-The purpose of any framed building is to transfer the loads of the structure plus any imposed loads through the members of the frame to a suitable foundations .
- Framed buildings are particularly suitable for medium and high rise structures and for industrialized low rise building like single store factory building.
Categories of framed structures
i. Plane frames
§ Fabricated in a flat plane and are usually called trusses or girders according to their elevation shape.
§ They are designed as a series of connected rigid triangles which given a light weight structural member.
§ Main uses are in roof Construction and long span beams of light loading.
ii. Space frames.
§ Similar in conceptions to a plane frame but Designed to span in two directions as opposed to the one direction spanning of the plane frame.
iii. Skeleton frames.
§ Basically these are a series of rectangular frames placed at right angles to one another so that the loads are transmitted from member to member until they are transferred through the foundations to the subsoil.
Choice of materials
The materials most suitable for the construction of mult- storey structure frame are,
§ Steel
§ Reinforced concrete
§ Also timber- in domestic type building up to three storeys high.
§
The principle factors influencing the choice between steel and concrete are ;
i. Cost
ii. The availability of materials and labour
iii. Speed of erection
iv. Possibility or otherwise of standardizing the sizes of the structural members
v. Size and nature of site
vi. Fire resistance required
THE STEEL FRAMED STRUCTURES
The steel frame structures are designed with the connections between the separate members treated as either;
§ Non – rigid or
§ Fully rigid joints.
The multi -storey steel frame is build up of hot rolled mild steel sections standardized in shape and dimensions in the following categories;
i. Beam
ii. Channel
iii. Angle
iv. Tree section
The beam section is the basic of the frame for both beams and stanchions. However this sections is guarded with special standard call BSS
“HOW STEEL CONNECTION DONE”
The connections between the members of a normal steel frame are made by means of welding and bolts with angle , cleats and plates
METHODS OF CONNECTIONS
1) STANCHION BASE CONNECTION
The foot of a Stanchion must be expanded by means of base plate which will act as an inverted cantilever beams.
Two types of base
§ The slab base or bloom base
§ Gusseted base
Slab base
-Consists of a base plate thick enough to resist the moments caused by the bearing pressure.
Gusseted base
§ consists of a base plate stiffened by gusset plates which acts as ribs
2) STANCHION CAPS CONNECTION
§ A cap must be provided to the stanchion of the beams rest on top of it.
3) BEAM TO STANCHION CONNECTIONS
§ Done where the beam rests on top of the stanchion.
4) BEAM TO BEAM CONNECTING
§ The direct compression connection in which one beam bears directly on the top flange of the other, is the most economical.
5) WELDING
-Two methods are normally employed in structural work
§ Oxy – acetylene welding
§ Metal arc welders
ADVANTAGES OF STEEL FRAME STRUCTURE
i. It is easy to construct e.g speed of erection is simple
ii. More economical companied to other types of frame structure for places where is easy to get
iii. Comprising different standardized shape and dimensions.
iv. Suitable of any span.
THE REINFORCED CONCRETE STRUCTURE
Can be formed into walls as well as into beams and columns to form a skeleton frame, and floor slabs.
Classification of Concrete
§ Insitu concrete
§ Precast concrete
§ Prestressed concrete
§ Rigid or portal frames
INSITU CAST CONCRETE STRUCTURES
Where all the constituent concrete materials have been brought to the site , mixed and placed in formwork erected in the position the concrete will be finally occupy in the complete structure.
-where construction is done through full continuity of construction of columns, beams and slabs; that kind of construction is called monolithic or fully continues contraction.
Advantages of monolithic or fully continues contraction.
i. reduced deflections in members
ii. Reduced bending moments distributed more uniformly
iii. In the case to beams there is a less rapid increase in dead weight with increase in span.
PRE-CAST CONCRETE STRUCTURES
Defined as a component cast in for work in a position other than that which it will finally occupy in the completed structure and which after removal from the forms and maturing requires to be placed and fixed in position.
THE MATERIALS OF REINFORCED CONCRETE
1. Reinforcement steel
§ Is used either as mild steel or higher tensile steel bars or wires.
2. Aggregates
§ Various materials are employed as aggregate, two types
§ Heavy aggregates
§ Lightweight aggregates
3. Sand
4. Cement
§ pozzolana cement
§ ordinary Portland cement
REINFORCED CONCRETE MEMBERS
1. Reinforced concrete columns
§ A Colum is vertical member carrying the beams and floor loadings to the foundation and is a compression member.
2. Reinforced concrete slab
Two basic of reinforced concrete slabs
i. Flat slab floors or roofs
ii. Beams and slab floors or roofs
3. Reinforced concrete wall
§ The reinforced concrete load bearing wall used of as the enclosing wall to a building is the alternative to its use as a divining element in the concrete box frame.
§ The wall areas over openings act as beams and those areas between openings as columns.
PRESTRESSED CONCRETE
Prestressing is the process of imparting to a structural member a compressive stress in those zones which under working loads, would normally be subject to tensile stresses.
In fact, is a process of recompressing by means of which the tensile stress produced by the applied load are counteracted by the compressive tresses set up before the application of the load.
METHODS OF PRESTRRRESSING
Pre – tensioning
In this system high- tensile steers wires are tensioned before the concrete is cast round them, and then when the concrete has attained sufficient strength, the wires are released.
Figure pg. 213
Post – tensioning
In this system the concrete is cast and permitted to harden before the steel is stressed.
Normall can be introduced after the concrete has set by casting in bars or duct tubes at the appropriate positions which are extracted before the steel is inserted.
Figure pg 213 C,
RIGID OR PORTAL FRAMES
§ Is continuity of structure due to the stiff or restrained, joint between the parts.
§ The use of rigid frame construction overcomes these disadvantages to a very large span.
§ It can be used up to span of 36m span.
(Figure pg 291 Structure and fabric part II)
Types of Rigid frames or portal frames
1. Fixed or hinge less portal
2. Two pin rigid frame
3. Three pin- rigid frame
FIXED OR HINGELESS PORTAL
This is a fixed- base frame with the feet rigidly secured to the foundation blocks and with all other joints rigid.
Attach figure
TWO PIN – RIGID FRAME
In this form hinged joints at the base are introduced to reduce the foundation of any tendency to rotate.
Attach figure
THREE PIN – RIGID FRAME
In this form a for other hinged joint is introduced at the crown or mid- point of the spanning member
TOPIC 2 : WALLS
TYPES OF WALLS:
Retaining walls.
§ The basic function of a retaining wall is to retain soil at a slope which is greater that it would naturally assume, usually at a vertical or vertical position
§ The natural slope taken up by any soil is called its angle of eposes and is measured in relationship to the horizontal.
§ Angle of pose for different soils range from 45% to near 0 for wet days soil, but most soil an average angle of 30 but is usually taken.
Design principles:
§ The design of any retaining wall is basically concerned with the lateral pressure of the retained soil and any subsoil water.
§ It must be designed to ensure that :
a)overturning does not occur
b) sliding does not occur
c) The soil on which the wall rests is not over loaded.
d) The materials used in construction are not overstressed.
The factors to be considered during calculating the pressure exerted at any point on the walls.
i. Nature and type of soil
ii. Height of water table
iii. Subsoil water movement
iv. Type of wall
v. Materials used in the construction of the wall.
Earth pressures;
The designer should also consider the effects of two forms of earth pressure
1. Active earth pressure
2. Passive earth pressure
Retaining wall terminologies
Types of Retaining walls
1. Mass Retaining walls
§ sometimes called gravity walls
§ it relying upon their own mass together with the friction on the underside to the base to overcome the tendency to slide or overturn
§ It is generally economically up to height of 1.800m.
§ Mass walls can be constructed of semi- engineering of quality bricks bedded with mortar in the ration of 1:3 of cement mortar or of mass concrete.
Refer drawings
2. Cantilever walls
§ usually of reinforced concrete and work on the principles of leverage
§ Normally basic forms can be considered
§ Base with large heel so that the mass of earth above can be added to the mars of the wall for design purpose.
§ Cantilever wall with a large toe can be used if the first one is not suitable.
Refer drawings
3. Counter fort retaining walls.
§ These walls can be contracted of reinforced or priestesses concrete
§ Normally is suitable if the height is over 4.500m.
Refer figure
4. Recast concrete crib retaining walls
§ crib walls are designed on the principle of a mass retaining walls
§ They consist of a frame work or crib of precast concrete or timber units within which the soil is retained.
4 Masonry wall
Under this we have the following sub types;
Solid masonry walls;
§ Include all walls made by brick or blocks
§ It is normally good for small scale buildings for all types of buildings for all types of building ie. Single storey or multistory up to five storey in eight where planning requirements.
§ Are not limited by its use
§ Masonry walls were calculated on a scientific basis great height required great thickness of walls.
§ But in order to reduce the thickness of the walls, even if there is great height e.g. five storey building the following factors should be considered.
i. The choice suitable plan form
ii. Maintaining a suitable proportion f height to width of bilge to keep wind stresses to a minimum.
iii. Running concrete floor stars through to the outface of external walls to reduce the centricity of floor load.
iv. Strength of masonry walls
§ Strength of masonry walls
§ Normally depends primarily upon the strength of the units and of the mortar.
§ Also quality of the workman ship
§ Determination of walls thickness
§ Traditional types of builders, the ethicalness of the walls determined through:
§ Looking into the height and the length of the walls as speculated in building regulations
§ Also the height and the width of the buildings
§ The span and the area of the floors relating to the wall.
§ The imposed loads on various part of the structure and the width of any openings in the walls.
-In the case of large and taller buildings the wall thickness are determined means of calculations.
Under building regulation through BS5628 defined certain terms in relation to the process of calculations.
ØSlenderness ration;
-This is defined as the ratio of effective height to effective thickness
ØEffective eight;
-This is based on the distance between adequate lateral supports provided by floors and roof and depends upon the degree of support they are assumed to provided.
ØEffective thickness;
-This is the actual ethicalness of a solid wall excluding plaster, rendering or
any other applied finish or covering.
Reinforced masonry walls
§ Reinforced masonry, usually in the form of reinforced bind work or block work.
§ This type of wall is normally used to withstand tensile and shear stresses in addition to the compressive tresses which it is capable of bearing alone.
§ It is suitable to place where there is effect of earthquake.
5. Diaphragm walls
§ there are external walls of load bearing bricks or block construction developed to deal with the structural problems in the rent in the design of walls to tell single- storey, buildings such as sports halls, and factories.
§ Due to high height of walls from 6m to 10m, the slenderness becomes a significant factor, to reduce it the application of Diaphragm walls is necessary.
6. Cross wall construction.
§ This is a particular for of load bearing wall construction in which all loads are carried by internal walls running at right angle to the length of the building.
§ The majority of building requires dividing up by the internal partitions or separation walls.
§
Advantages of this type of construction;
i. Simplicity of construction- the Wace consist of simple un broken runs of brick or block work or instu concrete .
ii. Projecting beams and columns are delimited.
iii. The external walls, being free from load may be designed with greater freedom in the choice of materials and finish.
iv. Also construction costs are low.
Design generally of cross wall construction;
- Cross walls could be spaced at regular or regularly repeating intervals along the building in order that a limited number of floor spans can be standardized in terms of ethicalness, reinforcement and formworks.
7. Panel walls
- These are wall normally formed between columns and beams of a frame building or to areas of calculated masonry walling.
- It is assumed to sustain no loads from floors and roof.
-compressive than transpires strength to withstand pressure from wind
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