High Tech Concrete for Complex Building Structures

MSc Architectural Engineering 2008-2009 TU Delft

Report – June 2009 Technical Research – Graduation MSc4 Architectural Engineering High Tech Concrete for Complex Building Structures

Student – Sagar Thorat Study Number – 1379674 Mentor - Prof. Ir. C. Van. Weeren


Contents

1. Introduction

2. Design Goals

3. Structural Research I. Load Conditions II. Typical Structural Systems III. Structural Concrete for Complex Buildings

4. Advances in Structural Concrete

I. Preface II. UHPC/Ductal® III. C.R.C IV. HRUHPC

5. Application to Architectural Design

I. Option for suitable structural system II. Simulation III. Finite Element Analysis - IDIANA

6. Conclusions

I. Remarks II. Recommendations III. References


1. Introduction Research question How can technical innovations in concrete be applied to create, innovative structures which can facilitate the design of freer and more ambitious projects? Objective To explore the new advances in concrete technology and explore the method of their application in a complex building structure, whereas on date steel has always been preferred due to its high tensile strength. Application of developing technology can change the image of concrete as being a robust, unpleasant construction material when compared to steel and wood. Example: Ultra high performance concrete can create thinner sections with higher stability making concrete structures more graceful. Relevance Vital for development, concrete is the most used material in the world, after water. Every year, close to 7 billion cubic meters of concrete are consumed throughout the world. This works out at more than 1m3 per person. Indeed, economic and demographic growth demands the construction of housing, offices, hospitals, roads, bridges, tunnels, airports, dams, ports, water distribution networks, etc., a whole range of applications that today make concrete an essential material. The recent intensification of research into cement aggregates and concrete has led to the development of an approach based on scientific understanding of these materials. This has been made possible by techniques or instruments developed over recent years: nuclear magnetic resonance, scanning or transmission electron microscopy, atomic force microscopy Nano indentation, synchrotron radiation, etc. All these tools have helped to reveal the highly technical nature of concrete in many respects (physical and chemical phenomena, sensitivity to external parameters, changes in mechanisms over time, etc.) and to develop knowledge of the mechanisms of concrete’s behaviour on a Nano metric scale. Thanks to a greater understanding of the material, it is now possible to produce better-structured concrete, with considerably improved properties compared with those obtained twenty years ago, making it possible to construct even more sustainable buildings and infrastructure. For example, it is possible to manufacture ultra-high performance concrete, which is ductile, more durable and more resistant, allowing previously unheard of structural applications; or self-compacting concretes, which are more aesthetic and easier to use on worksites.


2. Design Goals

I. Architectural design goals

• Minimizing building footprint for Architectural development within Urban Highways. • Due to narrow ground surface in Highway zones, activities will be concentrated at the higher

level in the buildings. • Setback space for future development of roadways or automated transport systems. • Minimizing the effect of Sound and air pollution.

II. Structural Design Goals

• Continuity in the structural members is desired. • Inclined columns/shear walls or Brace frame structure. • Structural members are concentrated where the building reduces in section to take the

compressive forces imposed on them (as seen in the figure below) • The material should be consistent throughout the height of the building

Fig 1: Left, Concept Sketches. Right, Concept view of buildings between highway roads.

Fig 2: Left, Concept Sketches. Right, Concept view of structural system between highway roads.


3. Structural research –

I. Loading conditions - Lateral stability

In high-rise buildings, lateral loads become an increasingly dominant parameter for the planning and

design of the whole building. Lateral stability systems are frequently fitted within the central core and

services have to be carefully integrated with this structure.

• Wind loads The dynamic properties of the main wind force-resisting system must be taken into account (because of its sensitivity to wind induced oscillations and other conditions where wind tunnel testing may be required:

a. Tall Buildings over 400 ft high.

b. Slender towers buildings with a height that exceeds five times the least horizontal dimension.

c. Flexible buildings that are prone to wind excited oscillations due to other reasons. Some designers claim that structures with fundamental periods greater than 1 should be designed for dynamic loads. Since ordinary buildings, not higher than 10 stories, have generally fundamental periods less than 1 second, they can be designed statically.

d. Hybrid buildings and buildings of unusual shape, which include open structures. The static design wind Pressure formula p = Ce Cq qs I (psf) Qs = 0.00256V² = Stagnation pressure V (mph/kmph) = basic wind speed.

• Seismic loads

Irregular buildings refer to a significant asymmetrical arrangement and discontinuities of geometry, mass, or the lateral force-resisting structure. Physical discontinuities (e.g., setbacks, offsets, cantilevers, re-entrant corners, large openings, and other abrupt changes) cause interruption of force flow and stress concentrations.

• Load combinations

Fig 3: Effect of building form on wind and seismic load distribution


II. Structural research – Typical structural systems

• Two Dimensional structures Bearing wall structures: combinations of single walls and connected walls and connected walls, cross walls, long walls, two way walls, stacked boxes Skeleton (frame) structures: rigid frame, braced frame, braced rigid frame, truss, flat slab, Vierendeel wall beam (interspatial, bridge type) Connected walls and frames Core structures: they may be considered three-dimensional from a structural point of view, but not necessarily integrate the entire building shape: cantilevered slab, bridge structures(multicore), cores with outriggers on top (suspension), at the bottom, and at intermediate levels Combinations of these systems

• Three-dimensional structures Staggered wall beams Cores plus outriggers plus belt trusses: single, double, and multiple outrigger systems Tubes: Vierendeel tube, deep-spandrel tube, perforated wall/shell tube, trussed tube, tube with belt trusses and head, etc. Mega structure: super frame, super diagonals Hybrid structures

• Typical combinations of structure systems are the following: Walls + core(s) Frames + core(s) and/or walls Tube + frame(s) or wall(s) Tube + core (tube-in-tube) Tube + tube (bundled tubes)

Fig 4: Equivalent lateral seismic load distribution


• Other possible combinations Vertical stacking of structures: connected towers of bridge type Series of super frames Internally braced structures Cellular structures Stayed structures Other mixed systems

Fig 5: High rise structural systems


III. Structural Concrete for Complex buildings

a. Preface

• Concrete structures from high quality materials can provide a superior combination of durability, sound control and fire safety needed in today's building market. Considering the current market factors of cost options, material supply and lower floor-to-floor heights, and available developer financing, concrete is often selected as the more cost effective material over steel.

• Concrete is very strong in compression and weak in tension, while steel is very strong in compression and tension. Since the framing members in a building must resist loads through a combination of compression and tension forces, concrete framing is reinforced with steel.

• Present Day advances in High strength concrete are put to Application especially to use it as

a structural load bearing skin. Application varies to medium and moderately high rise structures from 2 storey’s to 20 storey’s constructions.

Example 1:

• This new tower, 0-14, is designed by Reiser + Umemoto RUR Architecture. The building was designed for Dubai’s Business Bay and features 22 floors covered in a double skin facade. The outermost skin is constructed from 400mm, thick perforated concrete.

• Heavy reinforcement and dense

rebar’s are used on the edges of the openings to reduce the high tensile stresses.

• Concrete mix designs have to be tailored primarily for workability, avoiding the use of large aggregates and using Self compacting concrete.

• Non standard openings in the concrete wall are filled in with light and reusable materials which also act as a formwork.

Fig 6: Top left, view of site construction. Top right, internal view of the facade. Bottom assembly of reinforcement.


Example 2:

• The 9 storey building is designed By Toyo Ito & Associates, Architects + TAISEI DESIGN PAE

Steel frame (steel plate reinforced concrete), partly wall reinforced concrete. Construction Detail – 200mm Concrete wall is sandwiched between steel plates and reinforced with steel cross section like beams.

Fig 8: Working detail of floor and wall connection.

Fig 7: Left, Internal view of Facade, Right, structural configuration view


b. Pre stressed Concrete construction

• Pre stressed concrete seeks to mitigate concrete's natural weakness in tension by imposing a permanent compression load on the structural members.Pre-tensioned concrete is cast around already tensioned tendons. This method produces a good bond between the tendon and concrete, which both protects the tendon from corrosion and allows for direct transfer of tension. The cured concrete adheres and bonds to the bars and when the tension is released it is transferred to the concrete as compression by static friction.

• Pre stressed hollow core slabs are widely used in High rise buildings. They provide longer

spans; shallow depth and the ability to carry heavy loads are easily accommodated. A series of Pre stressed hollow core slabs will provide a basic diaphragm capable of resisting lateral loads in the form of lateral earth pressures, wind loads or seismic loads by a grouted slab assembly provided proper connections and details are installed.

• Post-tensioned concrete is cast in place at the job site; thus there is almost no limit to the

shapes that can be formed. The floor slabs can have thinner concrete sections and/or longer spans between supports. Curved facades, arches and complicated slab edge layouts are often a trademark of post-tensioned concrete structures. A major advantage of Post-tensioning is that beams and slabs can be continuous, i.e. a single beam can run continuously from one end of the building to the other. Structurally, this is much more efficient than having a beam that just goes from one column to another.

c. Key issues

• Micro Cracks Due to Normal stresses

Reinforced concrete can be considered to have failed when significant cracks occur. Cracking of the concrete section cannot be prevented however the size of the cracks can be limited and controlled by reinforcement. But as a result of inadequate quantity of rebar, or rebar spaced at too great a distance, the concrete can develop wider cracks. The concrete then cracks either under excess loading or due to internal effects such as early thermal shrinkage when it cures. Cracking defects can allow moisture to penetrate and corrode the reinforcement.The challenge facing designers and builders of concrete, is controlling the appearance of random cracks, which are unsightly and require long-term maintenance. Cracks appear because forces develop in the concrete that exceed its tensile capacity. These tensile forces develop from the restraint of structural member movement by the applied loads, walls or other building components, from volume changes due to water reduction within the concrete matrix, from and from temperature effects. For this reason steel rebar or mesh is usually added to the member which to limit the width of random cracks that form between control joints. This reinforcement prevents cracking, to a certain limit. The thinner the section of the concrete member, laying of the reinforcement and maintaining effective cover from edge of concrete becomes a critical problem

• A plain concrete member has very little in bending, but as concrete fails in tension in a brittle

mode, there would be no warning of impending collapse. These problems are overcome by reinforcing the concrete beam with ductile steel bars. When over stressed the steel yields, and before failing will undergo a degree of strain hardening. Consequently, if suitably reinforced concrete beam is subjected to over loading, and the steel yields the deflection of the beam will increase notably, and it will crack grossly on the tension face. Furthermore due to strain hardening of the steel reinforcement, the beam could sustain a small further increase in load before collapsing. This ductile behaviour of reinforced concrete is a very important characteristic, not only giving visible warning of overloading, but also making it very tolerant of simplified methods of analysis, and allowing practical arrangement of reinforcement. For reinforced concrete to exhibit ductile behaviour, it must contain a minimum percentage of reinforcement. This may be understood by considering an unreinforced concrete beam of rectangular cross section. When subjected to a bending moment, this beam will fail in a brittle mode when the tensile stress in the concrete reaches its limiting value. We may call this the limiting bending moment.


If the beam were to be reinforced with less than a critical percentage of steel, the moment of resistance of the reinforced section, assuming that the concrete in cracked in tension and all the bending strength is provided by the steel reinforcement, M ultimate of steel would be less than that of the unreinforced beam. The beam would still fail at M ultimate concrete in a brittle mode, the steel instantly yielding and rupturing. If the amount of reinforcement is greater than this critical value, when the moment reached M ultimate concrete, the concrete would rupture, but the moment would be held by the reinforcement. The critical percentage of the high yield reinforcement for a bending member is generally assumed to be about 0.15 percent of the concrete area, placed close to the extreme fibre in tension. If mild steel is used, the percentage is increased in proportion to the yield strength of the bars.

• Workability - Another key issue is how to effectively place the concrete. In many locations, this is hampered by dense steel rebar. This is particularly the case around steel at the base of columns or junctions that rise from hinged connections in the structure. To overcome this problem, concrete mix designs are tailored primarily for workability. Pea gravel mixes, which have no large aggregate, are used to enable concrete to be flowed around the reinforcing steel.

Examples of such type of construction methodology: Phæno by Zaha Hadid Architects is the largest building constructed from self-compacting concrete in Europe and is significant as a reference object. Without the new type of concrete, the diverse forms of phæno – its jagged angles, looming curves, fractured planes and daring protrusions – would have been difficult to achieve.

Building Structures by Santiago Calatrava Architects

Fig 9: Top, Zaha hadid Building in Germany. Bottom, Santiago Calatrava stations.


4. Advances in Structural Concrete A new formulation approach by using ultra-fines materials supported by strong development of new admixtures open the way over the last twenty years to amazing processes in concrete technology. The range of performances and characteristics that are today covered by concrete have been expanded in various directions from ordinary concrete up to ultra high performance concrete or self compacting concrete, etc. High strength concrete, however, remains basically a brittle material requiring the use of passive reinforcement. A technological breakthrough took place at the turn the 90’s with the development of the said Reactive Powder Concrete (RPC) [1], offering compressive strength exceeding 200 MPa and flexure strength over 40 MPa, showing some ductility.

I. Ductal® Technology Based on the RPC initial research, the Ductal technology was developed. It is a new construction material technology belonging to UHPFRC family, with very high durability, compressive strength, flexural resistance with ductility and aesthetics. Ductal refers to a simple concept, minimising number of defects such as micro-cracks and pore spaces that allow achieving a greater percentage of the potential ultimate load carrying capacity defined by its components and providing enhanced durability properties.

• Greater Compressive strength Enhancement of homogeneity by Elimination of coarse aggregates Enhancement of density by Optimization of the Granular mixture Enhancement of microstructure by post-set heat treatment

• Ductility

Enhancement of ductility by incorporating adequate size fibres’ These fibers can be made of steel (Ductal®-FM), made of organic material (Ductal®-FO) or combination of both steel and organic material (Ducta® AF).

(Ductal®-FO) – High mechanical resistance is not required, only Durability and Aesthetics (Texture/Colur) (Ductal®-FM) – The first developed mix is designed for structural applications where high bending and direct tensile strengths are required. These mechanical properties are achieved by using short steel fibres. A content of 2% volume of 13-15mm length fibres with diameters around 0.2mm is considered optimum till date.

Fig 10: Left, Virtual Image of steel fibres. Right, Ductal Concrete Mix with fibres.


Fig11: Top, Ductal® behaviour in compression. Bottom, Ductal® Behaviour in bending.


• Applications

This UHPC is developed with an objective that additional reinforcement will no longer be necessary, which seems interesting for particularly small sized constructions but is not yet to apply to medium and large sized constructions, here for it is considered to use additional reinforcement and/or pre- or post tensioning. The elimination of passive reinforcement makes it possible to use thinner sections and a wider use of innovative and acceptable cross sectional shapes.

• Example 1- Toll Gate Millau Viaduct

The roof is 98m long and 28m wide, with a maximum thickness of 85 cm at centre. Its structure is like an aircraft wing and is made of match cast prefabricated segments, 2m wide, connected together by an internal longitudinal pre stressing.

Fig12: Top, Installed roof of the Toll Gate. Middle, prefab segments stored on site. Bottom, Flexible on aite formwork assembly.


II. CRC – Compact reinforcement concrete is the designation for a special type of fibre reinforced concrete with high strength (150-400 MPa) developed in 1986. CRC is used with a combination of steel fibres and conventional rebar’s. Since 1993 CRC has been used in a number of structural applications in Denmark – typically for precast elements such as balcony slabs and staircases.

III. HRUHPC – Heavy reinforced Ultra high performance concrete

The use of a HPC and an UHPC in combination with large amounts of reinforcement (fibres, rebar’s and/or wire) and known under the name of Compact Reinforced Concrete (C.R.C) as originally developed by Hans Henric Bache in 1986. The C.R.C principal makes it easier to design medium and large size constructions since the main reinforcement (traditional rebar’s, high strength rebar’s, carbon rebar’s, wire, etc.) can be exactly calculated and placed in the area where they are necessary. The main reinforcement will help to avoid segregation of the fibres and will also orient the fibres much more efficiently. The C.R.C principal makes it possible to predict the behaviour of small, medium and large sized constructions under different loadings very accurate especially when scaling up from small models are used for modelling the actual construction. This makes it thus also possible to deal with high local stresses in constructions, accidental overloading and impact resistancy on any level. The HRUHPC seems to have extremely good fatigue resistance even under continuous high loads. Various research projects and applications are executed during the last few years. Examples 1- Rehabilitation and re strengthening of Orthotropic Bridge Decks in the Netherlands In the Netherlands there are approximately 80 orthotropic steel bridges, most of them build in the seventies and the eighties of the last century. During the last 6 years a very large research project, initiated by the Civil engineering division of the Ministry of Transport, Public works and water management in the Netherlands, is carried out to investigate the reason for the Fatigue cracks in the old bridges and to develop practical solutions for cost effective rehabilitation and renovation.

Development of a new HRUHPC wearing course is underway in major Institutions around the Netherlands

Fig13: Left, Precast Balconies for residential building. Right, Precast staircase.


Example 2- Prefab Bridge deck for the Kaag Bridges A solution of HRUHPC prefab panels was chosen as the best solution to replace the existing wooden bridge decks on the bascule bridges in the motorway N44 in the Netherlands [23]. Investigations initiated by the Civil engineering division of the Ministry of Transport, Public works and water management in the Netherlands. Different tests were made with the producer of the panels, Hurks Beton at Veldhoven, to control the workability of the UHPC in combination with the densely placed reinforcement and the necessary angle of heel. Testing of two small panels in the lab from TNO Building research revealed the flexibility properties of the plate which clearly bulged under the load. This project demonstrates on a small scale what is possible to reach with the HRUHPC: light, thin and ductile constructions with strength possible as that from structural steel.

Fig14: Left, Typical Orthotropic Bridge Deck in the Netherlands. Right, Precast slab Installation

Fig15: Left, Casting of the RUHPC overlay. Right, testing of the RUHPC Overlay on deckplate with fatigue cracks on TU Delft.


5. Application to Architectural Design The Proposed design aims to analyse a complex building structure to facilitate the creation of new Functional and physical environments. The cross section of the building changes consistently along the entire plan of the building. Physical Attributes of a Typical Building Part (Dimensions are in Meters)

Fig 16: Perspective of Proposed Building Development within Highway

Fig 17: Left, Cross section of Building. Right, Plan, of Building.


I. Options for suitable Structural system

a. Concrete skin/shells + series of concrete wall

b. Stiff Floor Construction + series of concrete wall Shown in the sketch below are the floors which can act like horizontal ties to unify the shear walls and act like a box unit. Shown in the figure on the right is a concept to optimize the structure by providing architectural openings and minimize material for the shear walls.

Fig 16: Left, View of structural assemble. Right, changing cross section.

Fig 17: Left, View of structural assemble. Right, analyzed cross section for further simulation


I. Simulation

• Load Transfer Scheme

• Architectural and Structural Design Improvisations The shear walls are optimized for a more smooth curvature to minimize stress concentrations at junctions where the geometry changes as these local stress concentrations on the surface due to notches or changes in geometry are responsible for fatigue failure.

Fig 17: View

Fig 19: Optimized sections


• Configuration of Shear walls and Stiff Floor construction The distance between the shear walls varies from 13 to 16 meters.

Fig 21: Left, Architectural Model. Right, Structural Model.

Fig 20: Plans


• Proposed flooring system Infra + Floor for Horizontal stiffness

• A Combination of Steel Beam and RCC Ceiling Slab is developed for Integration of Services.

• Standard profiles can span up14 meters.

• Physical simulation Model A scaled model made of approximate material mass densities was developed to test the structural system. This designed system works on the principal of counter balancing shear walls held together by stiff floor construction which provides flexural rigidity to the walls.

Fig 22: Sequential construction of the floor unit.

Fig 23: Experimental Model


II. Finite Element Analysis - IDIANA Technical Data Input

• Material Properties High Strength Concrete Young’s Modulus E = 3e x 10 N/m2 Density σ = 2400 kg/m3 Poisons ratio ν = 0.2 Maximum permissible tensile stress = 3 Mpa = 3e x 6 N/m2 Maximum permissible Compressive stress = 35-50 Mpa

• Material Properties Ultra High Performance Concrete Young’s Modulus E = 5e x 10 N/m2 Density σ = 2500 kg/m3 Poisons ratio ν = 0.2 Maximum permissible tensile stress = 8 Mpa = 8e x 6N/m2 Maximum permissible Compressive stress = 150-180 Mpa

• Graphical Model

The 3D Model from Rhinoceros Software was exported to IGES Format to be imported in I Diana Finite element analysis Software.

For structural simulation of the system the geometry of the slabs and shear walls were further divided into calculable mesh. Fixed Foundation Constraints were applied at the base of the shear walls.

Fig 23: Graphical Model of calculable mesh


• Load Case considerations Load Case 1- Wind Pressure 1 KN/m2 on the Facade: Transferred Directly to the Shear wall as 14 KN/m along its 60 meter height. Load Case 2- Pay Load + Dead load = 3.5 KN/m2 Transferred from the floor on to the Shear walls.

a. Test Condition 1 High Strength Concrete : 250 mm thick Shear walls with no openings Combined Load case 1+2

• Global Displacement Max = 0.07 meters

Fig 23: Left, Applied Load Case 1. Right, applied Load Case 2


• Fig left: Local Stresses in Vertical Z Direction(Shear walls) Max Tensile = 0.8e x 7 N/m2, Max Compressive = 0.16e x 8 N/m2

• Fig Right: Local Stresses in Y Direction, Max = 0.15e x 7 N/m2

• Fig left: Distributed Forces in X Direction, Max Tensile force = 0.14e x 7 N/m2, Max Compressive force = 0.32e x 7 N/m2

• Fig Right: Distributed Forces in Y Direction, Max Tensile = 0.82e x 6 N/m2, Max compressive = 0.22e x 7N/m2


b. Test Condition 2a High Strength Concrete : 250 mm thick Shear walls with openings Load Case 1 (Pay Load a+ Dead Load)


• Fig left: Local Stresses in Vertical Z Direction Max Tensile = 0.9e x 7 N/m2, Max Compressive = 0.23e x 8 N/m2

• Fig Right: Local Stresses in XY Direction, Max Tensile = 0.11e x 8 N/m2, Max Compressive = 0.9e x 7 N/m2

• Fig left: Distributed Forces in X Direction Max Tensile force = 0.97e x 6 N/m2, Max Compressive force = 0.25e x 7 N/m2

• Fig Right: Distributed Forces in XY Direction, Max Tensile = 0.11e x 7 N/m2, Max compressive = 0.64e x 6N/m2


• Fig Right: Distributed Forces in YY Direction, Max Tensile = 0.16e x 7 N/m2, Max

compressive = 0.47e x 7N/m2 • Fig Right: Bending Moment in XY Direction, Max = 0.40e x 5 N/m2

c. Test Condition 2b High Strength Concrete: 250 mm thick shear walls with openings

and a stiff floor construction. Load Case 1 (Pay Load + Dead Load)


• Fig left: Local Stresses in Vertical Z Direction(Shear walls) Max Tensile = 0.67e x 7 N/m2, Max Compressive = 0.17e x 8 N/m2



• Fig left: Distributed Forces in X Direction Max Tensile force = 0.89e x 6 N/m2, Max Compressive force = 0.27e x 7 N/m2



• Fig left: Max Bending Moment in XY Direction = 0.47e x 5 N/m2

d. Test Condition 2c High Strength Concrete : 250 mm thick Shear walls with openings Combined Load Case 1+2



• Fig left: Local Stresses in Z Direction Max Tensile = 0.15e x 8 N/m2, Max Compressive = 0.33e x 8 N/m2

• Fig Right: Local Stresses in XY Direction, Max Tensile = 0.13e x 8 N/m2, Max Compressive = 0.1e x8 N/m2

• Fig left: Distributed Forces in Y Direction, Max Tensile force = 0.38e x 7 N/m2, Max Compressive force = 0.81e x 7 N/m2


• Fig Above: Max Bending Moment in XY Direction = 0.39e x 5 N/m2


e. Condition 2d High Strength Concrete : 250 mm thick Shear walls with openings and stiff floor construction

Combined Load Case 1+2


• Fig left: Local Stresses in Z Direction Max Tensile = 0.1e x 8 N/m2, Max Compressive = 0.29e x 8 N/m2





•

• Fig left: Distributed Forces in XY Direction, Max Tensile force = 0.14e x 7 N/m2, Max Compressive force = 0.98e x 6 N/m2

• Fig Right: Max Bending Moment in XY Direction = 0.7e x 5 N/m2

f. Condition 3a Ultra High Performance Concrete: 250 mm thick Shear walls with openings.



• Fig left: Local Stresses in Z Direction, Max Tensile = 0.15e x 8 N/m2, Max Compressive = 0.33e x 8 N/m2







g. Test Condition 3b Ultra High Performance Concrete: 250 mm thick shear walls with openings and stiff floor construction.




• Fig left: Local Stresses in Z Direction, Max Tensile = 0.15e x 8 N/m2, Max Compressive =

0.32e x 8 N/m2 • Fig Right: Local Stresses in XY Direction, Max Tensile = 0.11e x 8 N/m2, Max Compressive =

0.1e x 8 N/m2

• Fig left: Distributed Forces in X Direction, Max Tensile force = 0.17e x 7 N/m2, Max

Compressive force = 0.5e x 7 N/m2 • Fig Right: Distributed Forces in Y Direction, Max Tensile = 0.3e x 7 N/m2, Max compressive =

0.8e x 7N/m2




6. Conclusions

I. Remarks

• The Proposed structural system with counter Balancing shear walls and stiff floor construction is considered as an effective way for Load Transfer.

• For Load Case 1(Dead load + Payload) The Resulting Deflections, Local Stresses and Distributed Forces are within the ultimate stress capacities of High strength concrete which is used widely for various Architectural projects in present situation.

• For Load Case 2 (Dead Load + Pay load + Wind Load) the structural system shows increased deflections for, the Stresses are above the ultimate stress capacity of Present day concrete. But the stresses are well within control and further improvements by stiff floor construction and the ductility effect by adding conventional reinforcement or pre stressing will make it possible to use the structural system effectively. The structure and its critical components are analysed below to understand the effective factors responsible for failure and improvement.

a. Mass Damping effect – In Test Condition 1 without openings maximum deflection in the

Shear walls is 7 cm but In Test Condition 2c with openings the deflection increases to 14 cm. Wind load is applied in both conditions. Mass of shear wall without openings is = 4268 KN Mass of shear wall without openings is = 3704 KN Reduction of mass per wall 15 %

b. Wind Load effect – In Test condition 2a for shear wall with openings, without applied wind load the Deflection is 9 cm but in Test Condition 2c with applied wind load the deflection increases to 14 cm. Evident increase in the Local stresses is observed from 0.67e x 7 N/m2 to 0.15e x 8 N/m2. The maximum stresses are observed in the areas with a narrow structural width which is due to the eccentric loading of the complex structural shape. Wind Load acts perpendicular to the axis of the shear wall and produces increased Tensile Forces at the bottom narrow cross section of the shear wall.

Fig 24: Left, Global Displacement Test Condition 1. Right, Global Displacement Test Condition 2c

Fig 25: Local stresses for Test conditions 2a and 2c


c. Floor Stiffness Effect – The in plane stiffness of the floors appears to have a significant effect on the structural behaviour. In addition to the steel beams, a floor slab 200mm thick was introduced.

• The Local stresses are reduced to about 13.5% in Test Conditions 2a to 2b without wind load and 15% in test conditions 2c to 2d with wind load. Increased effect from the stiff floor is observed when wind load is applied.

d. Modulus of Elasticity

• In Test Condition 3a Further Simulation with UHPC (improved ductility/Young’s Modulus)

Shows Effective improvement in reducing the Deflection. Test condition 2c with, E= 30Mpa Max Displacement = 14 cm Test condition 3a with, E= 50Mpa Max Displacement = 8.5 cm

• The combined effect of floor stiffness and Modulus of Elasticity in Test Condition 3b decreases the deflection even further. Test condition 2d with, E= 30Mpa Max Displacement = 11 cm Test condition 3b with, E= 50Mpa Max Displacement = 6.5 cm

Fig 26: Local stresses for Test conditions 2a and 2b

Fig 27: Local stresses for Test conditions 2c and 2d


• The Bending Moments at critical section is also reduced due to improved ductility. Test Condition 2d Max Bending Moment Mxy = 0.39e x 5 N/m2 Test Condition 3b Max Bending Moment Mxy = 0.32e x 5 N/m2

II. Recommendations

• From the conducted research it is observed that the advantage of High Performance concrete is not for the stability of the shear wall. It is the improved ductility/Young’s Modulus that shows Effective improvement in reducing the tensile forces and deflection also is minimized. This stability can be obtained by providing appropriate steel reinforcement or even with increased thickness of the shear wall. The shear walls can also be widened and narrowed till the ultimate strength limit of the applied concrete mix. The concrete mix can be further altered to achieve the desired strength capacities.

• Furthermore the functional requirements of a structure have to considered, when choosing the structural member sizes, for example a thicker wall for a residential building is beneficial for physical performance of the building. On other hand, to achieve larger open spaces and aesthetics in exhibitions, museum or auditoriums thinner concrete members would be of importance.

• Height of the building is an important factor that affects the thickness of structural concrete members, tall buildings have thicker walls. To achieve less thickness, the concrete is further reinforced with different steel profiles or bars and also sandwiched between steel plates to minimize the tensile forces at the edges of narrow concrete members.

• The application of High performance concrete with steel fibres offers the possibility to minimize the usage of steel profiles in thin concrete structures.

• Steel fibers with appropriate proportions can be used as a means to minimize random cracking especially in thinner concrete members. When steel fibers are added to plain concrete, an inherently brittle material is transformed into a ductile composite. This steel fiber reinforced concrete initially inhibits the propagation of cracks then maintains a measure of control after visible cracks appear. Steel fibers can provide the same level of crack control as steel rebar or mesh. Guidelines can be formed in order to select the correct amount of steel fiber reinforcement to equal steel rebar or mesh used as reinforcement to minimize random cracking.

Fig 28: Bending Moments for Test conditions 2d and 3b


• Creep and shrinkage are probably the most outstanding characteristics of High Performance concrete. For normal concrete the creep coefficient can reach up to 3 to 4, for High performance concrete the creep co efficient is reduced. Creep co efficient of Ultra High performance concrete is less than 0.8 and when a thermal treatment is applied, the creep is as low as 0.2. When using pre stressing technology, the pre stress losses are substantially reduced.

• In future it should be possible to pre/post tension cantilevered concrete structures like post tensioned steel frames currently used for cantilevered constructions.

• Beyond the needs of a typical building, rising straight up from ground, a leaning building requires an enhanced lateral-force system in the direction of the cantilever and an enhanced lateral force system in the direction of the cantilever and an enhanced torsion system on account of eccentricity of transverse lateral forces, the overhang creates a substantial gravity induced over turning moment.

• To compensate for gravity-induced lateral deflection, many approaches are possible. These include cambering the structure, post tensioning and increasing the stiffness of the structure.

• The Mass Dampening effect plays an important role as the Height of the Building increases, because of increased structural dynamic resonance. Eccentric loads can be stabilized by counter balancing with additional mass.

• Post tensioning is a proven technique for the control of gravity – induced deflections. In concept, post tensioning is introduced into those elements of the structural system that are under gravity induced tension so as to place these members in compression. By post-tensioning the cantilevered part of the leaning building, it is possible to compensate for all or part of the lateral deflection induced by gravity loading.

• In order to control deflections, it is essential that the post tensioning system be load-balanced, which requires a significant increase in concrete strength. And even with an increase, the wall thickness need to increase throughout the height of the tower and particularly at the anchorages of the post tensioning.

• Lateral deflections in concrete structures are more severe because the deflection may increase with time; on account of the long-term creep properties of concrete.

III. References

• Monographs

Bennett, D 2002. Innovations in Concrete, London: Telford Taranath Bungale S. 1998. Steel, concrete, and composite design of tall buildings, Boston: McGraw-Hill Professional. Wolfgang Schueller 1996. The design of building structures, New Jersey: Prentice Hall

• Journals

Peter, Buitelaar. René, Braam. Niek, Kaptijn 2004, Reinforced High Performance Concrete Overlay System for Rehabilitation and Strengthening of Orthotropic Steel bridge decks. Orthotropic Bridge Conference Sacramento, CA. USA 2004. Reference Journals from The second international symposium on ultra high performance concrete, Kassel Germany March 5-7 2008: Jacques, R. First recommendations for Ultra-High Performance Concretes and examples of application Alain, Simon. Daniel, L. Jerome, P. Ziad, H. Design and construction of the world first Ultra-High Performance Concrete road Bridges Udo, W. Michael, S. State of the art report on Ultra High performance concrete of the German committee for structural Concrete (DAfStb)


Paul, A. Mouloud, B. DUCTAL® Technology: a large spectrum of properties, a wide range of applications.

• Websites http://library.tudelft.nl (Technical University Delft Library) http://en.structurae.de/ http://www.concretecentre.com http://books.google.com http://www.google.com http://en.wikipedia.org

High Tech Concrete for Complex Building Structures

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