
- end bearing.
it is useful where near-surface soil is unsuitable for heavy loads. Doc-20260827-wa0002_260908_0011…
6. Drilled shaft/caisson
a large-diameter deep foundation constructed using drilling methods.
it can carry large column loads and transfers load through shaft and/or toe resistance. Doc-20260827-wa0002_260908_0011…
selection
foundation selection depends on:
- structural load.
- soil bearing capacity.
- soil type.
- groundwater.
- settlement.
- building height.
- earthquake conditions.
- economy and construction feasibility.
13. Black cotton soil — foundation and construction procedure — 10 marks
covers: assignment-1 q6.
black cotton soil is an expansive soil that undergoes significant swelling when wet and shrinkage when dry.
this volume change can cause:
- foundation movement.
- cracks.
- differential settlement.
- uplift.
- reduction in bearing capacity.
the notes state that black cotton soil can undergo approximately 20–30% volume change, with cracks developing due to expansion and contraction. Doc-20260827-wa0002_260908_0011…
suitable foundation
the notes indicate that foundation selection in black cotton soil depends on the severity and site conditions. For deeper problematic soil, pile/deep foundation solutions may be appropriate; other solutions include suitable raft or under-reamed pile systems depending on conditions.
construction procedure given in the notes
- foundation trenches are dug 30 cm deeper than the maximum depth of cracks.
- trench width is selected so that allowable bearing pressure does not exceed approximately 15 t/m² as stated in the notes.
- a gravel bed is placed in the trench.
- the gravel is properly compacted.
- a 50 cm layer of concrete is laid over the gravel.
- masonry is constructed over the concrete up to plinth level.
- the sides of the masonry are filled with sand.
- proper drainage should be provided to control water entering the expansive soil.
14. Load estimation on roof and floor slabs — stepwise procedure
step 1 – identify the slab
determine:
- slab dimensions.
- thickness.
- support condition.
- floor/roof usage.
step 2 – calculate slab self-weight
calculate the self-weight using:
self-weight = thickness × unit weight of material
for rcc, the notes give approximately 24 kn/m³. Tos v unit 01 part i 2026
step 3 – add floor finishes
include the weight of:
- flooring.
- screed.
- ceiling/plaster where applicable.
- waterproofing for roofs where applicable.
step 4 – add wall/partition load
where walls or partitions are supported by the slab, their load should be included according to the structural arrangement.
step 5 – add imposed/live load
select the appropriate load based on occupancy.
for example, the notes list:
- residential rooms: 2 kn/m²
- residential passage/stair/store: 3 kn/m²
- store room: 5 kn/m²
- office: 2.5 kn/m². Tos v unit 01 part i 2026
step 6 – check concentrated loads
where applicable, concentrated/point loads must also be considered.
step 7 – roof-specific loads
for roofs consider:
- imposed roof load.
- rainwater accumulation.
- dust load.
- snow load where applicable.
- roof finishes/waterproofing.
the notes state that for flat/sloping/curved roofs up to 10°, the imposed load is 1.5 kn/m² where access is provided and 0.75 kn/m² where access is not provided. Tos v unit 01 part i 2026
step 8 – obtain total slab load
total slab load = dead load + finishes + wall/partition load + live load + applicable additional loads
step 9 – transfer load
the slab load is transferred to supporting beams/walls and then to columns/foundations.
15. Load estimation on beams — stepwise procedure
the beam receives loads from the slab, walls and its own self-weight and transfers them to columns or other supports.
step 1 – determine beam dimensions
identify:
- beam width.
- beam depth.
- beam span.
step 2 – calculate beam self-weight
beam self-weight = width × depth × unit weight × length
the notes specifically identify beam self-weight as a load to be considered per running metre. Tos v unit 01 part i 2026
step 3 – calculate slab load transferred to beam
determine the tributary area of slab contributing to the beam.
convert the slab area load into beam load:
beam load from slab = slab load × tributary width
step 4 – calculate wall load
if a wall rests on the beam:
wall load = wall thickness × wall height × unit weight
the notes identify wall load per running metre as one of the loads to be considered. Tos v unit 01 part i 2026
step 5 – add finishes and other permanent loads
include applicable permanent loads supported by the beam.
step 6 – add imposed load
include the live/imposed load transferred from the slab.
step 7 – obtain total beam load
total beam load = beam self-weight + slab load + wall load + other applicable loads
step 8 – transfer to columns
the beam transfers its load through its supports to columns and subsequently to the foundation.
the notes explain the overall load path as:
slab → beam → column → foundation.
sketch: draw a simply supported beam with slab load downward, wall load downward, beam self-weight downward and reactions upward at columns.
16. Seismic base shear — meaning and effect — 10 marks
seismic base shear is the total horizontal design earthquake force acting at the base of a building due to seismic ground motion.
it represents the overall lateral earthquake demand that the structural system must resist.
importance
- it is an important parameter in earthquake-resistant structural design.
- it represents the total design lateral force at the building base.
- the force is distributed to different floors/elevations.
- it produces lateral shear and overturning effects.
- columns, beams, shear walls and foundations must resist these effects.
- larger building mass generally produces larger seismic force.
- building height and dynamic characteristics influence seismic response.
- seismic zone affects the design seismic demand.
- soil conditions affect seismic response.
- building configuration and structural system influence force distribution.
effects on buildings
seismic base shear can produce:
- lateral displacement.
- storey drift.
- bending in columns and beams.
- shear forces.
- overturning.
- increased foundation forces.
- damage to non-structural components if detailing is inadequate.
the unit 01 notes emphasise lightness, simple/symmetrical configuration, strength in both directions, ductility and protection of non-structural components as earthquake-resistant design principles.
17. Static equivalent method for base shear — stepwise procedure
the static equivalent method represents the earthquake effect by an equivalent set of static lateral forces acting at different floor levels.
step 1 – determine building parameters
identify:
- building height.
- number of storeys.
- floor masses/weights.
- structural system.
- seismic zone.
- soil/site condition.
step 2 – determine seismic weight
calculate the effective seismic weight of the building from the applicable permanent loads and the relevant portion of imposed loads.
step 3 – determine fundamental natural period
estimate the fundamental time period according to the applicable provisions of the seismic code.
step 4 – determine design horizontal seismic coefficient
determine the design seismic coefficient based on:
- seismic zone.
- importance factor.
- response reduction factor.
- soil/site characteristics.
- fundamental period.
step 5 – calculate design base shear
the basic relationship is:
vᵦ = aₕ × w
where:
vᵦ = design base shear
aₕ = design horizontal seismic coefficient
w = seismic weight of building
step 6 – distribute base shear vertically
the total base shear is distributed among the floors according to their seismic weights and heights as required by the applicable code procedure.
step 7 – apply forces to structural system
the calculated lateral forces are applied at the appropriate floor levels.
step 8 – check structural response
the structure is checked for:
- lateral forces.
- storey shear.
- bending.
- drift.
- stability.
- foundation effects.
18. Load path of a building
important extra topic — useful for q2, assignment q7/q8 and load numericals.
the load path explains how loads travel through a structural system.
roof/floor load → slab → beam → column → foundation → soil
slab : receives floor/roof loads and transfers them to supporting members.
beam : receives loads from slabs and walls and transfers them to columns/supports.
column : carries mainly axial compression and transfers the load toward the foundation.
foundation : spreads/transfers structural load to the soil safely.
19. Important extra: wind load
this is not a separate question in the shown paper, but it is important because it is part of the unit 01 load classification.
- wind load is primarily horizontal.
- it results from movement of air relative to the earth.
- it depends on geographical location.
- building height affects wind pressure.
- terrain roughness affects wind speed.
- building size and shape affect wind action.
- local topography affects wind characteristics.
- design wind speed is obtained using the relevant factors.
- the notes give:
vz = vb × k1 × k2 × k3
- design wind pressure is given as:
pz = 0.6 vz²
- wind force on an individual member depends on external and internal pressure coefficients and exposed area.
20. Important extra: foundation selection criteria
foundation selection depends mainly on the relationship between structural loads and ground conditions.
important factors are:
- structural load.
- building height.
- construction material.
- soil bearing capacity.
- soil type.
- settlement.
- groundwater.
- earthquake susceptibility.
- depth of competent soil.
- economy and construction feasibility.
for low-rise buildings, shallow foundations may be suitable when adequate bearing soil exists near the surface. For high-rise or heavily loaded buildings, deep foundations may be necessary.
memory: load + soil + settlement + water + seismic condition → foundation selection
21. Important extra: factors affecting safe bearing capacity
- type of soil – cohesion, friction and shear strength influence sbc.
- water table – a high water table can reduce bearing capacity.
- density – denser soil generally has higher bearing capacity.
- void ratio – greater voids can reduce bearing capacity.
- porosity – affects seepage and soil strength.
- soil structure – arrangement and properties of particles influence behaviour.
- foundation dimensions and depth influence the pressure transferred to soil.
- settlement characteristics must be considered along with shear failure.
the notes specifically identify soil type, water table, density, void ratio and porosity as important factors.