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Basis of Structural Design

Course 6

Structural action:- Foundations

- General remarks on structural action

Course notes are available for download athttp://www.ct.upt.ro/users/AurelStratan/

Foundations

Most structures invariably rest on the ground

The best solution would be to place the supports of astructure on solid rock, but this is seldom possible

In most cases solid rocks lies deep in the ground, withsofter and weaker soil layers above it

Relatively high stresses in the superstructure have to besafely transferred to the much softer and weaker soil.This is done through foundations

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Types of foundations

Isolated footing – Top soil layer is removed and a block ofconcrete, wider than the one which restson it, is placed on the ground

– Plan dimensions of the isolated footingneed to be larger than the ones of thecolumn, in order to have lower stressesat the foundation-soil interface

– Foundation dimensions should be largeenough to allow stresses acting on thesoil to be smaller than the soil strength

Continuous footing: when thestructural member to be supportedby the foundation is a wall, thefooting is realised continuouslybelow the wall, following theconcept of the isolated footing


Raft foundation: – When the soil is very poor, larger

area is required for the foundation,which extends over the full plandimension of the building

– Raft foundations were developed byRomans, who built them fromhydraulic concrete several metresdeep

– Modern raft foundations are muchthinner, as they area realised fromreinforced concrete

– Raft foundations can beconstructed as a series of boxes,with the walls in the basementcontributing to the strength of thefoundation and enabling thinnerslab

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Isolated and continuous footings, and raft foundationsare shallow foundations: – placed relatively close to the surface of the ground – loads are transferred from the building to the soil by providing

large enough area of the foundation in order to reduce stressesbelow the ones allowed by the strength of the soil


Pile foundations: – Soil properties get better as the depth

increases. When the soil near the surface isvery poor, pile foundations can be used.

– Pile foundations are made of tree trunks (inold times), steel or reinforced concrete (inmodern times)

– Loads are transferred to the soil throughshear stresses between the pile shaft andthe soil (major contribution) andcompression stresses at the bottom of thepile (minor contribution)

– Piles are long, enabling them to reachstronger and stiffer soil layers, or evensolid rock

– First pile foundations date back to Neolithicperiod, and were made of tree trunks

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Pile foundations: – Without pile foundations, cities like Veniceand Amsterdam, located due to strategicand economic reasons on marshes couldnot have been developed at all

– Wooden piles were usually of oak or, in thesea, of greenheart from Central America,which is particularly resistant to marineborers

– Pile foundations can be installed by eitherdriving them into the ground (wooden, steeland precast concrete) or drilling a shaft and

filling it with concretePiles are deep foundations, in whichloads are transferred to the soil byreaching deeper and stronger soillayers.


Cofferdam foundations – Cofferdam is an enclosure beneath

the water constructed to allowwater to be displaced by air for thepurpose of creating a dry workenvironment

– Were developed by Romans andremained mainly unchanged untilthe early 19th century

– Pneumatic caissons were theninvented, allowing underwaterfoundations to be excavated,keeping the water out by airpressure. Difficult and expensiveto operate.

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Failure of foundations

Complete failures of foundations are rare, though theymay happen – Example: Transcona grain silo, Winipeg , Canada. In October

1913, this grain silo started to tip over. It was loaded with over amillion bushels of wheat and was newly built. It continued to sinkslowly for over 12 hours until finally it was at an angle of 30degrees from vertical but still intact. The wheat was emptied fromthe bins, and work began to right it. By tunelling underneath it,they built new foundations down to the bedrock and then pushedit back into position. It is still in use today

Failure of foundations

Complete failure of foundations are rare, though they mayhappen – Example: Tilting of apartment buildings at Kawagishi-Cho,

Niigata, produced by liquefaction of the soil during the 1964Niigata Earthquake

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Foundation soil behaviour

The biggest problem of foundations is soil settlement,especially the differential settlement, of various parts of astructure, leading to cracking and distortion of thesuperstructureSoil can vary greatly in composition from one point toanother, even under the same structureSoil properties are greatly affected by ground waterSoil consists of a mass of solid particles (soil skeleton) ofsand and/or clay, more or less loosely packed, and thespaces between them filled with water

In an undisturbed soil the weight of the earth above iscarried by solid particles, and the water in pores is atnormal pressure of water at that level below the watertable


Soil skeleton is much more compressible than water, andwhen an additional load (e.g. from a building) comes ontothe ground, – At first, the additional compressive stress in the soil is carried

entirely by water because it is stiffer than the soil skeleton – The pore pressure increases and it is squeezed out sideways

from under the foundation – Pore water pressure drops gradually back to normal values at that

depth, as the soil skeleton is compressed enough to carry itselfthe loads

In fine clays the water escapes slowly and the process ofconsolidation under a foundation can take many years

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Problems due to settlement can arise when: – Soil property changes at different points under the same structure – When construction of the building proceeds fast (as is the case in

modern times) – When an additional heavy load (e.g. a tower in old times) is added

after the bulk of the structure is completed and has settled – Ground water is pumped out. Notorious instances: Venice and

Mexico-city

Example: Venice – Water supply in Venice originally came from mainland – Starting from 1910, this was increasingly supplemented from

boreholes up to 300 m deep

– General subsidence of buildings (100-200 mm) ⇒⇒⇒⇒ extremelydamaging to buildings as walls of most Venetian houses start atonly about 1 m above average sea level


Example: Mexico-city: – Most of the city is built on a soft bed 30-40 m deep of a dried-up

lake – Building settlement reached constant levels and was not a

problem – In the 19th century pumping started from deep wells to

supplement water supply – Today the ground level in the centre of the city is more than 6 m

lower than it was in 1900 – Old buildings, sewers and water pipes much affected

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General remarks on structural action

[1] Structures support loads inthe most direct way open tothem – bowstring truss: if the top chord

has the right shape for the givenloading, loads pass directly to thesupport, ignoring the webmembers

– a lateral load at the top of atriangular tower is transmitteddown the two main members whilethe inner bars are unstressed


[1] Structures support loads inthe most direct way open tothem – (a) the load applied at the top of a

column in the frame from thefigure goes directly to thefoundation through the column,while the rest of structure isvirtually unstressed

– (b) if the direct path is interrupted,the load path is much morecomplicated, and the stresses anddeflections are greatly increased

– Rule: provide paths as simple andas direct as possible for the loadsto pass to the supports

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Characteristics of a well-designed structure: – elements are few and well-disposed – their function is obvious, and – the whole effect inspires confidence

well conceived structure

ill-conceived structure


[2] The larger the structure, – the more important is the own weight of the structure in

comparison with other loads – the more important is that structural elements be arranged as

efficiently as possible

Example: simply supported beam bridge – moment larger at the midspan – provide more material at the midspan

to increase the moment resistance

– larger loads at the midspan

– larger moments

– inefficient structural configuration – Bridges using simply-supported beams are most often of

constant cross-section and are used for small spans only

Mmax

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Example: cantileverbridge – the moment due to dead

weight is largest at thesupport

– the material must beconcentrated at thesupports

– a load near the supportproduces only a smallincrease in moment

– efficient structure for largebridges


[3] Statically indeterminate structures support loads inthe stiffest mode open to them – very often, load paths can take two alternatives: direct

tension/compression or bending – a thin plate loaded transversally supports loads by bending but

direct (membrane) action develops rapidly as the plate deflects

– thin shells support transverse loads as far as possible bycompressive membrane forces rather than bending

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[3] Statically indeterminate structures support loads inthe stiffest mode open to them – sometimes simple change in a structure allows loads to be

carried in a more efficient way:• portal frame with a concentrated load at the ridge develops bending

stresses• if a tie is inserted between the two sides of the eaves level. The two

rafters and the tie form a triangulated structure. The loads aretransmitted through compression in the rafters, tension in ties,compression in beams, with negligible bending.


[4] Direct tension is preferable to direct compression – it is rarely possible to avoid compression – even in predominantly tension structures as suspension bridges,

tensile forces in cables must be balanced by compressive forcesin towers

– minimize the loss of efficiency due to compression by:•

keeping the compressive members short• use a material (e.g concrete) with lower strength, and therefore morestocky members less prone to instability

[5] In statically indeterminate structures, the stifferelements will attract larger forces – Example: portal frames are often

haunched near the corners• further increase of bending moments

at the corners though actual stressesreduce due increase of cross-section

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