Basis of Structural Design Course 6 Structural action: -Foundations -General remarks on structural...

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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 a

structure on solid rock, but this is seldom possible

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

� Relatively high stresses in the superstructure have to be safely 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 of

concrete, wider than the one which rests

on it, is placed on the ground

– Plan dimensions of the isolated footing

need to be larger than the ones of the

column, in order to have lower stresses

at the foundation-soil interface

– Foundation dimensions should be large

enough to allow stresses acting on the

soil to be smaller than the soil strength

� Continuous footing: when the

structural member to be supported

by the foundation is a wall, the footing is realised continuously below the wall, following the concept of the isolated footing


� Raft foundation:

– When the soil is very poor, larger

area is required for the foundation,

which extends over the full plan

dimension of the building

– Raft foundations were developed by

Romans, who built them from

hydraulic concrete several metres

deep

– Modern raft foundations are much

thinner, as they area realised from

reinforced concrete

– Raft foundations can be

constructed as a series of boxes,

with the walls in the basement

contributing to the strength of the

foundation and enabling thinner

slab

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� Isolated and continuous footings, and raft foundations

are 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 stresses

below 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 is

very poor, pile foundations can be used.

– Pile foundations are made of tree trunks (in

old times), steel or reinforced concrete (in

modern times)

– Loads are transferred to the soil through

shear stresses between the pile shaft and

the soil (major contribution) and

compression stresses at the bottom of the

pile (minor contribution)

– Piles are long, enabling them to reach

stronger and stiffer soil layers, or even

solid rock

– First pile foundations date back to Neolithic

period, and were made of tree trunks

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� Pile foundations:

– Without pile foundations, cities like Venice

and Amsterdam, located due to strategic

and economic reasons on marshes could

not have been developed at all

– Wooden piles were usually of oak or, in the

sea, of greenheart from Central America,

which is particularly resistant to marine

borers

– Pile foundations can be installed by either

driving them into the ground (wooden, steel

and precast concrete) or drilling a shaft and

filling it with concrete

� Piles are deep foundations, in which

loads are transferred to the soil by reaching deeper and stronger soil layers.


� Cofferdam foundations

– Cofferdam is an enclosure beneath

the water constructed to allow

water to be displaced by air for the

purpose of creating a dry work

environment

– Were developed by Romans and

remained mainly unchanged until

the early 19th century

– Pneumatic caissons were then

invented, allowing underwater

foundations to be excavated,

keeping the water out by air

pressure. Difficult and expensive

to operate.

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Failure of foundations

� Complete failures of foundations are rare, though they

may happen

– Example: Transcona grain silo, Winipeg , Canada. In October

1913, this grain silo started to tip over. It was loaded with over a

million bushels of wheat and was newly built. It continued to sink

slowly for over 12 hours until finally it was at an angle of 30

degrees from vertical but still intact. The wheat was emptied from

the bins, and work began to right it. By tunelling underneath it,

they built new foundations down to the bedrock and then pushed

it back into position. It is still in use today

Failure of foundations

� Complete failure of foundations are rare, though they may happen

– Example: Tilting of apartment buildings at Kawagishi-Cho,

Niigata, produced by liquefaction of the soil during the 1964

Niigata Earthquake

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Foundation soil behaviour

� The biggest problem of foundations is soil settlement,

especially the differential settlement, of various parts of a

structure, leading to cracking and distortion of the superstructure

� Soil can vary greatly in composition from one point to

another, even under the same structure

� Soil properties are greatly affected by ground water

� Soil consists of a mass of solid particles (soil skeleton) of

sand and/or clay, more or less loosely packed, and the

spaces between them filled with water

� In an undisturbed soil the weight of the earth above is carried by solid particles, and the water in pores is at normal pressure of water at that level below the water table


� Soil skeleton is much more compressible than water, and when an additional load (e.g. from a building) comes onto the 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 itself

the loads

� In fine clays the water escapes slowly and the process of consolidation 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) ⇒⇒⇒⇒ extremely damaging to buildings as walls of most Venetian houses start at

only 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 in

the most direct way open to

them

– bowstring truss: if the top chord

has the right shape for the given

loading, loads pass directly to the

support, ignoring the web

members

– a lateral load at the top of a

triangular tower is transmitted

down the two main members while

the inner bars are unstressed


� [1] Structures support loads in the most direct way open to them

– (a) the load applied at the top of a

column in the frame from the

figure goes directly to the

foundation through the column,

while the rest of structure is

virtually unstressed

– (b) if the direct path is interrupted,

the load path is much more

complicated, and the stresses and

deflections are greatly increased

– Rule: provide paths as simple and

as direct as possible for the loads

to 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: cantilever

bridge

– the moment due to dead

weight is largest at the

support

– the material must be

concentrated at the

supports

– a load near the support

produces only a small

increase in moment

– efficient structure for large

bridges


� [3] Statically indeterminate structures support loads in the 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 by

compressive membrane forces rather than bending

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� [3] Statically indeterminate structures support loads in

the 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 are

transmitted 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 forces

in 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 more stocky members less prone to instability

� [5] In statically indeterminate structures, the stiffer elements will attract larger forces

– Example: portal frames are often

haunched near the corners

• further increase of bending moments at the corners though actual stresses reduce due increase of cross-section

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