Thursday 26 February 2015

FRAMED STRUCTURES....................!!!!!




-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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