ARCHITECTURAL STRUCTURES REVIEWER.pdf

Concrete –a mixture of sand, gravel, crushed rock, or other aggregates held together in a rock-like mass with a paste of cement and water. Sometimes one or more admixtures are added to change certain characteristics of the concrete such as its workability, durability, and time of hardening. Reinfor...

Concrete –a mixture of sand, gravel, crushed rock, or other aggregates held together in a rock-like mass with a paste of cement and water. Sometimes one or more admixtures are added to change certain characteristics of the concrete such as its workability, durability, and time of hardening. Reinforced concrete - is a combination of concrete and steel wherein the steel reinforcement provides the tensile strength lacking in the concrete. ADVANTAGES OF USING REINFORCED CONCRETE AS A STRUCTURAL MATERIAL 1. It has considerable compressive strength per unit cost compared with most other materials. 2. Reinforced concrete has great resistance to the actions of fire and water and, in fact, is the best structural material available for situations where water is present. 3. Reinforced concrete structures are very rigid. 4. It is a low-maintenance material. 5. As compared with other materials, it has a very long service life. 6. It is usually the only economical material available for footings, floor slabs, basement walls, piers, and similar applications. 7. A special feature of concrete is its ability to be cast into an extraordinary variety of shapes from simple slabs, beams, and columns to great arches and shells. 8. In most areas, concrete takes advantage of inexpensive local materials (sand, gravel, and water). 9. A lower grade of skilled labor is required for erection as compared with other materials such as structural steel. DISADVANTAGES OF USING REINFORCED CONCRETE AS A STRUCTURAL MATERIAL 1. Concrete has a very low tensile strength, requiring the use of tensile reinforcing. 2. Forms (which are expensive) are required to hold the concrete in place until it hardens sufficiently. In addition, false work or shoring may be necessary to keep the forms in place for roofs, walls, floors, and similar structures until the concrete members gain sufficient strength to support themselves. 3. The low strength per unit of weight of concrete leads to heavy members. This becomes an increasingly important matter for long-span structures, where concrete’s large dead weight has a great effect on bending moments. 4. Similarly, the low strength per unit of volume of concrete means members will be relatively large, an important consideration for tall buildings and long-span structures. 5. The properties of concrete vary widely because of variations in its proportioning and mixing. Furthermore, the placing and curing of concrete is not as carefully controlled as is the production of other materials, such as structural steel and laminated wood. CEMENT AND WATER Used in binding aggregates (sand and gravel) Water/cement ratio greatly affects the strength of concrete CURING OF CONCRETE - performed by submerging the specimen underwater. This is done in order to prevent moisture loss. Rapid moisture loss leads to cracking and loss of strength of the concrete specimen Note : Ideally, the maximum strength of concrete is attained at the 28th day of curing. STRESS – STRAIN RELATIONSHIP OF CONCRETE Stress - Force applied at a certain surface area σ = F / A Strain - Change in length over the original length ε=ΔL/Lo Proportionality Limit - Stress is proportional to strain Hooke’s Law - The stress is directly proportional to strain up to the proportionality Limit 𝝈 =𝑬𝜺 where E is the Modulus of Elasticity Elastic Limit - The material returns to its original shape when the load is removed. Ultimate Compressive Strength - The highest stress on the stress-strain curve. Break Point / Rupture. - Failure occurs. The concrete cracks in tension. Design codes - provide detailed technical standards and are used to establish the requirements for the actual structural design. 𝑬𝒄 = 𝟒𝟕𝟎𝟎𝝀 𝒇𝒄 ; in MPa ; For Normal Weight Concrete (NWC) ; NWC = 2300 kg/m3 𝑬𝒄 = 𝒘𝒄𝟎. 𝟎𝟒𝟑 𝒇𝒄 in MPa ; for other weights, wc : mass density, kg/m3 Where : 𝜆–factor considered for the type of concrete 𝜆 = 1.00 if Normal Weight Concrete (NWC) 𝜆 = 0.75 if Light Weight Concrete (LWC) (below 2300 kg/m3) ULTIMATE COMPRESSIVE STRENGTH OF CONCRETE, fc’ the load-carrying capacity of the uncracked portions of the concrete reaches a maximum value Commercial Available fc’ of Concrete 17 MPa- Lowest value according to NSCP 2015 21 MPa- 3 ksi 28 MPa- 4 ksi 34 MPa- 5 ksi 1ksi = 1000 psi ` 1ksi = 6.895 MPa DESIGN ANALYSIS OF REINFORCED CONCRETE RC1 WORKING STRESS DESIGN (WSD) METHOD - The behavior of concrete is LINEAR ELASTIC. The consideration is up to the proportionality limit RC2 ULTIMATE STRESS DESIGN (USD) METHOD - The behavior of concrete is NON-LINEAR ELASTIC. The consideration is up to the ultimate strength. TYPES OF STEEL REINFORCEMENT LONGITUDINAL BARS (rebars) Plain Bar – smooth surface Deformed Bar – ridges and lugs; used to anchor the steel to concrete. PRE-STRESSED STEEL - High Tension Cable Wires STRUCTURAL STEEL SHAPES - Wide Flange, Angle Bar, C Section, Rectangular, Tubular, etc… Note : Reinforcing Steel Bars are strategically placed where concrete is weak in tension SHRINKAGE- Contracting of a hardened concrete mixture due to the loss of water/moisture. Shrinkage temperature bars are used. CREEP - Additional deformation because of the load applied for a very long time Structure- is the load-carrying part of all natural and man-made forms. It is the part which enables them to stand under their own weight and under the worst conditions of externally applied force. Designer- is one who conceives a structural part or a structural system which functions satisfactorily, is integrated successfully within the overall design and is appropriate for its purpose in terms of material and form. design process- Without a brief it is not possible to design, since there are no rules and no constraints. Therefore, no matter how sketchy, it is the brief which sets the basic framework for the designer. It provides the lead-in for the first analysis of the problem which then develops into an iterative process, with ideas being tested, modified, rejected, until an appropriate solution to the problem is reached. Optimum design - A designer should generally aim for the optimum solution in order to obtain the maxi- mum benefit with the minimum use of material within the constraints of strength, stiffness and stability. The result will be EFFICIENCY combined ideally with ELEGANCE AND ECONOMY. Influences on the designer The major influences on creative structural design are: Precedent – what's gone on Awareness - what's going on Practicality - how to do it Structural form- Structures take one of four basic forms which may exist singly or in combination. Solid - An homogeneous mass structure where the external surface is independent of the internal form - a three-dimensional solid body Surface - An homogeneous surface where the external and internal forms are similar - a two- dimensional panel Skeletal - A framework where the assembly of members gives a clear indication of the form usually using one-dimensional elements Membrane - A flexible sheet material sometimes reinforced with linear tension elements used either as single cables or as a cable net. A variation is the pneumatic where air under pressure is contained by a tension membrane skin Hybrid - A combination of two of the above forms of near equal dominance Strength- is the measure of the force required to break the material. A material can be strong or weak- see Mild steel - stiff and strong Sheet glass - stiff and weak Nylon rope - flexible and strong Rubber - flexible and weak Stiffness - The majority of structural materials behave in an elastic manner according to Hookers Law which states that elastic extension is proportional to load. When the load is removed, the material recovers its original length and shape. Different materials have different stiffness characteristics. They can be: stiff, flexible, stretchy, springy or floppy. This stiffness is defined for each material as the E-value -Young's modulus, named after its discoverer. E is the value of stress/strain and is a constant for a given material. Shape - is the third property which affects the performance of a material in a particular loading situation. In pure tension, shape does not matter, but in all other loading modes - compression, bending and shear - the cross- sectional shape affects performance. In general terms, for maximum performance, the material should be arranged in order to be as far away from the centre of the section as possible. Material behavior- Materials are either 'isotropic' or 'anisotropic' depending on their behaviour under load. Isotropic - Providing equal performance in all directions in both tension and compression Anisotropic - Providing differing performances in different directions and in compression and tension Example: Isotropic materials Metals - Including steel, aluminium, bronze, titanium etc. Anisotropic materials Timber - Different values for compression and tension. Different values for load parallel and perpendicular to the grain. Concrete and masonry - Good in compression, poor in tension. Steel reinforcement provides the tension element in reinforced concrete. Plastics and reinforced plastics - Usually stronger in tension than compression. A very wide range of performance according to type of plastic and reinforcement. NOTE: Aluminium can be extruded through most die shapes Masonry cut to squared stone or pressed + baked as bricks Concrete cost to shape in formwork moulds COMPOSITE MATERIAL PANELS  Aluminium skin + rigid plastics form core  Aluminium skins + phenolic resin coated paper core (Nomex)  Injection moulded GRP stiffenes as required with rigid isocyanate core  GRP skins on end grain balsa core  Vacuum formed superplastic aluminium with rigid foam core  ETFE Foil in a aluminium frame air- inflated Loads on structure - All structures develop internal forces which are the result of external applied loads and the weight of the structure itself. Permanent Dead load - The self-load of the object or part due to its mass Temporary Imposed load - The 'user' load which is removable and thus is a 'live' load Thermal load - The load induced by temperature change causing expansion or contraction of the object Dynamic load - A cyclical load caused by varying external conditions which cause the object to vibrate or oscillate Structures must always be designed for the worst anticipated combination of loading otherwise "unserviceability or failure can result. Overhead travelling crane creates surge Dynamic loading causes flexing of cables and bridges deck Wind induced oscillation on a tower.  To stand up and stay in place structures must be in equilibrium.  External loads act on a structure and induce internal forces, both loads and forces having magnitude and direction.  For equilibrium, reactions must act in an equal and opposite sense to the applied loads. There are three conditions which may have to be satisfied to achieve equilibrium depending on the form of loading.  V= 0 - The sum of vertical loads and reactions must equal zero  H = 0 - The sum of horizontal loads and reactions must equal zero  M = 0 - Clockwise moments must equal anti-clockwise moments Moment = load x distance of load from support or point of rotation P = Load (linear) R = Reaction (linear) M = Moment (bending or rotation) C = Sum Examples of equilibrium  Vertical - The load and reactions of an object sitting on the floor A horizontal structure carrying a vertical load producing end reactions  Horizontal - The tug of war where, for equilibrium, both teams must pull with equal force. The vehicle travelling horizontally which meets an obstruction  Rotational - The see-saw where the sums of the loads x their distance from the point of support must equal for balance  W is constant for obiect  As P increases, rotation increases  When action line of W falls outside base then obiect is unstable and falls over  For stability p, < wx  If y increases and x decreases, instability occurs when P, > Wx  P and W are drawn to scale to represent load magnitude and direction, R is resultant P = external load W = weight of obiect R = reaction > greater than < less than  Parallelogram of forces (vectors) - Forces acting at a point in a direction other than vertical or horizontal can be resolved into vertical and horizontal by a vector diagram drawn to scale or by trigonometry. Similarly, two forces can be resolved into a resultant by the same method. The opposite of the resultant forms the equilibrium force. Settlement and earthquake behavior - Both settlement and earthquakes can cause movements and distress in building structures. Settlement occurs due to compression of the soil under the foundations. Differential settlement occurs due to uneven bearing capacity of the soil or to uneven loading. Earthquakes give rise to horizontal ground movement and can also be the cause of settlement due to ground compaction. The elements  Strut - A slender element designed to carry load parallel to its long axis. The load produces compression.  Tie - A slender element designed to carry load parallel to its long axis. The load produces tension  Beam - Generally a horizontal element designed to carry vertical load using its bending resistance  Slab/plate - A wide horizontal element designed to carry vertical load in bending usually supported by beams  Panel - A deep vertical element designed to carry vertical or horizontal load Element behaviour – deformation The loaded behaviour of structural elements is dependent on internal and external factors. Infernal factors - type of material, cross- sectional shape, length, type of end fixity External factors - type of position and magnitude of load  Struts compress under load and can buckle if not stabilized laterally  Ties extend under load  Beams and slabs deflect due to bending  Panels deform due to in-plane load Element behaviour - stress When an element is loaded it becomes stressed.  Tensile - The particles of material are pulled apart and the element increases in length. A tie is in tension  Compressive - The particles of material are pushed together with a consequent decrease in length. A strut is in compression  Shear - The particles of material slide relative to one another  Torsion- A form of shear caused by twisting  Bending - A combination of tension, compression and shear. Beams are in bending 