FOUNDATION IN ENGINEERING

Foundations in Civil Engineering

Foundations in Civil Engineering

The John Hancock Building in Boston Storage tanks

Cable-stayed bridge

Offshore drilling platform

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Foundations

o Emergence of Foundation Engineering

o Uncertainties

o Rationalism vs. Empiricism

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Foundations

o Factors of Safety

o Building Codes

o Types of Foundations

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Foundations


Early foundation designs were based on precedent, intuition and common sense

Empirical rules usually produced acceptable results as long as they were applied to structures and soil conditions similar to those encountered in the past

However, . . . the results were often disastrous when builders extrapolated the rules to new conditions

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Foundations


New methods of building construction began to appear in the late 19th century

The introduction of steel and reinforced concrete led to a transition away from rigid masonry structures to more flexible rigid frame structures

New materials also permitted buildings to be taller and heavier than before

Good sites became occupied, builders were forced to consider sites with poorer soil conditions.

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The Eiffel Tower in Paris

o Built in 1887 - 1889

o An excellent example of a new type of structure in which old rules for foundation no longer applies

o Adjacent to the Seine River and is underlain by difficult soil conditions (uncompacted fill and soft alluvial soils)

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o Piers for the nearby Alma bridge (founded in this alluvium) had already settled by 1 m.

o Eiffel devised a new way of exploring the soils, which consisted of driving 200-mm diameter pipe filled with compressed air.

o Eiffel placed the foundations for the two legs furthest from the river on the shallow but firm alluvial soils. The bottom of these foundations were above GWT

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o He made the foundations for the other two legs much deeper so they too were founded on firm soils. This required 12m - excavation below the ground surface (6m below GWT).

o As a result of Eiffels diligence, the foundations have supported the tower for more than 100 years.

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Some examples of foundation failure

Canada (Oct. 18, 1913)

Leaning Tower of Pisa, Italy (Built 1173 1350)

Bridge failure due to ground failure

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Some examples of foundation failure

Definition

Panoramic view of the same warehouse with widespread variable differential settlements

Large angular distortion leading to severe damage to a warehouse 11/58

Foundations

As structures continued to become larger and heavier, engineers continued to learn more about foundation design and construction.

Instead of simply developing new empirical rules, they began to investigate the behavior of foundations and developed more rational methods of design thus establishing the discipline of foundation engineering

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Uncertainties

From an unknown structural engineer:

Structural engineering is the art and science of molding materials we do not fully understand into shapes we cannot precisely analyze to resist forces we cannot accurately predict, all in such a way that the society at large is given no reason to suspect the extent of our ignorance

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Uncertainties

o Limited knowledge of soil conditions

o Limitations in our understanding of the interaction between a foundation and a soil

o Difficulty in the prediction of the actual service loads that will act on the foundation

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Rationalism vs. Empiricism

o Design methods include a mixture of rational and empirical techniques

o Rational techniques are those developed from the principles of physics and engineering sciences

o Empirical techniques are based primarily on experimental data

o One of the keys to successful foundation engineering is to understand this mix of rationalism, empiricism, the strength and limitations of each, and how to apply them to practical design problems 15/58

Factors of Safety

o In spite of the many uncertainties in foundation design and analysis, the public expects the engineers to develop reliable and economical designs in a timely and efficient manner.

o We compensate these uncertainties by using factors of safety in our designs

o Qult load that will lead to an ultimate state

o The ASD is based on the inequality

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Factors of Safety (modified after Vesic 1975)

Category

Typical structures

Observations

Soil Exploration

Thorough Limited

A Railway bridges Warehouses Blast furnaces Retaining walls Silos

Maximum design load likely to occur often

Ultimate limit states with disastrous consequences

3 4

B Highway bridges Light industrial and public buildings

Maximum design load may occur occasionally

Ultimate limit states with serious consequences

2.5 3.5

C Apartment buildings Office Buildings

Maximum design load unlikely to occur often

2 3

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Building Codes

o NSCP Buildings / Bridges

o UBC

o ACI, AISC, API

o AASHTO

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Types of Foundations

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Performance Requirements

o Design Loads

o Methods of Expression

Allowable Stress Design (ASD) / Working Stress Design (WSD)

Load and Resistance Factor Design (LRFD)

o Strength Requirements

Geotechnical strength Requirements

Structural strength Requirements

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o Serviceability Requirements

Total Settlement

Differential Settlement

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o Performance Requirements

Constructibility Requirements

Economic Requirements

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Design Loads

o Dead loads (D) are those caused by the weight of the structure, including permanently installed equipment.

o Live loads (L) are those caused by the intended use and occupancy. These include loads from people, furniture, inventory, maintenance activities, moveable partitions, moveable equipment, vehicles, and other similar sources.

o Snow loads (S) and Rain loads (R) are a special type of live load caused the accumulation of snow or rain. Sometimes rain loads caused by ponding (the static accumulation of water on the roof) are considered separately.

o Earth pressure loads (H) are caused by the weight and lateral pressures from soil or rock, such as those acting on a retaining wall.

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Design Loads

o Fluid loads (F) are those caused by fluids with well-defined pressures and maximum heights, such as water in a storage tank.

o Earthquake loads (E) are the result of acceleration from earthquakes

o Wind loads (W) are imparted by wind onto the structure.

o Self-straining loads (T) are those caused by temperature changes, shrinkage, moisture changes, creep, differential settlement, and other similar processes.

o Impact loads (I) are the result of vibratory, dynamic, and impact effects. Impact loads from vessels are especially important in some bridge and port facilities.

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Design Loads

o Stream flow loads (SF) and ice loads (ICE) are caused by the action of water and ice in bodies of water, and are especially important in bridges, offshore drilling platforms, and port facilities.

o Centrifugal (CF) and braking loads (BF) are caused by the motion of vehicles moving on the structure. Centrifugal forces occur when the vehicle is turning, such as on a curved bridge, while braking forces are those transmitted to the structure when a vehicle brakes.

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Methods of Expression

Two methods of expressing and working with design loads

o Allowable Stress Design (ASD) or Working Stress Design Method

o Load and Resistance Factor Design (LRFD)

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Design Loads

ASD design load combinations [ANSI/ASCE 2.4.1]

o D

o D + L + F + H + T + (Lr or S or R)

o D + L + (Lr or S or R) + (W or E)

o D + (W or E)

Alternate method of evaluating wind and seismic loads

o 0.75[D + L + (Lr or S or R) + (W or E)]

o 0.75[D + (W or E)]

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Load and Resistance Factor Design

o aka Ultimate Strength Design method

o Applies load factors, g, most of which are greater than one, to the nominal loads to obtain the factored load , U.

o In the case of normal loads, the factored load PU is

= 1 + 2 +

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Load and Resistance Factor Design

o applies a resistance factor, f (aka strength reduction factor) to the ultimate capacity from a strength limit analysis

o nearly all resistance factors are less than one

o Design must satisfy the following criteria

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ACI Code

ACI 1999 defines the factored load as the largest of those computed from the following equation [ACI 9.2]

o U = 1.4D + 1.7L

o U = 0.75(1.4D + 1.4T + 1.7L)

o U = 0.9D + 1.4F

o U = 1.4D + 1.7L + 1.7H

o U = 0.9D + 1.3W

o U = 0.9D + 1.43E

o U = 0.75(1.4D + 1.7L + 1.7W)

o U = 0.75(1.4D + 1.7L + 1.87E)

o U = 0.9D + 1.7H

o U = 1.4(D + T) 30/58

ANSI/AISC Code

ANSI/ASCE defines the factored load as the largest of those computed from the following equation [ANSI/AISC 2.3.2]

o U = 1.4D

o U = 1.2(D + F + T) + 1.6(L + H ) + 0.5(Lr or S or R)

o U = 1.2D + 1.6(Lr or S or R) + (0.5L or 0.8W)

o U = 1.2D + 1.3W+ 0.5L + 0.5(Lr or S or R)

o U = 1.2D + 1.0E + 0.5L + 0.2S

o U = 0.9D + (1.3W or 1.0E)

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Load Factors

Loads AASHTO (1998)

ACI (2002)

AISC (1994)

API (1993)

MOT (1992)

NRC (1995)

Dead 1.25 1.95 (0.65 0.9)

1.2 (0.9)

1.2 1.4 (0.9)

1.1 1.3 (0.9)

1.1 1.5 (0.65 0.95)

1.25 (0.85)

Live 1.35 1.75 1.6 1.6 1.1 1.5 1.5 1.4 1.5

Wind 1.4 1.3 1.3 1.2 1.35 1.3 1.5

Seismic 1.0 1.4 1.0 0.9 1.3 1.0

Note: Values in parenthesis apply when the load effects tend to resist failure for a given load combination, that is, when the loads have a beneficial effect.

AASHTO American Association of State Highway and Transportation Officials

ACI American Concrete Institute

AISC American Institute of Steel Construction

API American Petroleum Institute

MOT Ministry of Transportation (Canada)

NRC National Research Council (Canada) 32/58

Example Problem

A column carries the following vertical compressive loads; PD = 2100 kN downward, PL = 1400 kN downward, and PW = 600 kN upward. Using the ASD load combinations, compute the design normal load for use in foundation design.

The column described above will be supported by a group of four steel H-pile foundations. These H-piles are similar to wide flange beams, and are driven vertically into the ground. The piles will be made of A36 steel (Fy = 248 Mpa) and the allowable compressive stress, Fa, is 0.50Fy. Considering only the stresses in the steel, determine the required cross-sectional area of each pile.

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Example Problem

Solve the previous problem using LRFD with the ANSI / ASCE load factors and a resistance factor of 0.70.

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Reliability-based design (RBD)

o The key concept in RBD is risk

o Risk refers to the likelihood (or probability) that a foundation system will fail to perform adequately.

o The safety margin S is defined as the difference between capacity and demand

S = C D

o The criterion S = 0 defines the limit of acceptable performance (a limit state). 35/58


o The events S < 0 defines system failure.

o The probability of failure (Pf) can be determined if enough information is available about the probability distribution of the capacity C and Demand D.

o The reliability index b is defined as the number of standard deviations of S between the mean, E[S] and S = 0.

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The probability distribution of the safety margin S, which is the difference between capacity C and demand D, and the concepts of probability of failure Pf and the reliability index b.

o Mathematically

where sS is the standard deviation of S and b is the inverse coefficient of variation of S.

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Strength Requirements

Foundation soil must be strong enough to support imposed loads

Once the design loads have been defined, we need to develop foundation designs that satisfy several performance requirements

o Strength requirements intended to avoid catastrophic failures

geotechnical requirements

structural strength requirements

o Serviceability requirements intended to produce foundations that perform well when subjected to service loads

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A bearing capacity failure beneath a spread footing foundation. The soil has failed in shear, causing the foundation to collapse

o Geotechnical Strength requirements

address the ability of the soil or rock to accept the loads imparted by the foundation without failing.

geotechnical strength requirements is satisfied by comparing shear stresses with shear strengths and designing accordingly

for spread footing foundations, geotechnical strength is expressed as the bearing capacity of the soil

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o Structural Strength requirements

address the foundations structural integrity and its ability to safely carry the applied loads.

structural strength analysis are conducted using either ASD or LRFD methods

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Seatwork

A proposed column has the following design loads:

EVEN COLUMNS:

Axial load: PD = 200 kN, PL = 170 kN, PE = 50 kN, PW = 60 kN (all compression)

Shear load: VD = 0 kN, VL = 0 kN, VE = 40 kN, VW = 48 kN

ODD COLUMNS:

Axial load: PD = 250 kN, PL = 150 kN, PE = 60 kN, PW = 40 kN (all compression)

Shear load: VD = 0 kN, VL = 0 kN, VE = 30 kN, VW = 58 kN

Compute the design axial and shear loads for foundation design using ASD.

Repeat the problem using LRFD with the ACI load factors. 41/58

Seatwork

EVEN COLUMNS:

A steel pile foundation with a cross-sectional area of 15.5 in2 and Fy = 50 k/in2 is to carry axial compressive dead and live loads, of 300 and 200 k, respectively. Using LRFD with the ANSI / ASCE load factors and a resistance factor of 0.75, determine whether this pile satisfies structural strength requirements for axial compression.

ODD COLUMNS :

A steel pile foundation with a cross-sectional area of 20 in2 and Fy = 60 k/in2 is to carry axial compressive dead and live loads, of 350 and 400 k, respectively. Using LRFD with the ANSI / ASCE load factors and a resistance factor of 0.70, determine whether this pile satisfies structural strength requirements for axial compression.

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Serviceability Requirements

Foundation that satisfy strength requirements will not collapse, but they still may not have adequate performance.

Serviceability requirements are intended to produce foundations that perform well when subjected to service loads. These requirements include:

settlement

heave

tilt

lateral movement

vibration

durability

Settlement must be within acceptable limits

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Serviceability Requirements

Modes of settlement (a) uniform; (b) tilting with no distortion; (c) distortion

o Settlement

the vertical downward movement of foundations

zero settlement is ideal, but not an attainable goal

the foundation engineer must answer the following questions

how to determine the amount of tolerable settlement

how to design the foundation to accommodate this requirement

Total vs. Differential Settlement

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The Leaning Tower of Pisa

Schematic cross-section of intervention by under-excavation (elevation in meters)

Situation of the tower of Pisa in May 1993. 46/58

Total Settlement

Total settlement in a spread footing foundation

o Total Settlement, d

the change in foundation elevation from the original unloaded position to the final loaded position

some of the problems experienced during excessive total settlements

connections with existing structures

utility lines

surface drainage

access

aesthetics

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Total Settlement

Type of Structure (in) (mm)

Office buildings 0.5 2.0 (1.0 is the most common value)

12 50 (25 is the most common value)

Heavy industrial buildings 1.0 3.0 25 75

Bridges 2.0 50

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Total Settlement

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Total Settlement

Total settlement in a spread footing foundation

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o Differential Settlement, dD the difference in total settlement between two foundations

or between two points on a single foundation

some of the problems experienced during excessive total settlements

cracks in walls

Jamming in doors and windows

poor aesthetics

if allowed to progress, it can threaten the integrity of the structure

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allowable differential settlement, da

da = qaS where

qa = allowable angular distortion

S = Column spacing (horizontal distance between columns)

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Type of Structure

Steel tanks 1/25

Bridges with simply-supported spans 1/125

Bridges with continuous spans 1/250

Building that are very tolerant of differential settlements, such as industrial buildings with corrugated steel sliding and no sensitive interior finishes

1/250

Typical commercial and residential buildings 1/500

Overhead traveling crane rails 1/500

Buildings that are especially intolerant of differential settlement, such as those with sensitive wall or floor finishes

1/1000

Machinery 1/1500

Buildings with unreinforced masonry load-bearing walls

1/2500

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Example Problem

A steel-frame office building has a column spacing of 20 ft. It is to be supported on spread footings founded on a clayey soil. What are the allowable total and differential settlements?

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Seatwork

Even Columns:

A one-story steel warehouse building is to be built of structural steel. The roof is to be supported by steel trusses that will span the entire 70 ft. width of the building and supported on columns adjacent to the exterior walls. These trusses will be placed 24 ft on center. No interior columns will be present. The walls will be made of corrugated steel. There will not be any roll-up doors. Compute the allowable total and differential settlements.

Odd Columns :

The grandstands for a minor league baseball stadium are to be built of structural steel. The structural engineer plans to use a very wide column spacing (25 m) to provide the best spectator visibility. Compute the allowable and differential settlements.

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Foundation designs are usually more conservative than those in the superstructure for the following reasons:

o Assessments of the soil and rock conditions include considerable uncertainty

o Foundations are not built with the same degree of precision as the superstructure

o Structural materials (piles) may be damaged during installation

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Foundation designs are usually more conservative than those in the superstructure for the following reasons:

o Uncertainty in the nature and distribution of the load transfer between foundation and the ground

o The consequences of a catastrophic failure are much greater

However gross over-conservatism is not warranted. This is also a type of failure; the failure to produce an economical design.

Engineers must strive to achieve the optimum balance between reliability (safety) and cost.

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Summary

o The foundation engineer must determine the necessary performance requirements before designing a foundation

o Foundations must support various types of loads

o Design loads may be expressed using the ASD or the LRFD.

o Strength requirements are those that are intended to avoid catastrophic failure. There are two kinds: geotechnical strength requirements and structural strength requirements

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Summary

o Serviceability requirements are those intended to produce foundations that perform well when subjected to the service loads.

o Settlement is often the most important serviceability requirement.

o Durability is another important serviceability requirement. Foundations must be able to resist the various corrosive and deteriorating agents in soil and water.

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Summary

o Foundations must be buildable

o Foundation designs must be economical.

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FOUNDATION IN ENGINEERING

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