Reinforced Concrete Construction Low And Medium Rise Residential Engineering Essay
Masonry is referred to brick and masonry construction started twenty thousand years ago in the “Stone Age” and through history people used clay to build their houses by drying the clay in the sun till it is hardened and form clay masonry.
They used masonry in building bridges, domes, and churches. In the eighteenth century a great improvement was made to masonry construction especially in bridge design and the parabola was then invented, moreover eighteenth century saw a great output of iron and new methods of production were invented. Cast iron columns and beams were made to support a brick floor, but masonry construction was taking the lead in huge construction projects and buildings. (Crozier, 1999)
During this time understanding of materials was their main objective and mathematical analysis of structures. Studying the thrust line and making sure it remains in the middle third of the masonry element cross section. In the early of the nineteenth century masonry bridges and viaducts were constructed to support the railways expanding. (Crozier, 1999)
The latter of nineteenth century reinforced concrete was developed and engineers found that it is easier to build bridges using reinforced concrete than masonry and gives them more flexibility in shaping the bridges. Masonry buildings was still used in buildings but due to its’ huge wall thickness was not preferred in high buildings, for example a building can be six stories with a wall of two meters thickness which was not economical and takes a lot from the building area. (Crozier, 1999)
At this time masonry was used in other ways as for example cladding of huge area of steel frames and decorations for the outer Skelton of a reinforced concrete building. Later engineers started to learn more about masonry and started to develop its’ structural design, by this development the wall bearing masonry was used to develop residential buildings with smaller thickness with number of stories up to fifteen. Engineers understood the concept of forming plastic hinges and how important are joints between elements. (Crozier, 1999)
1.2 Types of loads subjected on masonry structures
Masonry walls are similar to concrete which is designed and built to carry only compression forces and they are weak on the tension side, that’s why lateral loads cause a huge threat on masonry building (wind load and earthquakes) there are two main types of loads; permanent like dead load, semi permanent as live load and transient load as earthquakes and wind load.
Engineers developed the reinforced masonry construction to lower the risk of tensile forces subjected to masonry buildings, moreover designers studied the arrangement of bricks in planar walls to increase the strength of the bricks in resisting permanent and semi-permanent loads. (Crozier, 1999)
Loads transferring in masonry elements is a main objective for the designer to develop a durable and safe structure, therefore a designer must ensure a number of load paths through the structure to ensure safety if accidently any supporting element is damaged for example, when a column accidently destroyed a new load path must be offered for transferring loads through beams. (Crozier, 1999)
1.3 Examples on masonry structures
Brown County Jail
Figure 1.3.1: The brown county jail (1902) in USA
picsBridge [Alcantara].gif
Figure 1.3.2: Bridge at Alcantara in Spain (105 AD) (Crozier, 1999)
picsVault.gif
Figure 1.3.3: Masonry Vault (Crozier, 1999)
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Figure 1.3.4: Giza pyramids (2560 BC)
1.4 Advantages and Disadvantages of masonry construction
Advantages
Disadvantages
When well designed provide attractive, permanent and durable structure.
Masonry need a lot of very skilled labour to finish the job which might not exist unlike concrete where very skilled labour is not important.
When constructed there is no need for finishing with plastering or scraping.
Load bearing partitions and floors must all be done on the same time that is why engineers are always on a very critical path unlike steel and concrete.
Great choice for cladding as it provides a good weather protection, and great sound, and thermal insulation and fire resistance.
Masonry can be cracked easily through settlement of foundation on unstable soil.
People prefer masonry because it is warm and friendly unlike concrete.
As mentioned before masonry is weak in tension and cannot handle strong lateral loads like wind and earthquakes
Masonry has varieties of units and sizes depending on the masonry structure needed to be built.
Masonry is hard to clean unlike concrete.
Masonry provides an economical alternative to concrete in load carrying and provides cladding to large area such as factories made of steel frames.
Masonry is very critical in detailing to ensure safety and to be economical.
Masonry is fast in building and with skilled labour it can be finished faster than concrete.
Table 1.4.1: Advantages and Disadvantages of masonry construction (Crozier, 1999)
2) Materials and forms of masonry:-
2.1 Structural forms of masonry
-There are various types of masonry structural forms as piers which are designed to carry only compression force and they form as a part of the wall. Single leaf wall which is a planar wall made of hollow or solid masonry in different alignments and thicknesses and has different types as straight and zigzag leaf wall, steel are aligned horizontally in bed joints and vertically in cores of the hollow masonry. (Crozier, 1999)
Figure 2.1.1: Planar wall type (Crozier, 1999)
-Diaphragm walls which consists of two brick walls with a crossing beam and can be reinforced or unreinforced depending on the subjected loads. (Drysdale, Hamid, Baker, 1999)
Figure 2.1.2: Diaphragm wall (Drysdale, Hamid, Baker, 1999)
-Cavity walls are two same or different brick walls spaced and connected to each other wall ties made of steel and the cavity is then filled with grout and reinforcement to form the wall. (Crozier, 1999)
Figure 2.1.3: Cavity wall (Crozier, 1999)
-Veneer walls which mainly resists lateral load are formed to brick wall connected from its’ back with concrete or timber frame using steel ties. (Drysdale, Hamid, Baker, 1999)
Figure 2.1.4: Veneer wall (Crozier, 1999)
-Columns are vertical member built with either clay bricks or concrete blocks, they are designed to carry compression forced and built as a separate member from the wall. Column can be unreinforced if it is only subjected to vertical loads, but in case of bending on any of its’ principal axis reinforcement is essential. (Drysdale, Hamid, Baker, 1999)
Figure 2.1.5: Masonry columns (Drysdale, Hamid, Baker, 1999)
-Masonry beams are horizontal member used to carry loads upon an opening or a lintel which will be discussed later. Masonry beams are always reinforced to be able to resist tensile forces which come from bending moments as in concrete, lintels are constructed using special types of concrete masonry units or concrete hollow blocks and then grouted over the reinforcement. (Drysdale, Hamid, Baker, 1999)
Figure 2.1.6: Masonry beams (Drysdale, Hamid, Baker, 1999)
2.2 Materials and masonry units used in masonry construction
-There are different types of masonry units used in construction depending on the structure and the load applied from the structure or from nature. Starting by basic which is the clay masonry which can be solid with mortar or cored with reinforcement and grout, clay masonry is clay dried by fire and left to harden which relatively produce high compressive strength. Clay bricks can be found as solid or as hollow. Clay brick may be reinforced to increase its’ capacity to resist compressive loads. (ECP 204, 2005)
Compressive strength for solid and hollow clay bricks
Type
Solid clay brick
Hollow clay bricks
dimensions (mm)
Length
Width
Height
Length
Width
Height
Mm
mm
mm
mm
mm
mm
250
120
60
235
120
250
250
120
65
Â
Â
Â
250
120
95
Â
Â
Â
250
120
115
Â
Â
Â
250
120
130
Â
Â
Â
Compressive strength (N/mm^2)
Average five bricks
one brick
Average five bricks
one brick
Load bearing bricks
Not less than 8
Not less than 7
Not less than 2.5
Not less than 2
Non load bearing solid brick
Not less than 4
Not less than 3.5
Non load bearing vertical hollow brick
Not less than 4
Not less than 3.5
Non load bearing horizontal hollow brick
Not less than 3
Not less than 2.5
Based on the Egyptian code of practice (ECP 204, 2005)
Table 2.2.1: compressive strength for solid and hollow clay bricks
-Concrete units which is simply made of sand, aggregate, cement and water and its’ compressive strength can vary depending on the raw materials used, and size of the block and it is relatively high. It can be found as solid or hollow, and can be reinforced to carry tension loads or to increase the compression resistance. Concrete unit has high compressive strength but by adding mortar the compressive strength of the whole prism decreases. (ECP 204, 2005)
Compressive strength for solid and hollow concrete blocks
Type
Solid concrete brick
Hollow concrete block
dimensions (mm)
Length
Width
Height
Length
Width
Height
mm
mm
mm
mm
mm
mm
250
120
60
400
100
200
250
120
120
400
120
200
250
120
200
400
120
250
Â
Â
Â
400
200
200
Â
Â
Â
400
250
200
Compressive strength (N/mm^2)
Â
Average ten bricks
one brick
Average five bricks
one brick
load bearing solid units
7
5.6
7
5.6
load bearing hollow and cored units
5
4
5
4
Non load bearing solid units
2.5
2
2.5
2
Non load bearing hollow and cored units
2
1.6
2
1.6
Based on the Egyptian code of practice (ECP 204, 2005)
Table 2.2.2: compressive strength for solid and hollow concrete blocks
-Mortar is a very important type of materials used in masonry construction; mortar is made of cement, lime, sand and water. It is used as a bonding material between the masonry units and also it is a good weather insulator. Mortar has different thicknesses depending on the design, but it is mostly to be used with ten millimetres thickness. Mortar is very weak in tension therefore under tension loads exceeding the tension capacity of both the bricks and mortar (prism) failure occur in mortar by splitting of mortar joint way before a failure occurs in the brick. (ECP 204, 2005)
Mortar components percentages and compressive strength
Mortar number
Mortar components percentage with respect to volume
Average compressive strength after 28 days (N/mm^2)
Portland cement or Blended cement
Hydrated lime
Sand (measured when hydrated and not compacted)
1
1
From 0.0 to 0.25
From 2.25 to 3.00 times the total volume of cement and lime
15
2
From 0.25 to 0.5
10
3
From 0.5 to 1.25
5
4
From 1.25 to 2.5
2
Based on the Egyptian code of practice (ECP 204, 2005)
Table 2.2.3: Mortar components percentages and compressive strength
-Grout is composed of cement, sand, aggregate and water, grout fills the cores and cavities of the masonry units to increase its’ gross area, and increase strength of masonry. Grout also may be used to form a bond between masonry units and steel reinforcement and it has a minimum compressive strength of fourteen (N/mm^2). (ECP 204, 2005)
Grout types, components percentages and compressive strength
Grout Type
Grout components percentage with respect to volume
Â
Â
Cement percentage with respect to mass
Hydrated lime
Aggregate percentage
Size of the biggest aggregate (mm)
Minimum compressive strength
Small
Big
(N/mm^2)
Grout for small thickness till (50 mm)
1
0.0 to 0.1
2.25 to 3.0
–
5
Bigger than 14
Grout for large thickness bigger than (50 mm)
1
0.0 to 0.1
2.25 to 3.0
1.0 to 2.0
19
Based on the Egyptian code of practice (ECP 204, 2005)
Table 2.2.4: Grout types, components percentages and compressive strength
Reinforcement form a great importance in masonry construction process, it can highly increase the strength of masonry, highly resist tensile stresses that may occur from non concentric loads and also controls masonry cracking in suitable conditions. Reinforcement is placed on the hollow units and then filled with grout to form the bond. The only concern from combining steel with masonry is corrosion, therefore it is highly recommended to plate the steel bars with anti corrosion material to prevent this process from occurring. The maximum diameter of steel bars used in masonry construction is twenty five millimetres. (ECP 204, 2005)
2.3 Lintels
masonry structures is different form reinforced concrete structures; the main elements carrying load in concrete structures are columns, beams slabs and foundation, brick walls in concrete structures act as a covering materials between columns and they do not participate in carrying load with the mentioned elements in opposite of masonry structures where loads are mainly carried through the load bearing walls, therefore openings (lintels) in these walls may reduce the ability of the wall to carry applied load and may lead to collapse of the structure. To get to a conclusion loads applied on the lintels must be well taken into consideration.
A lintel beam is the solution to carry the loads coming from the upper own weight of the wall load, loads applied from the upper roof and floors and the lintel own weight. Lintel beam is mostly designed as a simple beam and which transfer loads to the supports. Lintel beams does not carry the whole load upon it due to the transfer of loads through the wall in an arched direction to be more specific the arching action, therefore lintels carry a triangular load upon it of forty five degrees, if the vertex of the load triangle is located within the wall, therefore the loads of the masonry side will be neglected and the only loads carried are the lintel own weight and the triangular load of the wall above, but if the vertex of the triangle is upon the wall height and extended to the roof therefore the lintel must carry the full load above it. (Drysdale, Hamid, Baker, 1999)
Figure 2.3.1: Lintel load distribution (Drysdale, Hamid, Baker, 1999)
2.4 Types of masonry construction
2.4.1 Unreinforced masonry:-
Unreinforced masonry is simply constructing masonry elements without using neither reinforcement steel bars nor steel stirrups. Unreinforced masonry is mainly used in low seismic regions where lateral loads are small moreover unreinforced masonry can be only used in the construction of low and medium rise buildings to lower the effect of wind loads subjected on the structure as the case of the current research paper. The thickness of the wall of the unreinforced masonry is important for the location of the thrust line. Unreinforced masonry are mainly subjected to compressive vertical loads and this rely on the high compressive strength of different types of masonry units on the other hand tensile stresses may occur which may cause the masonry to crack and then fail, therefore tensile strength of masonry must be higher than applied tensile load. (Drysdale, Hamid, Baker, 1999)
2.4.2 Reinforced masonry:-
Reinforced masonry is simply adding reinforcement bars and steel stirrups to masonry units. Reinforcement is mainly used to resist flexure tension and shear therefore the alignment of steel bars is important, usually vertical steel bars are used in masonry wall to resist tension force produced from bending moment and horizontal steel bars are used to resist shear. Reinforced masonry is mainly used in medium to high seismic regions due to steel that is strong in resisting tension forces. Grout is an important element is reinforced masonry, and it is poured in the cores of masonry units to surround reinforcement and perform a strong bond between steel bars and masonry unit. (Drysdale, Hamid, Baker, 1999)
3) Behaviour and design methodology of masonry:-
3.1 Physical properties of masonry units
Masonry units have different shapes and sizes depending on the need of construction, but mostly they are rectangular shape with length, width and height and they are laid on its’ length dimension parallel to the in plane of the wall and it is called stretcher, the vertical front side is called the face and the two vertical side are called ends and the side which is used for laying masonry units on it is called the top. Length and height of masonry are always standard and the only variable dimension is the width of the brick, mortar used for bonding bricks together mostly has a standard thickness of ten millimetres and is placed on the top, bottom and ends side of the brick and is called mortar joints, Areas used for mortar placement are called beddings and has two types; face shell bedding which is the covering of only the face shell with mortar and full bedding which to cover the total gross area with mortar. Not all masonry bricks are solid; bricks are found as cored or hollow units as in the case of concrete hollow blocks which consists of cells which are the hollow units, face shell which is the parts of the opposite faces of the cells, end webs which are the parts at the end of the block and central web which is between the cells. A block may be called solid if the degree of solidity is seventy five percent or more and are called hollow if it is less than seventy five percent, degree of solidity is the net area of the block divided by the gross area multiplied by hundred. (Drysdale, Hamid, Baker, 1999)
Figure 3.1.1: Masonry unit labels (Drysdale, Hamid, Baker, 1999)
3.2 Behaviour of masonry prism under axial compression
3.2.1 Prism characteristics and behaviour
As to understand the behaviour of masonry elements under axial compression loading as the case of the current research paper, going through laboratory tests is the best and effective way to illustrate various behaviours. Masonry elements are composed of masonry units such clay or concrete, bonding element such as mortar, grout and when needed reinforcement these elements form an orthotropic behaviour which means that mechanical properties are different through the axis of symmetry of masonry element unlike concrete which have the same properties on each axis and this is called isotropic. (Drysdale, Hamid, Baker, 1999)
Axial compression test is made on prisms which is one unit length and thick and can be built at several heights of two, three or four units and as mentioned before with mortar joints of ten millimetres between the units, usually it is aligned in stack bond which means that units are placed on the same direction. Prisms are tested to determine the axial compressive strength of masonry on a smaller scale which is then used for creating tables used for design, and prisms is tested on a machine that applies vertical load to the prism till failure occurs and then compressive strength is then measured. (Drysdale, Hamid, Baker, 1999)
Figure 3.2.1.1: Masonry prism (Drysdale, Hamid, Baker, 1999)
Prisms under axial vertical compression have two modes of failure relative to the height. The first mode of failure is conical shear and this happens when the prism height is two times the thickness of masonry unit, but this type of failure does not happen in real masonry element such as a masonry wall because the height of the wall to the thickness is different.
The second mode of failure is vertical cracking through masonry prism and this happens when using four to five bricks height which will be the same mode of failure that happens in a masonry wall. Masonry unit under vertical compression usually tries to deform laterally and therefore it will be subjected to biaxial tension, but mortar under same case of loading tries to expand from all direction but masonry unit of higher strength than mortar prevent it from deforming and performing triaxial compression which leads mortar to burden more compressive load than its’ capacity. When the compressive load increases mortar forms tensile stresses on the masonry unit leading to vertical cracks on masonry unit and this is typically what happens on load bearing masonry wall with full bedded area. (Drysdale, Hamid, Baker, 1999)
Figure 3.2.1.2: Forces acting on masonry prism (Drysdale, Hamid, Baker, 1999)
Adding grout to masonry hollow units increases the strength because of grout carrying part of the compressive load applied to the prism, on the other hand grout put a great threat to prism strength due different material properties between masonry unit and grout, incomplete grout compaction which will create gaps between grout and plastic shrinkage which is the fast water loss of grout surface before placing which leads to cracks and this is due to low skilled labour and lack of measurements taken before and through grout placement. These factors can lead grout to produce lateral forces on the unit which will lead to the failure of the prism before it reaches the full compressive capacity. (Drysdale, Hamid, Baker, 1999)
3.2.2 Prism and factors affecting strength
There are several factors affecting the strength of the masonry units and therefore the strength of the prism. These factors depend on the height of the specimen, dimensions and strength of the unit and bonding material (mortar), and strength of the grout being used. These factors may result in increasing or decreasing the strength of prisms of different types of units under vertical compression loading, therefore taking into consideration these factors will lead to a precise measuring of the compressive strength and capacity.
The height of the unit plays an important role in determining the strength of prism, as mentioned before there are two modes of failure which depends on the height of the prism that depends on the unit height therefore tests showed that increasing the height of the unit decreases the lateral deformation of it and this due to the increase of weight and dimensions of the unit which will make it more stiff, more over when the unit strength increases the prism strength increases and therefore the prism compressive strength shows an increase, on the other hand high strength clay bricks does not effect much the strength of the prism unlike concrete blocks. One may consider that the only important parameter of the masonry unit is the compressive strength, unfortunately this is not the right case; tensile strength is of the same importance because as mentioned before failure through vertical cracks in prisms and walls occurs due to tensile stresses formed on the unit due to mortar expansion, therefore tensile capacity of the unit limits the failure of the prism. (Drysdale, Hamid, Baker, 1999)
Hollow and cored units made of concrete and clay influences the unit strength and therefore the prism strength. Solid units’ compressive strength is higher than hollow units and this due to the uniformly distributed compression stresses on the total area (gross area) of the solid unit unlike hollow units of similar dimension; stresses due to vertical compression are distributed only on the net area (area of cores subtracted from gross area) which leads to a lower compressive capacity of the unit, more over mortar bedded area which carries part of the compressive load of the prism will decrease in case of hollow or cored units and therefore the prism compressive strength will decrease, tests showed that a decrease of fifteen percent of the bedded area will influence a decrease of forty five percent on the vertical sections resisting compression stresses. (Drysdale, Hamid, Baker, 1999)
Bonding material is one of the most important parameters affecting the prism strength. Mortar strength influence the strength of the prism by increasing it to a limit, on the other hand using mortar of high strength more than needed for ensuring a durable structure will not effect much the prism strength and will affect negatively on the durability and strength of the structure. high strength units usually covers over the mortar strength, mortar is always formed on site of construction, therefore low skilled labour may not ensure the precise workability needed of mortar used in construction. (Drysdale, Hamid, Baker, 1999)
Mortar thickness as mentioned before is usually ten millimetres and increasing the thickness of mortar usually happens on construction sites due to low skilled labour, therefore clay masonry prism is affected by decreasing the compressive strength due to different material properties, on the other hand concrete prisms is not much affected by the increase of mortar thickness due to their similar material properties, therefore to overcome this conflict a standardised thickness is used. Face shell bedding in hollow or cored unit increases the compressive strength of the prism due to the lower tensile forces produced on the masonry unit. (Drysdale, Hamid, Baker, 1999)
3.2.3 Stress and strain of masonry
Figure 3.2.3.1: Stress strain curve of masonry materials (Drysdale, Hamid, Baker, 1999)
The upper figure shows the vertical stress and strain behaviour for types of masonry materials which are units, mortar and prism. It is obviously seen that masonry units can handle large amounts of vertical compression stresses due to the applied load with small amount of vertical strain and then a sudden brittle failure occurs when reaching the maximum strain.
Mortar can handle smaller amount of vertical compression stresses than the unit and a larger amount of vertical strain. To get to a conclusion mortar vertical deformation is bigger than that of the unit for the same value of vertical stress as illustrated in the above figure. When combining both mortar with units a testing prism is formed, the figure above shows that a prism can handle smaller amounts of vertical compression stress than the unit itself and this shows why the unit compressive strength is larger than the prism. (Drysdale, Hamid, Baker, 1999)
Tests on prisms showed that concrete and clay masonry have a nonlinear behaviour on the stress and strain curve. Tests showed that concrete masonry strain in bigger than that of clay masonry and studying the behaviour of masonry showed that for different types of mortars the range of strain found is close to the strain range of concrete which is from (0.002-0.003) and this shows that masonry behaviour is the same as concrete under vertical compression load. To get to conclusion masonry is a strong and economic replacement for concrete with respect to bearing of compression loads. (Drysdale, Hamid, Baker, 1999)
3.3 Modes of failure of masonry walls
3.3.1 Failure of masonry wall under vertical compression load
As discussed before in vertical compression prism test, failure of masonry prism is similar of that of masonry wall. Under vertical compression load exceeding the ultimate compression capacity of the masonry element, vertical cracks occurs due to the expansion of mortar performing tensile stresses on the masonry unit, vertical cracks is the first step of failure, and when the masonry element reaches beyond the ultimate capacity vertical splitting occurs on the masonry wall leading to a complete collapse or failure. (Crozier, 1999)
C:UsersMitryDesktopScreenshot_2012-11-30-13-37-53-1.png C:UsersMitryDesktopScreenshot_2012-11-30-13-37-53-2.png
Figure 3.3.1.1: Failure of masonry wall under vertical compression (Crozier, 1999)
To be more practical this type of failure occurs due to concentric vertical loads which means that compression load passes through the centre of gravity (CG) and this case does not ever happen in real practice, therefore due to the edges of the slab rotation on the wall face an eccentricity of load occurs and the load becomes eccentric which means that there is a horizontal distance between the load and the centre of gravity of the masonry wall called eccentricity (e). The eccentricity induce bending moment on the masonry wall creating a compression failure on the masonry element inside face, and when the tensile capacity of the wall is exceeded on the member outside face a failure occurs which is buckling. This shows that not only the compressive strength of the masonry element is important, but also the tensile strength, therefore a minimum eccentricity is always put into consideration by the engineer to overcome such failures. (Crozier, 1999)
3.3.2 Masonry shear wall failure
To be more specific this discussion is on unreinforced concrete masonry shear wall subjected to lateral loads such as wind load. A masonry shear wall act as a cantilever that is mainly fixed from the base and its’ ends are free and subjected to uniform lateral load, to study the failure modes of a shear wall, a behaviour of the shear wall must be studied with laboratory tests on specimens, mainly shear failure occurs due to the combination of compressive loads and shear stresses induced by lateral loads, more over bed joints are the only subjected joints to shear stresses in the masonry shear wall, therefore failure always occurs in the bed joint. When the compressive stresses normal to the bed joint due to vertical load is smaller than shear stresses parallel to the bed joint due to lateral load, therefore slipping of bed joint occurs and therefore shear failure occurs, on the other hand if the compression load is increased relative to shear stresses, slip and splitting of bed joint occurs. (Crozier, 1999)
Figure 3.3.2.1: failure mode of a prism under shear (Drysdale, Hamid, Baker, 1999)
When lateral loads (wind) are subjected to a shear wall it performs tension force on the close side of the wall which is the wind ward and compression force on the far side which is the lee ward, therefore preventing tension failure from occurring on the wind ward side a suitable amount of dead load (own weight of shear wall) must be put into consideration to put the shear wall in a state of equilibrium. In practice due to lateral loads an overturning moment is induced and with low axial compression loads, shear walls are subjected to three main types of failures; cracks occur due to tension failure and then followed by diagonal crack through the wall length, and the toe is subjected to compression failure, and as mentioned before slipping of mortar joint occur and this is called slipping failure. (Crozier, 1999)
Figure 3.3.2.2: Shear failure of masonry wall (Crozier, 1999)
3.3.3 Masonry wall failure due to bending
Walls are subjected to flexure and this due to eccentricity of vertical compression loads as mentioned before and lateral forced. There are two main types of bending failures which are horizontal bending and vertical bending, testes has proved that masonry vertical bending strength is much higher than that of the horizontal bending. The behaviour of bending on any masonry element depends on the vertical or horizontal supports.
Horizontal bending occurs when the supports are led on the vertical sides of the element and bending occurs in plane. Mortared prepend joints which are the vertical sides of the masonry units bedded with mortar are the main reason of horizontal bending due to the bond between the unit and the mortar, shape and dimensions of the units and prepend joints strength in tension, therefore failure occurs through the mortared prepend joint and it is called toothed failure. (Crozier, 1999)
Wall H
Figure 3.3.3.1: Horizontal bending failure of masonry wall (Crozier, 1999)
Vertical bending occurs when the supports are led on the horizontal sides of the element and bending occurs out of plane. This type of failure occurs at fifty percent of wall height and this is due to rotation of along the bed joints of the wall due to lateral loads. Combined both vertical and horizontal bending due to supports found on both vertical and horizontal sides can lead to a mode of failure of rotation along bed joints with cracks formed diagonally through the entire wall. (Crozier, 1999)
Wall V
Figure 3.3.3.2: vertical bending failure of masonry wall (Crozier, 1999)
3.4 Design equations
All design equations that are going to be discussed in this section are based on the Egyptian code of practice for design and construction of masonry wall (ECP 204-2005). This section represents design equation and limitation for unreinforced and reinforced masonry elements with respect to allowable compressive stresses, allowable eccentricity of vertical loads, allowable tensile stresses and allowable shear stresses.
3.4.1 Unreinforced masonry:-
3.4.1.1 Allowable compressive stresses
Allowable compressive stresses in unreinforced masonry walls depend on slenderness ratio, eccentric loads, characteristic compressive strength (f’m), effective area of masonry wall cross section and load combinations. Allowable compressive stresses must not exceed neither (Fa) for concentric case of loading nor (Fb) for loading due to bending moment. In case of masonry walls subjected to both compressive stresses due to concentric loads and bending moment or eccentric loading, therefore there are two conditions that must be taken into consideration. (ECP 204, 2005)
1-
2- P
-is the calculated compressive stress on the wall due to concentric load (N/)
– is the allowable compressive stress on the wall due to concentric load (N/)
– is the calculated compressive stress on the wall due to bending moment (N/)
– is the allowable compressive stress on the wall due to bending moment (N/)
-P is the designed compression load (Newton)
– is the load which induces buckling due to Euler buckling equation (N)
The following equations state the calculation of (Fa), (Fb) and (Pe) that the calculated compressive stresses must not exceed.
3-
4-
5-
-f’m is the characteristic compressive strength of the wall (N/)
-h is the effective height of the wall (mm)
-r is the radius of gyration in the plane perpendicular to the applied load () (mm) where (A) is the cross sectional area in contact with mortar
-Em is the modulus of elasticity of masonry wall (N/) where (Em=700f’m) for clay masonry and (Em=900f’m) for concrete masonry
-e is the eccentricity of vertical load (mm)
-I is the second moment of area or moment of inertia ()
As mentioned before eccentricity is the distance from the center of gravity of the wall to the vertical applied load, therefore the Egyptian code of practice limits the mentioned distance to an extent of not inducing tensile stress greater than the allowable stated by the code as to prevent failure to occur. (ECP 204, 2005)
Slenderness ratio is calculated through the following equation [ ] which explains the ratio of the effective height to the effective length multiplied by the slenderness coefficient where [ ] which explains the ratio of effective length or effective height whichever is bigger to the radius of gyration and this coefficient is used to account the fixation of the end supports, moreover to determine the effective height and the effective length the code states that they are the distances from the center lines of the vertical and horizontal supports. As stated by the Egyptian code of practice slenderness ratio must not exceed a value of ninety. (ECP 204, 2005)
3.4.1.2 Allowable tensile stresses
Masonry is weak in tension and strong in compression, therefore working with unreinforced masonry elements depend on compression load and limiting tension load that is induced by bending moment in the out of plane of the wall, therefore the Egyptian code of practice put limitations for allowable tensile stresses that must not be exceed found in table (3-1,ECP 204-2005). There are two situations that tensile stresses are not allowed and tensile capacity is considered to be zero in masonry buildings which are tensile stresses resulted from concentric loading and shear walls. (ECP 204, 2005)
3.4.1.3 Shear stresses
Shear forces induce shear stresses on masonry element that when combined with vertical compression stresses that may induce bending moment may lead to failure of the masonry wall as discussed before, therefore the Egyptian code of practice set limitations for the applied shear stress as not to exceed the allowable. The calculation of shear stress due to an applied shear force is done through the following equation. (ECP 204, 2005)
a) Walls subjected to shear force and bending moment perpendicular to plane of the wall
1-
– q is the calculated shear stress (N/)
– S is the first moment of area ()
– Q is the design shear force (Newton)
– b is the effective width of the cross section (mm)
– It is the moment of inertia ()
-Calculated shear stress is then limited by the Egyptian code of practice by not exceeding the smallest value of the following equations.
2- (N/)
3- (N/)
4- 0.5 (N/)
– = 0.2 (N/) for ungrouted running bond wall
= 0.32 (N/) for grouted running bond wall
= 0.08 (N/) for stack bond wall
– N is the vertical load applied on the shear surface (Newton)
– An is the net area subjected to the vertical load ()
b) Walls subjected to shear force and bending moment horizontally
5- < smaller of
6- (N/)
7- 0.5 (N/)
3.4.2 Reinforced masonry:-
This section concentrates about the design of reinforced masonry structures where tensile capacity is ignored for masonry; on the other hand steel reinforcement is responsible for bearing tensile stresses that are subjected to masonry element due to loads in the in plane and out of plane.
3.4.2.1 Allowable tensile stresses in reinforcement steel bars
1- For deformed steel bars (Fs=0.5 Fy ≤ 165) (N/)
2- For steel wires (Fs=0.5 Fy ≤ 200) (N/)
3- For smooth steel bars and stirrups (Fs=0.4 Fy ≤ 140) (N/)
– Fs is the allowable tensile stress (N/)
– Fy is the steel yield strength (N/)
3.4.2.2 Allowable compressive stresses in reinforcement steel bars
-Compression resistance is ignored in the reinforcement bars if it is not fixed from the sides to prevent it from buckling, bit if it is fixed from the sides therefore the following equation is used to calculate the allowable compressive stresses. (ECP 204, 2005)
4- Fsc= 0.4 Fy≤ 165 (N/)
– Fsc is the allowable compressive stress (N/)
3.4.2.3 Allowable compressive loads in masonry
To calculate the allowable compressive load for masonry with ignoring steel compression resistance refer to allowable compressive loads and stress equations of unreinforced masonry that was discussed before, and for masonry columns or reinforced masonry bearing wall where steel reinforcement is fixed the allowable compressive load can be calculated through the following two equations. (ECP 204, 2005)
5- Pa= ( ) (1-() for ≤
6- Pa= for
– Pa is the allowable concentric compressive load (Newton)
– Ast is area of longitudinal steel bars (
3.4.2.4 Calculate tensile stresses on reinforced masonry due to bending moment
-Calculate the tensile stresses on reinforced masonry rectangular section due to bending moment for masonry and steel reinforcement from the following equations. (ECP 204, 2005)
7-
8-
9- k=
10- J = 1-
– is the calculated compressive stress on the maximum fibbers of the cross section due to bending moment (N/)
-is calculated tensile stress on the reinforcement steel due to bending moment (N/)
-b is the wall width (mm)
-d is the effective depth from the face subjected to compression to the center of steel reinforcement subjected to tensile stresses (mm)
-M is the bending moment subjected on the section
-n is the ration of the steel modulus of elasticity to masonry modulus of elasticity (n=)
– is the ratio between steel area to the net area of the masonry cross section
-As is the area of steel subjected to tension.
3.4.2.5 Calculated shear stresses
11- q =
-q is the calculated shear stress (N/)
-b is the effective width of the wall (mm)
-Q is the shear force subjected on the wall (Newton)
3.4.2.6 Allowable shear stresses
-It is not necessary to use shear reinforcement in the element if the calculated shear stress (q) does not exceed the allowable shear stress (). (ECP 204, 2005)
12- ≤ 0.3 (N/)
13-
14-
15-Ast=
– Equations thirteen and fourteen are used for walls subjected to bending moment and shear force in plane (shear walls and masonry beams)
– is Allowable shear stress (N/)
– M is the maximum bending moment on cross section (N-mm)
– Ast is the area of steel stirrups)
– S is the distance between the stirrups (mm)
4) Reinforced masonry construction as opposed to reinforced concrete construction:-
Masonry construction as mentioned before started hundreds of years ago before engineers and scientists invented reinforced concrete. Reinforced concrete structures are durable and have high compressive and tensile strength due to the strong bond between concrete and reinforcement, therefore reinforced concrete has high strength in bearing high wind and earthquake loads. Reinforced concrete materials are cement, aggregate, sand, water and reinforcement and they are available in most if not all the markets around the world, and on the other hand masonry structures are durable and permanent and have high compressive strength and low tensile strength if it is unreinforced. Moreover masonry materials are masonry units, steel reinforcement if needed and mortar which is composed of cement, hydrated lime, sand and water. To compare masonry to reinforced concrete with respect to time, materials and labour; going through construction steps and procedures of both types of structures is a must.
The current research paper concentrates mainly on residential buildings in low and medium rise compounds, therefore wind and earthquake pressures are limited, therefore unreinforced and reinforced masonry are this research paper main objectives and to be more specific load bearing masonry wall. Load bearing masonry walls are types of masonry elements used to carry vertical compression loads. Load bearing masonry construction steps are excavation for foundation alignment and placement made of reinforced concrete followed by isolating works using bitumen, second step is the load bearing wall composed of masonry units either clay, concrete or load bearing facade units which does not need any plastering or finishing bonded together with mortar and units are aligned mainly in running bond and erected in several heights, third step is the reinforced concrete slab constructed in three steps which are wooden shattering, steel alignment and concrete pouring, final step is the finishing which consists of electrical work, sanitary work, flooring and outside and inside plastering if needed.
Reinforced concrete construction steps are soil excavations for reinforced concrete foundation alignment and pouring followed by isolating works using bitumen, second step is wooden forms or shattering for steel alignment and concrete pouring of columns, third step is wooden forms for steel alignment and concrete pouring of beams, fourth step is reinforced concrete slab which is constructed on reinforced concrete beams, final step is the finishing which consists of electrical work, sanitary work, flooring and outside and inside plastering.
4.1 Time of completion
Masonry and concrete construction are used all over the world, depending on the type of structure and region of construction. In Egypt concrete is taking over most if not all the structures built, therefore masonry construction is not well known due to several cases that will be discussed later. The information that is going to be provided in this section is based on self investigation from “Alrwad Company for Construction”.
As discussed previously; construction procedures are not the same for concrete and masonry, moreover masonry showed a faster performance in construction as opposed to concrete. Masonry with respect to time is fast due to the unneeded form work, non existence of beams and column in load bearing walls, steel reinforcement can decrease the time consumed if designed and placed with alternative way, to be more specific steel has two types of basic alignments, which are vertical and horizontal alignments of steel bars, therefore to decrease the time consumed in steel alignment, a method of joint reinforcement can be used which is a steel mesh of horizontal bars of minimum of two and inclined or vertical bars are laid on the horizontal bars welded together and put as on unit on the bed joints of the units and then covered with mortar, this type of reinforcement is very effective in decreasing time consumption as not the same case if vertical and horizontal steel bars are used.
http://constructionmanuals.tpub.com/14043/img/14043_229_1.jpg
Figure 4.1.1: Joint reinforcement method (Joint reinforcement, 2013)
Excluding the slab and foundation construction which is common between masonry and reinforced concrete construction procedures, a masonry building crew is divided into three main specializations; unit alignment worker, steel alignment worker and mortar mixing worker. The most important worker is the unit placing worker where the time of completion is depending on the amount of blocks that this worker can place per day, to be more specific a worker can place one to one and a half cubic meters per day for example if the unit dimensions are forty centimetres length, twenty centimetres width and twenty centimetres height, therefore a worker can place ninety four units a day (volume of one block =0.4×0.2×0.2 =0.016, number of blocks = = 94 blocks/day). This shows that time in masonry construction depends on the worker effort, steel alignment techniques and size of the work.
On the other hand a concrete crew is divided into five main specializations; form worker, steel alignment worker, concrete mixing worker on site if ready mix concrete is not used, concrete pouring worker, scraping and mechanical vibrator worker. Form work worker can achieve one and half cubic meters daily, steel alignment worker can align three hundred kilograms of steel daily and concrete worker can pour any volume of ready mix concrete in few hours, therefore using ready mix concrete pouring is a factor of decreasing time consumption. Columns and beams do not exist in load bearing masonry structures, therefore these two elements consume time due to building of wooden forms, steel alignment and concrete pouring. The main type of work that consumes time in reinforced concrete construction is the form work which does not exist in masonry load bearing construction, form work may be done using either wood or steel forms and must be well fixed, more over all other construction works completion time depends on the form work completion.
Comparison between reinforced concrete and reinforced masonry construction with respect to time
Â
Â
Â
Reinforced concrete structures
Reinforced load bearing masonry structures
Â
Elements
work steps
Work included
Quantity / Day
Work included
Quantity / Day
1
Foundations
a)Form work
Yes
1.5
M^3/day
Yes
1.5
M^3/day
Â
Â
b)Steel alignment
Yes
300
KG/day
Yes
300
KG/day
Â
Â
c)Concrete pouring
Yes
Unlimited
—–
Yes
Unlimited
—–
2
Columns
a)Form work
Yes
1.5
M^3/day
No
—–
—–
Â
Â
b)Steel alignment
Yes
300
KG/day
No
—–
—–
Â
Â
c)Concrete pouring
Yes
Unlimited
—–
No
—–
—–
3
Beams
a)Form work
Yes
1.5
M^3/day
No
—–
—–
Â
Â
b)Steel alignment
Yes
300
KG/day
No
—–
—–
Â
Â
c)Concrete pouring
Yes
Unlimited
—–
No
—–
—–
4
Slabs
a)Form work
Yes
1.5
M^3/day
No
—–
—–
Â
Â
b)Steel alignment
Yes
300
KG/day
No
—–
—–
Â
Â
c)Concrete pouring
Yes
Unlimited
—–
No
—–
—–
5
Load bearing walls
a)Unit alignment
No
—–
—–
Yes
1.5
M^3/day
Â
Â
b)Mortar placement
No
—–
—–
Yes
Unlimited
—–
Â
Â
c)Steel alignment
No
—–
—–
Yes
300
KG/day
6
Vibrator
—–
Yes
—–
—–
No
—–
—–
7
Finishing
—–
Yes
—–
—–
Yes
—–
—–
Table 4.1.1: Comparison between reinforced concrete and masonry with respect to time
4.2 Cost of materials
Materials of reinforced concrete structures are different from those of reinforced masonry structures, in this section material types and cost will be discussed in details based on the Egyptian materials market. Materials used in reinforced concrete structures are steel and concrete made of cement, aggregate, sand and water, on the other hand materials used in masonry are steel, grout and masonry units weather it is clay or concrete. All information that is going to be mentioned in this section is based on personal investigation from “Alrwad Company for Construction”.
Reinforced concrete components are cement with price of six hundred Egyptian pounds per ton, aggregate with price of one hundred and thirty Egyptian pounds per cubic meter, sand with price of twenty Egyptian pounds per cubic meter, and water which depend on the location of the construction site. Let us consider a concrete mix of two hundred litters of water with a price of thirty Egyptian pounds, three hundred fifty kilo grams of cement with a price of two hundred and ten Egyptian pounds, eight tenths cubic meter of aggregate with a price of one hundred and four Egyptian pounds, and four tenths cubic meters of sand with a price of eight Egyptian pounds, therefore the price of one meter cube of reinforced concrete is three hundred and fifty two Egyptian pounds. Steel has a price of four thousands one hundred and twenty five Egyptian pounds per ton; it is usually assumed one hundred kilo grams of steel reinforcement in one cubic meter with a price of four hundred and thirteen Egyptian pounds, therefore the total price of one cubic meter of reinforced concrete including steel bars is seven hundred and sixty five Egyptian pounds. Wooden forms or shattering have different types and priced, but the most commonly used types are plywood and white wood with a price of three thousand Egyptian pounds per cubic meter. Transportation and wastages are excluded from the prices mentioned above.
Masonry units have two different types as mentioned before, but in this research paper we will be concentrating on concrete units. Let us assume a dimension of forty by twenty by twenty centimetres for a concrete hollow unit with a price of three thousand five hundred and fifty Egyptian pounds per one thousands block, therefore there are sixty two blocks in one cubic meter with a cost of two hundred and twenty one Egyptian pounds. Mortar components are cement, sand and water, therefore one cubic meter of cement is approximately equal to one thousand four hundred and forty kilo grams with a price of eight hundred and seventy Egyptian pounds, sand of one cubic meter with a price of twenty Egyptian pounds per cubic meter; let us assume one meter cube of a mortar mix of a ratio one to two and half, therefore cement content is thirty three tenths cubic meter with a price of two hundred and ninety Egyptian pounds, sand content is eight thousand three hundred and twenty five tenths cubic meter with a price of seventeen Egyptian pounds, and water with a price of thirty Egyptian pounds, therefore one cubic meter of mortar price is three hundred and thirty seven Egyptian pounds.
Usually masonry elements such as load bearing walls are composed of eight tenths masonry units and two tenths mortar, therefore one meter cube of masonry is equal to two hundred and forty five Egyptian pounds. Grout is rarely used in Egypt due to its’ high price, therefore contractors and engineers substitute grout with concrete with a price of three hundred and fifty two Egyptian pounds per cubic meter and it depends on the dimensions of the hollow hole. Adding steel to masonry of one hundred kilo grams for each cubic meter, therefore one cubic meter of reinforced masonry has a price of six hundred and fifty seven Egyptian pounds. To get to a conclusion masonry materials are cheaper than concrete, and that shows the benefits of masonry construction and how it is costly effective. All units prices based on “Cementa Egyptian Company” for the year of two thousand and twelve.
Price list of different types of materials used in reinforced concrete and reinforced masonry construction
Reinforced concrete
Reinforced masonry
Â
Materials
components
price (LE)
units
Â
Materials
components
price (LE)
units
1
Concrete
a)Cement
600
ton
1
Masonry unit
Concrete hollow units (40x20x20) cm
3550
1000 blocks
Â
Â
b)Aggregate
130
M^3
Â
Â
Â
Â
c)Sand
20
M^3
Â
Â
Â
Â
d)Water
Varies
Litters
Â
Â
Â
Â
Â
Â
Â
Â
Â
Â
Â
Â
2
Steel
—–
4125
ton
2
Steel
—–
4125
ton
Â
Â
Â
Â
Â
Â
Â
Â
Â
Â
3
Wooden Form
a)Plywood
3000
M^3
3
Mortar
a)Cement
870
M^3
Â
Â
b)White wood
3000
M^3
Â
Â
b)Sand
20
M^3
Â
Â
Â
Â
Â
Â
Â
c)Water
Varies
Litters
All prices are based on personal investigation in Alrawad construction company for year 2012
Table 4.2.1: Cost of materials used in reinforced masonry and concrete construction
4.3 Cost and experiences of labour
Masonry as a construction technique is rarely found in Egypt due to the lack of experiences of the labour in this field, therefore reinforced concrete is taking the lead in most if not all the construction projects in Egypt. The lack of experiences of the labour in Egypt is due to the untrained craftsmen whom take their construction experiences through practicing and knowledge they take from the past craftsmen who mainly gained their experience through also practicing, but they never learned any new construction techniques through professional schools for preparing craftsmen. (International Labour Organization, 2001)
Labours are divided into two main categories which are skilled labour and unskilled labour. Skilled labour is a worker who has techniques that suits the project needs such as steel alignment worker, masonry alignment worker, and form worker. Each of these workers has a crew to work with or to be more specific this crew work for him such as mortar mixing worker, steel formation worker, and form formation worker; these workers or crew are responsible for preparing the materials for the alignment and they do not have any construction techniques and just follow the crew leader, and they are called unskilled labour.
In reinforced masonry construction three types of labours are responsible for the process; whom are as mentioned before skilled labour who is responsible masonry alignment weather in stack bond or running bond with a salary of one hundred and sixty Egyptian pounds for one thousand block, the second type of labour is also skilled labour who is responsible for steel alignment with a salary of one hundred and sixty Egyptian pounds for one thousand blocks steel alignment, the third type of labour is unskilled labour who is responsible for mortar mixing and steel cutting with a salary of eighty Egyptian pounds for each one thousand blocks. All cost information mentioned is based on self investigation from “Alrwad Company for Construction”.
In reinforced concrete construction there are four types of labour whom are responsible for the construction process of structure; the first type of labour is skilled labour who is responsible for steel alignment with a salary of one hundred and sixty Egyptian pounds for each three hundred kilo grams of steel alignment, the second type of labour is unskilled labour who is responsible for concrete pouring with a salary of sixty to two hundred Egyptian pounds for each cubic meter, the third type is skilled labour who is responsible for the form work with a salary of fifty Egyptian pounds for each square meter, the fourth type of labour is unskilled labour who is unskilled labour who is responsible for mortar mixing, steel cutting and form cutting and formation with a salary of eighty Egyptian pounds per day.
Salaries of labour working in both reinforced masonry and reinforced concrete construction
Â
Skilled labour
Â
Unskilled labour
Â
Type
Cost
Currency
Type of work
Â
Type
Cost
Currency
Type of work
1
Steel alignment
160
LE
For 1000 blocks
1
Steel cutting worker
80
LE
For each day
Â
Â
Â
Â
Â
Â
Â
Â
Â
Â
2
Masonry alignment
160
LE
For 1000 blocks
2
Mortar mixing worker
80
LE
For each day
Â
Â
Â
Â
Â
Â
Â
Â
Â
Â
3
Form worker
50
LE
For each M^2
3
Form formation worker
80
LE
For each day
Â
Â
Â
Â
Â
Â
Â
Â
Â
Â
4
Sanitary worker
50
LE
For each M^2
4
Concrete mixing worker
80
LE
For each day
Â
Â
Â
Â
Â
Â
Â
Â
Â
Â
5
Electricity worker
50
LE
For each M^2
5
Vibrator worker
30
LE
For each day
6
Steel alignment
160
LE
For each 300 kg
Â
Â
Â
Â
Â
7
Concrete pouring
60 to 200
LE
M^3 (Depends on element type)
Â
Â
Â
Â
Â
Based on Alrwad Company For Construction
Table 4.3.1: Salaries of labour working in both reinforced masonry and concrete construction
References page
Crozier, D. (1999). Masonry structures. Victoria: Author.
Drysdale, G., Hamid, A. &Â Baker, R. (1999). Masonry structures: behaviour and design. (2nd ed.). Colorado: Masonry society.
Housing Department. (2005). Egyptian code of practice for masonry design and construction. Cairo: Author.
International Labour Organization. (2001). The construction industry in the twenty first century. Geneva: Author.
Joint reinforcement. (2013). Retrieved from http://www.faculty.delhi.edu/hultendc/A220-Week2-Lecture-Web.html
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