design challenges of tall buildings - aci · pdf fileconcrete in the middle east foreword...

DESIGN CHALLENGES OF TALL BUILDINGS

Presented by: Rodolphe Mattar

CONCRETE IN THE MIDDLE EAST

FOREWORD

• Beirut’s urban landscape has not been as closely associated with the presence of very tall buildings as in cities in the United States or Asia for example.

• Yet, in recent years, we have seen a noted development in Beirut’s skyline.

• Over half of the Lebanese population lives in cities;

• There is more demand than ever for space in cities, and tall buildings offer the most efficient use of land and space.

• In this presentation, we will consider the various challenges involved in the design and construction of tall buildings, along with their economic impact.

Rodolphe Mattar DESIGN CHALLENGES OF TALL BUILDINGS


FOREWORD (cont…)

The main challenge of any tall building, like any regular structure, is to design it in a safe, efficient and economical way; and this is achieved by :

• Minimizing the amount of material to be more

sustainable.

• Reducing loads and maximizing the use of net available space

• Completing the design within a short timeframe.

While ensuring the overall stability and comfort of the building.



DESIGN CONSIDERATIONS

• Structures are designed to resist the gravitational and lateral forces, that they will be called on to sustain during their construction and subsequent service life.

• With tall buildings, designers must deal with special requirements, not normally encountered with conventional constructions.

• Requirements such as: shrinkage, creep, elastic shortening, drift, human comfort, etc...



SHRINKAGE, CREEP, AND ELASTIC SHORTENING

• In very tall concrete buildings the cumulative vertical movements due to creep and shrinkage may be sufficiently large to damage non-structural elements, especially in the upper levels of the building.

• In assessing these long term deformations, the influence of a number of factors must be considered, particularly: – The concrete properties – The loading history – The age of the concrete at load application – The amount of reinforcement in the members concerned

• The differential movements due to creep and shrinkage must be

considered structurally and accommodated in the architectural details at the design stage.

• In the construction phase, in addition to creep and shrinkage, elastic shortening will occur in the vertical elements of the lower levels due to additional loads imposed by the upper levels as they are completed.



STIFFNESS AND DRIFT

The provision of adequate stiffness particularly lateral stiffness is a major consideration in the design of a tall building for several important reasons: • Deflections must be maintained at a

sufficiently low level to allow the proper functioning of non structural components such as elevators and doors.

• The structure must be sufficiently stiff to prevent dynamic motions becoming large enough to cause discomfort to occupants.

• It may be necessary to add dampers to control and reduce the oscillatory movements of the building.



HUMAN COMFORT CRITERIA

• If a tall flexible structure is subjected to lateral or torsional deflections under the action of fluctuating wind loads, this results in oscillatory movements of the structure.

• Most tall buildings at the top levels can easily move several centimeters in either direction without damage to their architectural components.

• However, these wind induced sudden movements (accelerations) are felt by the building’s occupants and can cause human discomfort.

• These motions that have a psychological or physiological effect on the occupants cause a building to become undesirable or even un-rentable despite being structurally sound and safe.



FACING THE CHALLENGES

Challenges encountered when designing tall buildings can be dealt with by:

• Creating an adequate structural

concept

• Reducing the overall mass of the building

• Carefully selecting the construction materials



CHALLENGES - CONCEPT

An adequate structural concept requires:

• A close coordination between the different disciplines: – Architectural – Structural – Electro-Mechanical

• A Lateral Force Resisting System such as:

– Shear Walls Structure – Wall-Frame Structure – Braced-Framed Structure (recently being implemented)

• The elimination of expansion joints to decrease the

period of the structure

• The use of Dampers (i.e.: Tunnel Liquid Damper) to decrease the period of the structure



DESIGN CHALLENGES – MASS REDUCTION

• Ways to reduce the building’s overall mass – The use of Post-Tensioned

elements

– The use of Lightweight Partitions • Lightweight Masonry Blocks • Gypsum Boards • Lightweight Gypsum Boards

– The Use of Lightweight Screed

– The Use of Lightweight Gravel



CHALLENGES – MASS REDUCTION (S)

• Mass Reduction vs. Seismic Loads:

– The Decrease in Seismic Forces

– The Decrease in Drift and

Displacements

– Reduces the Vertical and Horizontal Elements Sizes

– Reduces Costs



CHALLENGES – MASS REDUCTION (W)

• Mass Reduction vs. Wind Loads:

– The Decrease of the Building’s Period

– The Decrease in Wind Lateral Forces • Avoid the use of equivalent static methods • Wind Tunnel Test are necessary

– Meet and Exceed The Comfort Criteria

– Decrease the Displacements

– Reduce Vertical and Horizontal Elements

Sizes

– Reduce Costs



CHALLENGES – MASS REDUCTION (F)

• Mass Reduction vs. Foundations

– Decrease the Raft/Mat

Thickness

– Decrease the Settlements

– Reduce Costs



DESIGN CHALLENGES – MATERIAL PROPERTIES

Tall buildings design require:

• High Performance Concrete – The use of Concrete with high Compressive Strength: 50 MPa, 60 MPa, 70

MPa, 80 MPa, …

– Reduce the Vertical and Horizontal Elements Sizes

– Increase the Elastic Modulus of Concrete (Ec)

– Increase the Overall Rigidity

– Increase the Concrete Durability

– Decrease the Period of the Building



DESIGN CHALLENGES – MATERIAL PROPERTIES

• Steel Reinforcement:

– The use of Steel with high

Tensile Strength: Grade 70 and up

– Reduce Vertical Elements Sizes

– Reduces Costs



CONCLUSION

• Understanding the behavior of a skyscraper is an

art and often requires in-depth understanding and years of experience in the design of tall buildings.

• Issues may arise above ground with the proximity

to adjacent structures;

• And below ground where considerations must be taken with regards to the effects of foundations on surrounding sites and services.



THANK YOU


design challenges of tall buildings - aci · pdf fileconcrete in the middle east foreword...

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