Short Notes: Design of Beams | Short Notes for Civil Engineering - Civil Engineering (CE) PDF Download

 Page 1


 
 
 
Design  of  Beams 
 
Contents 
Introduction 
Beam types 
Lateral stability of beams 
Factors affecting lateral stability 
Behaviour of simple and built - up beams in bending  (Without vertical stiffeners) 
 
Design strength of laterally supported beams 
Design strength of laterally unsupported beams 
Shear strength of beams 
Maximum deflection 
Design of Purlins 
 
   
 
 
 
 
Page 2


 
 
 
Design  of  Beams 
 
Contents 
Introduction 
Beam types 
Lateral stability of beams 
Factors affecting lateral stability 
Behaviour of simple and built - up beams in bending  (Without vertical stiffeners) 
 
Design strength of laterally supported beams 
Design strength of laterally unsupported beams 
Shear strength of beams 
Maximum deflection 
Design of Purlins 
 
   
 
 
 
 
Introduction 
Beams are structural elements subjected to transverse loads in the plane of bending 
causing BMs and SFs. Symmetrical sections about z-z axis are economical and 
geometrical  properties of such sections are available in SP (6) The compression flange of 
the beams can be laterally supported (restrained) or laterally unsupported (unrestrained) 
depending upon whether restraints are provided are not. The beams are designed for 
maximum BM and checked for maximum SF, local effects such as vertical buckling and 
crippling of webs and deflection. Beams can be fabricated to form different types of c/s 
for the specific requirements of spans and loadings. Section 8 shall be followed in the 
design of such bending members.  
 
Types of beam cross sections  
 
 Beams can be of different cross sections depending on the span and loadings and 
are shown below -   
 
 
Page 3


 
 
 
Design  of  Beams 
 
Contents 
Introduction 
Beam types 
Lateral stability of beams 
Factors affecting lateral stability 
Behaviour of simple and built - up beams in bending  (Without vertical stiffeners) 
 
Design strength of laterally supported beams 
Design strength of laterally unsupported beams 
Shear strength of beams 
Maximum deflection 
Design of Purlins 
 
   
 
 
 
 
Introduction 
Beams are structural elements subjected to transverse loads in the plane of bending 
causing BMs and SFs. Symmetrical sections about z-z axis are economical and 
geometrical  properties of such sections are available in SP (6) The compression flange of 
the beams can be laterally supported (restrained) or laterally unsupported (unrestrained) 
depending upon whether restraints are provided are not. The beams are designed for 
maximum BM and checked for maximum SF, local effects such as vertical buckling and 
crippling of webs and deflection. Beams can be fabricated to form different types of c/s 
for the specific requirements of spans and loadings. Section 8 shall be followed in the 
design of such bending members.  
 
Types of beam cross sections  
 
 Beams can be of different cross sections depending on the span and loadings and 
are shown below -   
 
 
 
C / S of Plate Girders 
Page 4


 
 
 
Design  of  Beams 
 
Contents 
Introduction 
Beam types 
Lateral stability of beams 
Factors affecting lateral stability 
Behaviour of simple and built - up beams in bending  (Without vertical stiffeners) 
 
Design strength of laterally supported beams 
Design strength of laterally unsupported beams 
Shear strength of beams 
Maximum deflection 
Design of Purlins 
 
   
 
 
 
 
Introduction 
Beams are structural elements subjected to transverse loads in the plane of bending 
causing BMs and SFs. Symmetrical sections about z-z axis are economical and 
geometrical  properties of such sections are available in SP (6) The compression flange of 
the beams can be laterally supported (restrained) or laterally unsupported (unrestrained) 
depending upon whether restraints are provided are not. The beams are designed for 
maximum BM and checked for maximum SF, local effects such as vertical buckling and 
crippling of webs and deflection. Beams can be fabricated to form different types of c/s 
for the specific requirements of spans and loadings. Section 8 shall be followed in the 
design of such bending members.  
 
Types of beam cross sections  
 
 Beams can be of different cross sections depending on the span and loadings and 
are shown below -   
 
 
 
C / S of Plate Girders 
 
 
 
 Simple I sections are used for normal spans and loadings with all the geometrical 
properties available in IS 800 : 2007.  
 
 All the other sections indicated in the figure are built up sections. These sections 
are used when the normal I sections become inadequate due to large spans and loadings. 
These sections are also used due to other functional requirements. 
 
 I section with cover plates are used when the loads are heavy and the spans are 
large. If the depth of the beam is restricted due to functional reasons, smaller depth I 
sections with cover plates can be used Additional cover plates increases the lateral load 
resistance with increase in I
YY
. The properties of ISMB and ISWB sections with cover 
plates are available in SP (6). (Z
PZ
 has to be obtained from calculations) 
 
Page 5


 
 
 
Design  of  Beams 
 
Contents 
Introduction 
Beam types 
Lateral stability of beams 
Factors affecting lateral stability 
Behaviour of simple and built - up beams in bending  (Without vertical stiffeners) 
 
Design strength of laterally supported beams 
Design strength of laterally unsupported beams 
Shear strength of beams 
Maximum deflection 
Design of Purlins 
 
   
 
 
 
 
Introduction 
Beams are structural elements subjected to transverse loads in the plane of bending 
causing BMs and SFs. Symmetrical sections about z-z axis are economical and 
geometrical  properties of such sections are available in SP (6) The compression flange of 
the beams can be laterally supported (restrained) or laterally unsupported (unrestrained) 
depending upon whether restraints are provided are not. The beams are designed for 
maximum BM and checked for maximum SF, local effects such as vertical buckling and 
crippling of webs and deflection. Beams can be fabricated to form different types of c/s 
for the specific requirements of spans and loadings. Section 8 shall be followed in the 
design of such bending members.  
 
Types of beam cross sections  
 
 Beams can be of different cross sections depending on the span and loadings and 
are shown below -   
 
 
 
C / S of Plate Girders 
 
 
 
 Simple I sections are used for normal spans and loadings with all the geometrical 
properties available in IS 800 : 2007.  
 
 All the other sections indicated in the figure are built up sections. These sections 
are used when the normal I sections become inadequate due to large spans and loadings. 
These sections are also used due to other functional requirements. 
 
 I section with cover plates are used when the loads are heavy and the spans are 
large. If the depth of the beam is restricted due to functional reasons, smaller depth I 
sections with cover plates can be used Additional cover plates increases the lateral load 
resistance with increase in I
YY
. The properties of ISMB and ISWB sections with cover 
plates are available in SP (6). (Z
PZ
 has to be obtained from calculations) 
 
 
 Two I sections with cover plates can be used when very heavy loads and spans act 
on the beam. The properties of these sections are not available in SP (6) and have to be 
calculated. 
 
 Two I sections placed one above the other are used when the loads are light with 
large spans, where deflection is the main criteria. The properties of these sections are not 
available in SP (6) and have to be calculated  
 
 Gantry girders are used in industrial buildings to lift loads and typical sections 
used are indicated in the figure. The properties of these sections  are available in SP (6), 
(Z
PZ
 has to be obtained from calculations) 
 
 Plate girders are used where the spans exceed 20m and the loads are heavy. The 
properties of these sections  are available in SP (6), (Z
PZ
 has to be obtained from 
calculations)  
  
 Box sections have large torsional rigidity and can be used as single cell, twin cell 
or multi - cell sections. The openings are advantageously used for service lines. 
 
 Castellated beams are special sections fabricated from I sections and are used for 
light loads and large spans. The openings are advantageously used for service lines. 
 
 In all built up beams, the fabrication cost is higher due to the provision of 
connections between the elements. 
 
Section Classification  
 
 There are four classes of section namely Plastic, Compact, Semi - Compact and 
Slender sections as given in IS 800 : 2007. [cl. 3.7.2 pp - 17] For design of beams, only 
Plastic and Compact sections are used.  
 
Lateral Stability of Beams  
 
 A beam transversely loaded in its own plane can attain its full capacity (Plastic 
moment) only if local and lateral instabilities are prevented.  
 
 Local buckling of beams can be due to web crippling and web buckling. They are 
avoided by proper dimensioning of the bearing plate and through secondary design 
checks. Flanges shall always satisfy the outstand to thickness ratio as per IS 800 : 2007 so 
that local failures of flanges are avoided. Plastic and Compact sections are used.    
 
 Lateral buckling of beams is the out of plane bending and  is due to compressive 
force in the flange and is controlled by providing sufficient lateral restraint to the 
compressive flange.