1.0 Introduction
The subject of this dissertation is to design a concrete ground floor slab on metal decking because there is a basement for storage purposes. Ground floor is being used as a reception area for commercial building. Furthermore, to design a core wall which provides building lateral stability against wind load. The design project involves significant use of code of practices Eurocode 2 for concrete design. Also involves evaluation of traffic load because there is road which passes next to building, load from retaining wall and wind load.
In design project first stage is carried out by architect to determine the layout of the building. The structural engineer then develops on stage one and determines the best structural arrangements to bring the architects to develop a conceptional design. Once the best and most economical arrangements have being decided, structural engineer moves on to the next stages which involves estimating of loads, analysis to determine the maximum moments and shear for design, design of sections and reinforcement for the structure if required any depending on maximum moment and shear and then detail drawings of the projects are formed. All these procedures can be followed step by step in this dissertation.
1.1 Reinforced Concrete structure
The dry mix of concrete contains coarse and fine aggregates and cement. When water is added this reacts with cement which hardens and binds into the concrete matrix. If the steel reinforcements are present concrete matrix bonds on to them. It is a very important task for the designer to choose the right grade of concrete. Exposure conditions and durability can have an effect when choosing the mix design and class of concrete. Minimum grade required for concrete is C30/35. However, in places where there are high salt levels in air for example near sea, higher grade is required. If designing for such conditions, BS EN 1992-1-1:2004 XC and XD exposure classes must be taken in account. If the reinforcement design is intended to work for more a 100 year then Table NA.3 is used for the cover to reinforcement. Furthermore, depending on the ground condition to protect concrete against sulphate present in the soil higher grade and sulphate resisting cement must be used.
Reinforced concrete history can be traced back to early 1830 when it was mentioned for the first time. However, first reinforced concrete was not use in building, but it was rather used to make a boat in France. In 1855, it created a sensation in an exhibition in Paris. Then onwards one of the earliest use of reinforced concrete was in a pair of cottages built in 1860s. in 1954 one of the property that used reinforced concrete in the ceiling was demolished and the reinforcing bars (rebar) were found to be still in good conditions. However, in late 1860s concrete was being widely used in England but most of it was reinforced as little attention was paid to the reinforced concrete.
The principle behind using rebars is that concrete is strong in compression but fairly weak in tension. On the other hand, steel is good in both compression and tension. Therefore, when a load is applied at mid-span of unreinforced beam, the top is in compression and the bottom of concrete is in tension and cracks will start to appear at the bottom of the slab. Therefore, when rebars are added in the concrete structure this overcomes the weakness of concrete in tensions and controls cracking.
1.2 Reinforcing steel
In the UK currently the most common type of reinforcement that is being used is grade 500 (500N/mm2 characteristic strength) which has replaced the grade 250 and grade 460 reinforcement steel. However, Eurocode 2 permits of up to Grade 600. Reinforced ribbed steels are classified into different classes (Bill Mosley, 2012)
Class A – cold worked bars used in mesh, normally with smaller diameter then 12mm and with the lowest ductility category.
Class B – most commonly used in reinforcing bars
Class C – highest ductility mostly used in places with high level home disaster e.g. Earthquakes
When designer selecting the reinforced steel, main characteristics required are as follows;
1.2.1 Tensile strength
Tensile strength of steel can be determined from stress-strain curve. Tensile strength of steel includes yield strength and elongation. In the curve where the load suddenly drops is known as “yield effect”. The ratio for tensile strength to yield strength (ft/fy) is the measure of ductility in steel. This shows the ability of steel to harden prior to failure. Carbon content in steel is the major factor that effects the ductility in steel. As the carbon content increase in rebar it becomes more ductile and less brittle therefore prior failure there will be a warning as the structure will start to sag. However, if the carbon content is low there will be no warning but a sudden failure of the structure.
Uniform elongation (Agt) is another measure of deformation prior to failure. In British standards the measure of ductility was elongation to fracture (A5) which is replaced by Agt which believed to be more appropriate design characteristics.
1.2.2 Bend
Nearly all rebars necessitate bending when being installed in to a concrete structure. As the rebars are fairly high strength steel and therefore when the radius of bend is too tight it may fracture the reinforce steel. In BS4449:2005 have specification of minimum mandrel diameter for bending high steel rebar. As temperature decrease steel touchiness decrease therefore the risk of failure on bending is increased when temperatures are lower. Therefore, in the design code there is a minimum bending temperature given to carry out the bending operation safely.
1.2.3 Bond
In reinforcement steel grade 250 used to be hot rolled mild steel which usually have a smooth surface therefore bonds are only form by adhesion with the concrete. However as mentioned above in reinforcement steel that grade 250 is not used and has been replaced, this is also because of the smooth surface which are not available for use in UK any more as it is not recognised by Eurocodes.
High yield rebars are manufactured with the ribbed surface which manufactured by controlled cold twisting of hot rolled bars. Ribbed bars are very good at bonding with concrete mix therefore when using these end hooks are not required. Ribbed steel can be bent through 180o without creating any crack on the surface and maintaining the high strength. (UK Cares, 2011)
1.3 Metal deck slab
In the industry, there are two types of decking available Re-entrant decking and Trapezoidal decking. The steel deck acts together with the concrete by bonding between the deck and concrete. The deck on each floor provides lateral stability by acting as a floor diaphragm transferring loads back to the stability cores. Shear studs are shot through the deck and act together with the beam after concrete is poured. Using composite floor system significantly reduces the construction time and is much lighter and stronger than other type of slabs. The steel decking also inhibits and longitudinal slips between steel and concrete itself and transverse moment between slab and the supporting beam.
1.4 Core wall
Core wall is a vertical cantilever structure in building construction. It can transfer lateral forces from slabs, roofs and exterior walls to the ground foundation parallel to their planes. In high rise building wind load, seismic effects (however in the UK seismic effects are not applicable) and in addition the self-weight which generates very strong torsional forces which can cause the structure to fail by shear. In addition, core wall also provides stiffness to building in the direction of their orientation.
1.5 Retaining wall structure
Retaining wall is a structure which is designed to withstand the load from retaining material which is normally soil. Most common type of retaining wall which is used is cantilever wall which is fixed from the bottom and top free end rests on slab. This prevents soil collapsing on to the foundations before and after work is finished therefore protects the foundation. When designing a retaining wall for a big building there will be multiple types of pressures to design for. Such as earth pressure, hydrostatic pressure, surcharge and in some cases where the retaining wall is nearer to the road traffic load must be taken in account as well
1.6 Aim and Objectives
1.6.1 Aim
The aim of this project is to work out the loading on retaining wall and then to do design calculations for the first-floor composite slab in the office and determine the number of extra reinforcement that will be required to put in the slab to make it durable for the purpose. Furthermore, carry out design calculations for the core wall of the building.
1.6.2 Objectives
I. To investigate the structural analysis of ground floor slab
II. To model the slab on Robot Structural Analysis software using the values from I
III. To carry out design calculations according to EC2 for the ground floor slab using values from II
IV. To investigate the structural analysis of core wall
V. To model the slab on Robot Structural Analysis software using the values from IV
VI. To carry out the calculations according to EC2 for the core wall using values from V
To design concrete structures, as in this dissertation the use of Eurocode 2: Design of Concrete Structure is a must. Eurocode provides the basis of design and requirements for safety, serviceability and durability of concrete structure. A structure must be safe against collapse and be serviceable in normal use. In addition, it must also be able to withstand the effect of unexpected weather or accidental damage which cannot be avoided by the structure.
Ultimate limit states are applied which are concerned with safety as it requires structural should not collapse due to loss of equilibrium or failure of any part. Structural member must be stronger to resist the actions applied on it. Actions consist of a combination of permanent (dead) load and variable (live) load. Characteristic values are specified on EN 1991-1-1:2002 in table 6.1 and 6.2 for self-weight and imposed load on building.
Design value of an action can be expressed as:
Design value = characteristics value x yF
yF is a factor that takes in account the significance of the particular limit state, uncertainties in modelling the effect of action and the possibility of unfavourable deviation of the action values from the representative values. In USL for the design of structural member yF = 1.35 for permanent loads and yF = 1.50 for variable actions.
3.2 Dead Load
Using Comflor 51+ TATA Steel single span deck table (TATA Steel, 2017). Floor deck 200mm thick, single span deck continuous slab, no props, 60 minutes of fire resistance. Span of the design is 2.5m and the metal deck can span 2.74m using A393 mesh. Normal weight concrete 4.75KN/m2 and addition finishes 0.5 KN/m2.
Therefore, as slab is analysed per meter
G_K=dead loads×1m
Traffic loads of 100KN/m are applied in to two places with 1m interval where the worst-case scenario is expected in the slab. In this case it is between the span of 10m and load is applied between 4-5 meters with 5-6 meters interval and load is applied between 6-7 meters.
In this scenario, there are four different variable actions therefore each variable action must be considered as a leading action because it’s very unlikely for all the actions to be applying maximum loads together. Therefore, four different combinations need to be applied to get the maximum sagging and hogging bending moment where each variable action is a leading factor. When the variable action is not a leading factor, recommended value of 0 factors for building needs to be applied (figure 1) when calculation ULS. Recommended values for 0 BS EN 1990