Cork Institute Of Technology Construction Essay

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This project is based on the analysis and testing of timber and concrete composite structures, with particular reference to composite floors. Essentially the timber and the concrete is joined together in such a way that they act compositely together a single unified structural unit By combining these two individual structures together one is able to use the best properties of each material the timber resists the high tensile stresses and the concrete the high compressive strengths. . The two different materials are joined together by a shear connector which provides a shear resistance to the concrete uplifting at the timber interface, and resists any horizontal movement between the materials.

TCCS has been generally used in the past by companies in Mainland Europe when generally there is a higher percentage of timber structured buildings. The old timber buildings may possibly be falling into a state of disrepair. And rather than replace the old joists in the building, shear connectors are placed into the old joists, the floor back up to lie flush if had started to sag, and a layer of concrete poured onto the existing floor. When the concrete dries it acts compositely with the old timber joists and once the acroprops are removed. This new TCCS structure has much enhanced properties and characteristics, the shear capacity, sound resistance, heat and fire resistance will all be increased. And the fact that only a thin sheet of concrete has been used will not really add a massive load onto the joists.

Two different companies are seen as the leading providers of "built to purpose" shear connectors, these are tecnaria and HBV. In the project I appraise both products and use one shear connector for use in the laboratory tests; many other less commonplace shear connectors are evaluated and reviewed to gain a certain insight into using this technology in a modern society where essentially doing more for less money is key.

The design of a TCCS floor - spreadsheet - specified connectors

Table of Contents

Chapter 1

Introduction

Introduction

TCCS (Timber concrete composite structures) is essentially the designing a timber member in a building to withstand more load than it was originally designed to resist, this is done by means of pouring a concrete screed on top of the joists so that when they harden the loads are transferred into the concrete slab which withstands the compressive stresses. The concrete and timber act in unison to withstand the stresses applied, the best properties of each material are utilised. The Timber withstands the tensile forces applied and the concrete takes the compressive stress generated from the flexure of the floor. In mainland Europe this has been done to renovated old timber floors but nowadays companies are producing TCCS floors out of the factory as they are cheaper, lighter and more environmentally friendly to produce to other competitors such as hollow core slabs. They also have the additional benefit of being far easier to install than cast in situ floors which require complex propping systems. To get the timber and concrete to act in unison shear connectors must be provided to join the two materials together at the timber- concrete interface. There are a very many number of shear connectors available on the market, some of which will be reviewed and described in depth in this project. Each if these different connectors has a range of stiffness and load capacities each of which fundamental for the design of TCCS floors

The project will involve a study of the shear connectors utilised in the design of TCCS, the concrete strengths and the strength classes of the timbers. The different types of connectors will be reviewed and the strength/weaknesses of each connector discussed with relation to Eurocode 5 . Laboratory tests will be carries out to determine the structural performance of the TCCS connectors considered.

Background

In recent year there has been successfully increased demand for larger, bigger and in general better buildings. In response to this slabs need to span increasing lengths but with this increase of length a proportional increase in depth also needs to be seen. Traditionally the answer to this would have been to simply provide the slabs no matter of cost, but in recent times a better approach to building design would be to make the slabs beams and columns thinner and lighter, not only does this decrease the overall weight of the buildings but it reduces costs as generally a quantity surveyor will price materials per weight. Reducing the weight of these buildings even more is seen to be the next step, This would have been achieved by using hollow core precast concrete slabs in design, but it is not always practical to use these. This leads me on to the aim of my project, to provide an alternative, lighter stronger floor which can be produced at a cheaper cost and compare it to some off the shelve units

In the two decades timber composite construction has found a very many structural applications in modern building construction, refurbishment of old timber floor, deck systems for bridges and even preformed composite floors built in factory. They are very popular along the European mainland simply due to the ease of access to cheap timber as a structural material. Despite all of these advantages though most of the Irish and UK construction industry don't know too much about the use of TCCS. Or the design of these structures and it is this fear of the unknown, which makes designers wary of depending on small shear connectors to hold up a building. Hopefully upon the introduction of Eurocodes and particularly Eurocode 5, will change the designers demeanour towards innovative materials such as this, and especially with clients looking for cheaper forms of construction, the popularity of TCCS should improve.

Concrete composite construction is the idea behind making an individual structural unit out of two or more different structural materials, generally these two materials will have different properties but when bonded together as a single element will take on the best properties of each material, for example concrete has great compressive stress resisting abilities, but simply doesn't perform as well in tension. Steel acts well in tension and compression but it is sometimes inefficient to use a solid steel beam so it makes sense to bond steel strand into a concrete beam, thus using and attaining the best properties of each ,material. In composite construction each individual materials properties are used to attain the best structure which will perform a certain task, etc. perform well in bending, being more cost efficient, act stiffer, less noise transfer .etc.

There are countless example of composite construction seen today, precast hollow core slabs with steel strands weaved into the mix, steel beams with concrete slabs joined together, timber beams with steel decking and slab cast on top etc. he most widely available form of composite construction is reinforced concrete. E.g. concrete beams with steel bars placed into the wet concrete to resist tension while the concrete resists the compressive forces. As newer technologies come on the market more innovative uses are found for materials such as polymers being used in construction

Objectives and Aims

The objectives that have been given in this report were part of the project proposal, but in the interests of fully understanding this project on TCCS a further in depth analysis of concrete composite floor is required rather than That is specified, This includes giving a clear and prompt description of TCCS, This description will give show how TCCS performs structurally and describe the advantages and factors (dimensions, shear strengths numbers of connectors etc.) that effect element. This research will be backed up by work done in the laboratory where tests will be carried out to show comparisons between variant load cases. One of the main objectives of this project will be the testing and specifically the testing of the shear connectors which provide the bond between the concrete and timber interface. The beams assembled will be tested by analysing how they react under known loading conditions, comparisons will then be drawn to the theoretical deflections calculated using the excel spread sheet created for this purpose. The beams load bearing capacity will also be compared to the load carrying capacity of a timber only beam which is TCCS free. This aims to prove the effectiveness and durability of a TCC floor.

Figure 1

Outline of Project and research methodology

To research the project while also paying attention to the aims of the research phase a number of research methods have been used to date. The internet is probably the one that has been the most useful while also the library database In CIT was a welcome addition to finding reference documents and published literate that was somewhat specific to the project. The IHS database was also used to source the relevant codes of practise and documentation for the construction and analysis of shear connecters specific to my project

http://ars.els-cdn.com/content/image/1-s2.0-S1296207412000933-gr5.jpg

Figure 1

Chapter 2

Timber-Concrete Composite Structures

Literature Review

The masters thesis by jjorg

Function of a timber-concrete composite

library database

Advantages of the timber -concrete composite system

As the owner of a building by using a TCCS structure you can be safe in the knowledge that you are essentially preserving the existing structure of the building as it combines the best properties of reinforced concrete and timber joists in a lightweight package

By using TCCS sound qualities and sound insulation is improved, this coupled with fire protection. In a fire situation the concrete and timber work effectively together while each serving an individual purpose. The concrete acts as an impermeable barrier by stopping fire from moving to one floor to the next while when the timber ignites the edges char and provide insulation to protect the core of the beams and shear connectors from the extreme temperatures experienced. This allows the structure to stand for longer in an emergency situation. The Timber concrete sturture also improves sound insulation camaped to to pure structural timberwork sound limits of 30-67db can be achieved.

The cost savings shown in comparison to a cast in situ concrete floor ensure savings up to 40% and even more in refurbishment projects. Coupled with cheaper building the composite structure can cope with higher working loads and possible depending on the load provide enough load capacity to deal with a change of use to the building itself

Simply from an ecological standpoint it makes perfect ecological sense, Timber is the only major building material which is truly sustainable. The same floor being cast in suit would have a huge ecological impact due to the amount of carbon created when making the deep concrete slabs. Both steel and concrete would be serious contributor to the crbon footp[rint of a project "The amount of CO2 emitted by the cement industry is nearly 900 kg of CO2 for every 1000 kg of cement produced" (^ Mahasenan & Steve Smith, 2003)

In the case of a refurbishment project its makes financial sense to reinforce the existing timber floor . Removal of the existing joists and the instialltion of concrete structures is very expensice, this is avoidided by mean of using aa TCCS system. The creaking and movement of existing members is also eliminated when fixing the joists by means fo a shear connector. This also elimantes the additional cost of having to get rid of anad dispose the the material removed if one had to gut the bilding

When the concrete is poured in situ directly on top of the timber floor ing the timber hence acts as a perment formwork as the concrete is bount between the two materioals. This type of cinstructuin is much faster than cheap er than normally placing acroprops to hold up a cast in situ slab when the formwork has to be removed once the concrete has reached a sufficient strength. http://www.answersingenesis.org/museum/images/docs/slabpour7.jpg

http://www.asme.org/getmedia/8683e36d-52c8-4d17-84bc-9cba7aa28bae/pouring_concrete.jpg.aspx?width=340

In reinforced construction usuall 35% of the concrete lies below the neutral axix, This concrte is in the tensile zone which by viryue of the fact that concrete has minimal tensile strength means that the concrete simply acts as a dead load. TCCS reduces and eliminates this excessive dead load as the concrete all lies in the compressive zone of the TCCS floor where all its strength is used. This system is far more efficient in comparisomn to a cast in situ slab which has a massive carbon footprint.

Timber concrete composite floors have a greater effect on the total stability of the building due to them having such high in plane rigidity. This leads to the assumpition athta the rigitdy can be assumed infinite and this is an important design factor that make their use popular for buildings in areas where seismic activity and earthquakes are common.

The ashtetical appearance of timber flooring is far more beautiful and asteciallly pleasing than concrte flooring , It has a natural warmth and character that is second to noneit is one of the most poplular choices for flooring and for good reason, Nothing beats the beauty of timebr on the side of a house and is naturally a much more friendly material than steel or concrteTimber flooring is one of the most popular and enduring choices for flooring and for good reason. Nothing beats the beauty of a wooden floor in the home and timber floors suit all styles of homes from traditional right through to ultra modern.  There are different types of timber flooring available to you and some of these include laminates, bamboo, solid timber, or engineered timber.  Depending on the type of flooring, it can be nailed directly to bearers and joists, or laid over existing floors.

http://www.wales-holidaycottage.com/images/gallery/full/2010062120071112.jpg

library database

Shear connectors

library database

Adhesives

Adhesives are an alternative solition to using mechanicical connections in the interface between the concrte and the timber beam, Effectively the the adhesive achives its connection by binding the two materials together, generally the contact area of the connector will be the entire surface area of the concrete slab wheras with a steel shear connector you would have a tony shear connection area in comparision tio the area you are binding together. One advantage to using this typr of connector is that the shear and bending forces will be distrubiued perfectly evenly throught the cross sectional area of the floor. An adhesive connector ensures that there is no relative movemen between the interface and this guarantees a igh stiffness modulus and henceforth a decrease in deflections. Althought this is an advanget to an adhesive connector few problems still exist which reduce the populatiry of adhesives in comparison to mechiancal connectors. Once a small amount of slip occours between the interface the bond will break and the system no longer works, it reverts back to two indibidial members acting non compositly and cause the beam to possibly fail. A better more useful solution would be to use a mechanical connector in combination with the adhesive

Figure 2 Continuous glued in-plate connectors

Dowel type Fastners

The basic idea behind dowel type fatners is to insert them half way into the wooden beams, a concrte screed is then poured around and on top of the connectors that are extededed upards. Nails, boltsbars and screws are all anysled and classified as dowel type connectors The concrte is allowed set and cure. Once the concrete has set the connecters will transfer the forces so that there should be no slip between the materials and bond sould occour so that they act in union. For analysis purposes the dowels should be analysed as not being rigid. Ie provide a moment release at the location if one was analysisng the structure in staadpro. These dowel type connectors need to withstand high shear forces at the interface which could cause the connectors to simply shear in half. The load capacity of the section can be increased by decreasing the space between each connector. One disadvantage to this though is that this leads to higeher costs as more connectors will be needed.

Figure 2 Dowel type connectors: a)nails b) screws c) glued rebar d)screws inclined

Design procedure of a timber-concrete composite

library database

Historical background

During the 1940's the first ever design model for composite beams was created as result of a series of small scale tests in the United Stas. 10 years later two matameatics of th name Mohler and Newmark propsed a linear model for calculating bending test results. They based their work on proveriously derived equaions of equilibrium which was able to take into account interlayer slip between the interface of the two connected mateirals connected mechanicaly. This model is know as of today as the y- method was originally intended to be used for simply supported beams. Then in the ealrie 80's Goducky proposed a model which which was capable of takeing interlayer slip into account but also was able to account offor long term affects by assuming some long term declines, eg long term creep and deteriosation of the alastic moduli of the materials.

The first attempts to combine wodd and concrete together mechanically occoured in the earlys 1920's - 30's , Otto C=Schaub was the first inventor yto apply for a patent that was a TCC element in 1939 Then in the 1960 the first project was completed using Timber concrete composite elements. Its was called Hotel Zum Guten Hirten" It was an old hotel where the floor was after detioriating, in a n effort to restore it a concrete floor for cats in situ on top of the exhisting joists to enhance its performance. Once Goduky presented his deisgn model in 1984 the use of tcc as a technology became more widespread. When more tests were completed that time they showed that the composite action was able to increase the ovwerall loading capacity of the structure by a factor of 2.5.

Then in 1985 a new fastner known as a double headed screw was invented for the primary use in TCC systems. A Glued connection system was

introduced in 1993 by Bathon to provide a rigid

connection between timber and concrete. Then in 1993 in the Persian gulf timber bridges were covered with concrete to protect them from hot and humid environmental condition. This prevented the timber beams from rotting, But when Saki demonstrated that by using steel nails bolted in at 45 degrees to bind the concrte and the timber together a composite action was achived which lead to a beter and stonger deck system.

In 1994 more research was done by dienel who investigated fire insulation properties of tcc sturctures. His work also covered the effets that TCC had on sound insulation and the properties of it. The in 1197 Meierhofer showed in his work that by installing the bolt connectros at an angle to the interface the load capacity of the section and the stiffness of the member can be increased. . In 2003 Fagicomo and Ceccotti investagted the longterm behaviour of TCCS with respects to creep, poor environemtal conditions and skrinkage over time.

Uses of TCCS

New construtions

Given the advantages that TCCS seem to have over precast elements as shown in Error: Reference source not found It would be well worth investigating the possiblility of mass prodiucing TCCS floor for the irhish constructuiin industry. Clearly it has been shown to be easier to place than a cast in situ floor and potentially would be produced in the same way that prestressed beams are. These floors could possibly be cast offsite and delivered on site already at 28 day strength and seriously cut down on construction time. http://www.uni-stuttgart.de/ke/img/Forschung/Info/M8-2.jpg

Figure 2 Pre Cast Slab Stored Off Site

Old bridges

Renovataion of olf timber flooring/ joists

How it works

POSITION FO NEUTRAL AXIS

SHRINKAGE

TEMPERATIRE CHANGE

CREEP AND RELAXION

SUND INSULATION PROPERTOIES

Survey of tested shear-connectors

library database

Nails

library database

SFS-screw (VB 48-7,5x100)

2.3.3 Insa Hilti ........................................................................................... 19

2.3.4 Lag screws ........................................................................................ 20

2.3.5 Reinforcement bars ........................................................................... 20

2.3.6 Flat steel lock connector .................................................................. 21

2.3.7 Dimple/plug ...................................................................................... 22

2.3.8 Dimple/plug reinforced with lag screw ............................................ 23

2.3.9 Dimple/plug reinforced with steel pipe ............................................ 24

2.3.10 Dimple/plug reinforced with lag screw and steel pipe combined (CHS) 24

2.3.11 Punched metal plate fasteners (Nail-plates) .................................... 25

2.3.12 Steel brace anchor ............................................................................ 26

2.3.13 Framing bracket ............................................................................... 27

2.3.14 Universal column (UC) .................................................................... 27

2.3.15 Shaped perforated steel sheet ........................................................... 28

2.3.16 Steel mesh ......................................................................................... 29

2.3.17 Adhesive ............................................................................................ 30

2.4 Survey of currently employed timber-concrete composite solutions ..... 31

2.4.1 The HBV-system ............................................................................... 31

2.4.2 M-section-system solution ................................................................ 31

2.4.3 The SEPA 2000-system solution ....................................................... 33

Evaluation of timber-concrete composite solutions

library database ................................. 37

General

library database......................................................... 37

Execution

library database.................................................... 38

Testing

.......................................................

library database................................................... 40

General

Manufacturing of beam

Beam layouts

Beam 1

Beam 2

Beam 3

Preparation of timber

Preparation of concrete

Concrte cube test

Making and curing of concrete cubes

Testing cubes

Concrete cube results

Test Results

Beam 1 deflections

Beam 2 deflections

Beam 3 deflections

Beam 2 vs 3

Graph of differneces between 1 and 2

Timeber only beam 3 vs composite beams

Calcualtopn od theoretical deflections

Theoretical deflectoions vs actual deflectiosn

5.5.1 Case 1: tecnaria ....................................................... 62

5.5.2 Case 2 HBV system

............................................... 73

Execel spreadsheet

INsturcuons in use

Testing program

Coach screw connectord

Beam layot

Conclusions & Discussion

Errors

In manufactuee

Errors in texting of bneams

Errors in alculation of theoretical deflections

Reccomendations

Table Of Figures

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