Development of tall buildings in Thailand

Dr. Karoon Chandrangsu

How tall is the building that can be built in Bangkok subsoil with current technology?

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Nowadays, Structural Analysis and Design for tall buildings are relied upon the capability of the 3D computer program.

Currently, the tallest building in Bangkok is at the height of 315m and it is made of reinforced concrete.

95% of high rise towers in Thailand are situated in Bangkok with the most difficult subsoil.

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Key factors for the design of tall buildings

1) Structural system

1.1) Depending on aspect ratio Residential in Bangkok – Aspect ratio 12-15 coupled shear wallOffice in Bangkok – Aspect ratio 8-12 moment frame and shear core interaction

1.2) Construction cost versus time Reinforced concrete structure - most popular, cheaperSteel structure - higher cost but faster time, cost of fire proof materialPrefabricated reinforced concrete structure - higher cost but faster time, required good workmanship

Minimizing self weight of structural system is also the key for building to go taller.

Moment frame – shear wall interactionCoupled shear wall

Special structural systems are required in order to sustain wind loads

1) Wall shaped columns and large central lift core

Special Structural Systems for Lateral Loads in very tall and slender buildings

2) Outrigger floors

– Outrigger trusses link the columns in the front and in the back so they effectively deform together

• Effects of Outriggers Trusses

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– lateral sway can be significantly reduced by adding outrigger floors

3) Vertical Prestressing

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2) Loads considered in the analysis and design of tall buildings

- Gravity loads prescribed by regulations Thailand is in the process of issuing new regulations for live load to be used in the design

- Seismic loads in accordance with New ministerial law is about to be enacted the revised zone and correspondent magnitude of seismic design parameters

- Wind loads in accordance with Thai standard for Wind Calculation and Building Response (มยผ -1311, Year 2007), issued by the

Department of Public Works and Town & Country Planning, Ministry of Interior.

New characteristic of wind forces to be used in the design of tall buildings will be issued.

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3) Strength of construction materials

Concrete - As of now, local suppliers can provide Grade C80 as maximum strength.

Reinforcement – SD 50 is available and widely used for diameter higher than DB25.

Steel profile – Grade SM520 with fy = 4000 ksc is widely available.

Light weight concrete for slab application –With the use of special aggregates, local suppliers can provide f’c = 450 ksc, weight =

1800 kg/m3.

Minimizing self weight of structural system is also the key for building to go taller.

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4) Foundation

Bangkok soil profileLayers of soil typically seen in Bangkok area

Foundation of buildings in Bangkok normally consist of reinforced concrete mat foundation on piles.

Cast-in-place bored piles with diameter 1.2 -1.80 m. are normally used. These piles are designed to be seated on a very dense sand layer.

• Structural engineers often understand fully the super structure and can utilize structural software to assist in the design.

• However, the base of the building (or the foundation) is seated on the ground. The loads of the whole building will eventually transfer down to the bearing soil.

• Since Bangkok has poor soil condition, it is this factor that limits the height of the building to be built in Bangkok.

• It requires 2 disciplines; structural and geotechnical engineers, to fully realize the design of foundation system and understand soil structure interaction.

• Therefore, development in geotechnical field will allow building to go taller in Bangkok.

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Development of Piling Technology for tall buildings

• Recently, local piling contractor can install large diameter bored pile with the tip at -100 m.

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Full scale static pile load test was carried out to confirm the calculated capacity of the pile derived from data of subsoil exploration.

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Results from static pile load test

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Load-Settlement Curve

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Structural and geotechnical engineers must mutually agree upon design criteria which maintain the integrity of the super structure while the soil remains its sustainability through the service life of the building.

Analysis and design of mat foundation on piles

Design Criteria

1) The differential settlement between any two points on the building foundation shall not be more than L/400 where L = distance between the two points.

2) Under permanent loading, the pile shall not be overloaded by more than 10% of its allowable bearing capacity. For the case of building under transient loading, the pile shall not be overloaded by more than 30%.

Analysis and design calculation steps

The design of foundation of tall buildings can be divided into the following steps:

1) Initial design step: Determination of column loads, subsoil investigation, and single pile capacity analysis.

2) Detailed design step: Design of pile group including mat or raft sizing and the number of piles. This step also includes structural design step and analysis for settlement.

• The analysis and design methodology for building foundation is normally based on the model of plate on spring concept.

• The piled mat foundation is considered as thin and flexible plate resting on linear springs representing the supporting piles.

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Design algorithm

PART 1 To satisfy the strength requirements of the foundation, the algorithm follows the following steps:

Step 1 Obtain load distribution of the building on mat foundation: The computer model of the whole building structure skeleton in 3D is analyzed to get the total load transfer to the foundation under various gravity and lateral load combinations.

Step 2 Estimate the approximate weight of mat foundation and calculate number of piles required to support building load.

Step 3 Arrange piling layout and determine appropriate mat foundation size and thickness that meet required structural strength and constructability. Thickness of the plate will be determined by punching shear capacity of the plate.

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PART 2 The foundation design algorithm for settlement control will follow the following steps:

Step 1 To obtain the building foundation response under loading: The computer model of the whole building structure skeleton and mat foundation in 3D considering soil-structure interaction is analyzed to get the behavior of the mat foundation.

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Step 2 To check the reaction load on pile against the pile capacity criteria: In theory piles underneath heavy load structural elements on mat foundation such as tower center core, will sustain higher loads than the piles around perimeter of the mat foundation. Therefore, this concept results in concentrating numbers of piles underneath the core area and may lead to more numbers of piles required, and readjustment of piling layout.

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It is to be noted that the result of settlement of the foundation is considered as a short term settlement. This plate on springs type of analysis for pile group foundation cannot consider the interaction effect between piles in the group. Owing to the pile interaction effect, edge piles will behave stiffer than the inner piles even though they are of the same size.

Step 3 To iterate the analysis and design using the result and pile layout readjustment from step 2 until all stipulated criteria are satisfied.

Step 4 To obtain the response of the mat foundation in terms of internal forces and settlement profiles for further structural design and differential settlement control.

Plaxis software

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The use of BIM in the design of tall buildings