tb lecture04 design criteria

8/7/2019 TB Lecture04 Design Criteria

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EGNEGN--5439 Design of Tall Buildings5439 Design of Tall Buildings

Lecture 04Lecture 04

Design Criteria

L. Prieto-Portar, 2008


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Outline of the Design CriteriaOutline of the Design Criteria

The Basic Design Criteria

Limit States Design Philosophy

The Speed of Erection

LoadingSequential Loading

Strength and Stability

Drift Limitations

Stiffness

Human Comfort

Fire

Creep, Shrinkage, and TemperatureFoundation Settlement and

Soil-Structure Interaction


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(a) The building must not break in shear and (b) must not deflect excessively in shear.

1. The Basic Design Criteria for Tall Buildings.The pressurep =f(H2); in other words, the pressure on a 100-story building is 16x the

pressure on a 25-story building.


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The bending resistance of a building must provide, (a) that the building must not

overturn, (b) the columns must not fail in tension or compression, and (c) the deflectiondue to bending (drift index) must not be excessive.


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2. Limit States Design Philosophy.

The aim of this approach is to ensure that all structures and their constituentcomponents are designed to resist with reasonable safety the worst loads and

deformations that are liable to occur during construction and service, and to have

adequate durability during their lifetime.

The entire structure, or any part of it, is considered as having failed when it reachesany one of various limit states. Two types of limit states must be considered:

The ultimate limit states, corresponding to the loads to cause failure, endangering

lives and causing serious financial losses, the probability of failure must be low.

Theserviceability limit states, which involve the criteria governing the service life

of the building. Since the consequences are not catastrophic, a much higher

probability of occurrence is permitted.

A particular limit state may be reached as a result of an adverse combination ofrandom conditions. Partial safety factors are employed for different conditions that

reflect the probability of certain occurrences or circumstances of the structure and

loading existing. The implicit objective of the design calculations is then to ensure that

the probability of any particular limit state being reached is maintained below an

acceptable value for the type of structure concerned.


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3. The Speed of Erection Process.

The speed of erection is a vital factor in obtaining a return on the investment by

minimizing the cost of interest payments on the large capital costs involved in such

large-scale projects.

Most tall buildings are constructed in congested city sites with difficult access, andwith no storage areas.

Careful planning and organization of the construction sequence become essential.

The story-to-story uniformity of most multi-story buildings encourages constructionthrough repetitive operations and prefabrication techniques.

The progress in the ability to build tall buildings has gone hand in hand with the

development of more efficient equipment and improved methods of construction,

such as:

- Slip- and flying-formwork

- Concrete pumping

- The use of climbing tower cranes and large mobile cranes.


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4. Loading.

The structure must be designed to resist the gravitational and lateral forces, both

permanent and transient that will be sustained during construction and during the

expected useful life of the structure (from 60 to 100 years). These forces will depend

on the size and shape of the building, and its location. Load combinations depend on

the probable accuracy of estimating the dead and live loads, and the probability of the

simultaneous occurrence of different combinations of gravity loading, both dead and

live, with either wind or earthquake forces. The accuracy of these loads is included in

limit states design through the use of prescribed factors.

5. Sequential Loading.

For dead loads, the construction sequence should be considered to be the worst case. It

is usual to shore the freshly placed floor upon several previously cast floors. The

construction loads on the supporting floors due to the weight of wet concrete and its

formwork will greatly exceed the loads of normal service conditions. These loads mustbe calculated considering the sequence of construction and the rate of erection.

However, the designer rarely knows who the contractor will be, nor his method of

construction.


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If column axial deformations are calculated as though the dead loads are applied to

the completed structure, bending moments in the horizontal components (for

example, beams) will result from any differential column shortening.

Because of the cumulative effects of column axial deformations over the height of the

building, the effects are greater in the highest levels of the building. However, the

effects of such differential movements could be greatly overestimated because in

reality, during the construction sequence, a particular horizontal member isconstructed on columns in which the initial axial deformations due to the dead weight

of the structure up to that particular level have already taken place.

The deformations of that particular floor will then be caused by the loads that are

applied subsequent to its construction.

Such sequential effects must be considered if an accurate assessment of the structural

actions due to dead loads is to be achieved.


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6. Strength and Stability.

The primary requirement of the ultimate limit state design procedure is that thestructure have adequate strength to resist and remain stable under the worst

probable loads during its lifetime. This includes all critical load combinations,

augmented moments from second-order deflections (P-) plus an adequate reserve,

study all critical members whose failure may lead to a progressive collapse of part or

the whole structure. Finally, the whole building must be checked against toppling as arigid body about one edge of the base. Moments are taken about that edge with the

resisting moment of the dead weight of the structure to be greater than the

overturning moment by an acceptable factor of safety (FS > 3).


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7. Drift Limitations.

The parameter that measures the lateral stiffness is thedrift index. It is defined as the

ratio of the maximum deflection at the top of the building to the total height of the

building. In addition, each floor has an index called the inter-story drift index which

checks for localized excessive deformation.

There is no national code requirement for the drift index, but 1/400 is a traditionally

accepted limit.

Different countries use from 0.001 to 0.005 (1/1,000 to as low as 1/200).

Lower values are used for hotels and condominiums because the noise and discomfort

at those levels are unacceptable.

For conventional structures, the preferred range is 0.0015 to 0.0030 (in other words,

from 1/700 to 1/350).


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Deflections must be limited, in order to:

1)Prevent second-orderP- effects due to gravity loading, precipitating collapse;

2) Allow the functioning of non-structural components, such as elevators and

doors;

3) Avoid distress in the structure;

4) Prevent excessive cracking and consequent loss of stiffness;

5) Avoid any redistribution of load to non-load-bearing partitions, in-fills,cladding, or glazing;

6) Prevent dynamic motions from causing discomfort to occupants, or affecting

sensitive equipment.


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In the design process, the stiffness of joints, particularly in precast or prefabricated

structures, must be given attention to develop lateral stiffness of the structure and

present progressive failure.

Torsional deformations must not be overlooked, especially due to diurnal thermal

drift in steel frames.

As building height increases, the drift index should become lower to keep the topstory deflection to a suitably low level.

If excessive, the drift of a structure can be reduced by:

1) Changing the geometric configuration to alter the mode of lateral loadresistance;

2) Increasing the bending stiffness of the horizontal members;

3) Adding additional stiffness by the inclusion of stiffer wall or core members;

4) Achieving stiffer connections, by sloping the exterior columns;

5) In extreme circumstances, it may be necessary to add dampers, which maybe of the passive or active type.


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8. Stiffness.The lateral stiffness is a major consideration in the design of a tall building. Under the

ultimate limit state, the lateral deflections must be limited to prevent 2nd-orderP-effects from gravity loading to be large enough to precipitate a collapse. In addition,

serviceability requires these deflections not to affect elevator rails, doors, glass

partitions, and prevent dynamic motions to cause discomfort to the occupants and

sensitive equipment. This is one of the major differences of tall buildings with respect

to low-rise buildings.

9. Human Comfort.Buildings subjected to both lateral and torsional deflections (plus vortex shedding and

other usual effects) may induce in their human occupants from discomfort to acute

nausea. These are major factors in the final design of the building.

When a tall structure is subjected to lateral loads, the resulting oscillatory movements

can induce a wide range of responses in the buildings occupants, ranging from mild

discomfort to acute nausea. This may prove the structure undesirable or un-rentable.

There are no codified standards for comfort criteria. A dynamic analysis is required

to determine the response of the structure in order to determine its adequacy to the

comfort criteria.


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10. Fire.

One of the most extreme conditions placed upon a building is fire.

Fire is a primary concern during design. The temperature range and its duration

must be estimated from its probable cause and the materials present in the

building that could provide fuel for its continuation. Also of interest are possiblesources of ventilation, and egress from alterative paths must be considered. The

behavior of the different structural components must be known. For example,

mild steel at 700C is only 15% of the yield strength at 20C, and its elastic

modulus drops to only 45% of its original value.


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11. Creep, Shrinkage, and Temperature.

In very tall buildings, the cumulative vertical movements due to creep and shrinkage

may cause distress in the structure and induce forces into horizontal elements

especially in the upper regions of the building. During the construction phase, elastic

shortening will occur in the vertical elements of the lower levels due to the additional

loads imposed by the upper floors as they are completed. Cumulative differential

movements will affect the stresses in the subsequent structure, especially in the

building that includes both in-situ and pre-cast components. Buildings subjected to

large temperature variations between their external faces and the internal core, and

that are restrained, will experience induced stresses in the members connecting both.

Important factors in determining long-term deformations include:

1) Concrete properties;

2) Loading history;

3) The age of the concrete at the time of load application;

4) Volume-surface ratio and amount of reinforcement in the members

concerned;

5) Achieving a uniformity of stress in the vertical components will reduce any

relative vertical movement due to creep and elastic shortening.


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12. The Effect of Foundation Settlement upon the Tall Building.

The gravity and lateral forces on the structure will be transmitted to the earth

through the foundation system. Because of its height, a tall buildings columns may

be very heavy. In areas with bedrock, appropriate foundations can be shallow

foundations, drilled shafts, or deep basements.

In areas with poor soil conditions, differential settlements must be avoided. A typical

solution is the use of mat (or raft) foundation, where the weight of soil equals to a

significant portion of the gross building weight. This method is called partially

compensated foundation.

Overturning moments and resisting moments and shears must be checked. Minor

movements of the foundations are greatly exaggerated by a tall building, leading to

very large inclinations of the tower. If an overall rotational settlement of the entire

foundation occurs, the ensuing lateral deflections will be magnified by the height,

increasing maximum drift and incurringP- effects.


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13. Soil-Structure Interaction.

Soil-structure interaction involves both static and dynamic behaviour. The former is

generally treated by simplified models of subgrade behaviour, and finite element

methods of analysis are customary.

When considering dynamic effects, both interactions between soil and structure, andany amplification caused by a coincidence of the natural frequencies of building and

foundation must be included.

Seismic forces may develop excessive hydrostatic pressures, causing liquefaction of

the soil. These types of conditions must be considered and avoided.


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Common Daisy.

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