Steel and concrete have been the most dominant construction materials. However, both are susceptible to environmental factors with concrete segregating into its constituent elements while steel susceptible to corrosion. Before the discovery of reinforced concrete, steel was the most dominant construction material because of its numerous advantages. To begin with, steel is comparatively lighter to the other materials and as such can be used in areas where the soil has a lower bearing capacity. Secondly, the erection of steel structures is very fast because of the ease of fabrication.
Concrete may be regarded as very durable because of its constituent sand, aggregate properties. Furthermore, the material is very versatile. It is however heavier than steel and therefore not suitable in areas where the soil has a lower bearing capacity. The need for reinforced concrete arises from the individual weaknesses of concrete and steel. Concrete is strong in compression but weak in tension while steel is strong in tension but weak in compression (Arya, 2009).Therefore, combining these two materials results to a much stronger construction material.
Reinforced concrete is suitable for the construction of various structures such as retaining walls, single and multi storey buildings, containment structures na d retaining walls among others .Basically, there are three most dominant types of frames that are used in reinforced concrete design and are: The medium rise frame, the single storey portal and the multistory frame (McGinley & Choo, 1990).The multistorey type may contain a central core which is meant to ensure the building resists lateral as well as wind loads effectively (McGinley & Choo, 1990).
The design of a building mainly entails the different structural elements.These elements include beams, columns, slabs and walls: either load bearing or non-load bearing. To begin with, a column is a structural member that is subjected to compressive forces and transfers the loads from the beams to the foundation. Beams are structural members that support the slabs and transfer the loads from the slabs to the columns. However, it is important to note that there are primary as well as secondary beams. The secondary beams are located centrally while the primary beams are located at the edges of the slab. Finally, the slab is the structural element that supports the live as well as the dead load and transfers these loads to the beams.
There are two methodologies through which the building elements can be designed: ultimate limit state and serviceability limit state. This is known as the limit state design and is opposed to the plastic design used in designing of steel structures (McCormac & Brown, 2014).To begin with, the limit state is mainly concerned with cracking and deflection and models the building when it is in the working state (Arya, 2009). Therefore, when a building is subjected to excessive bending and cracking, the serviceability limit state is used as a check. On the other hand, the ultimate limit state is concerned with the building when it is subjected to a number of mechanisms that might eventually lead to its failure. Some of these mechanisms may include tension, compression mechanism, shear, torsion among others. Therefore, the state shows the potential for failure when the building is subjected to a couple of forces and mechanisms.
Design of the structural components of a reinforced concrete building
As stated before, the most important components are the columns, beams, slabs and the walls.Therefore, the aspect of design is very important to every engineer. Nevertheless, some important factors to consider before the design, factors that affect the durability of these concrete buildings include: the cover provided by the concrete to the reinforcement, the content of the cement as well as the water and cement ratio, the type of cement and the strength of the concrete. However, good engineering practices and the workmanship are equally as important when it comes to the durability.
Load combinations for the building
In the determination of the load requirements of building, it is essential for the designer to determine the types of loads prevalent in the building. Basically, there are three types of loads that the building may be subjected to: wind, dead and live loads. By definition, the dead loads are those that are permanent to the building and includes the self-weight of the structural components of the building, the partition and any other structural concrete that may be needed.
The imposed loads are those that are temporary to the building and depends on the type of occupancy. In this, the loads can either be snow loads, distributed loads, impact loads, inertia etc. Finally, there are the wind loads that are attributed to wind factors such as the wind speed, force coefficients, dynamic pressure and the pressure coefficients among others (McGinley & Choo, 1990).There are two forms of resistance provided to buildings because of wind loads: bracing and unbracing. In braced structures, the resistance is provided by the shear walls, stairs and the lift shafts. On the other hand, resistance in unbraced structures is provided by bending in the frame (McGinley & Choo, 1990).
The design of the structural elements is primarily influenced by the loading and as such, there are various load combinations necessary. In this, the designer has to consider loadings at different parts of the building while at the same time applying a factor of safety. Generally, the partial factor of safety is integrated in the design to ensure that the most severe case is used in the design ( Standard, 2000).
Each floor may be described by the following plan.
As it can be seen from the sketch, each floor is made up of 1 way slabs.
As with our building, the design will have to consider the permanent, imposed and superimposed loads. There are three types of combinations that may be present: dead load alone, the dead load+ the superimposed load, the dead load +imposed loads. The common denominator is the dead load because the weight of the structural should always be considered in the design.
The dead load of the building
For the beams, the maximum depth is 1000mm.Furthermore, our design calculations will be based on a unit width of the element. The other variable in the calculation is the unit weight of concrete.
To begin with, the design will be based on a unit strip of the element and as such, the beam width=1m.Furthgermore, because the depth of the beam is 1m, the total load imposed per unit strip=1*24=24kN/m2
The load form the slab: depth of the slab=0.2m per unit strip-therefore, the load by the slab=0.2*24=4.8kN/m2 per unit strip of the depth
The total dead load=28.8kN/m2
The imposed and superimposed loads are given as 3.0kN/m2 and 1.0kN/m2
Because there are three load cases:
AS1170.0:2002 clause 4.2.2
Load case 1=dead load only=28.8*1.35*5=194.4kN/m
Load case 2: dead load +imposed load= (1.2*28.8+1.5*3.0)5= 195.3kN/m2
Load case 3: dead +superimposed= (1.2*28.8+1.5*0.4*1.0)5= 175.8KN/m2
This is a general definition of the load cases for the whole building.
The serviceability state and strength limit states for the slab, column and beam designs
The serviceability limit state, as described before, describes the building under working conditions. The crack and deflection controls are done at this stage.
Slab: the maximum load that can be applied on the slab=195.3kN/m2
Therefore, the total load that can be applied over the whole span=195.3*5=976.5kN
Therefore, the shear force 976.5/2=488.25kN
Design shear= therefore=0.7*97.65= 68.355kN
This is the same shear strength as that of the s1ab and consequently transferred to the beam.
The bending capacity of th4e whole structure depends on the moment. The general rule for the moment=wl2/12
The results are 203.4375 kN/m2
22.214.171.124: The moments in the slab can be calculated from: ly/lx=1 therefore, By=Bx=0.024(four edges discontinuous)
Therefore, the moment=0.024*195.3*5=23.436 the bending moment is exceeded in both states
Bending moment and shear force distribution diagrams
In describing the moments:
The moment at the external support=
The moment midway between the external support and the first internal support=
The moment at the first internal support=
In describing the shear forces:
The shear force at the lower side of the support and at the upper side=