DESIGN OF STEEL & TIMBER

A Course on
Bachelor of Civil Engineering
(Purbanchal University)

Chapter-1:
INTRODUCTION
Lecturer:
Er. Rabin Bhattarai
Department of Civil Engineering
 Chapters Marks Distribution
Chap- 1 Introduction ( 6-8)
Chap-2 Types of Joint & design (8-10)
Chap-3 Design of Tension member (8-10)
Chap-4 Axially loaded Compression member (16)
Chap-5 Eccentrically loaded Compression member
Chap-6 Design of beams (16)
Chap-7 Design of composite & built up Beams
Chap-8 Design of plate Girders (8)
Chap-9 Design of Roof truss (8-10)
Chap-10 Timber structures
Chap 11 Design of Timber Structures (8-10)
 References Books
❖ L.S. Negi, Design of steel structure
❖ M Raghupathinath , Design of steel structure
❖ S Arya J.L Azmani, Design of steel structure
❖ Ramamurtham , Design of steel structure
❖ Ramchandra, Design of steel structure
❖ Dayratnam, Design of steel structure
Code
IS 800 :1984 ( steel)
IS 875:1987 (part 3 ) ( wind load)
IS 883:1994 ( Timber )
IS 808:1989 ( part I,II, IV,V ) for rolled steel beam channels
 1.0 Introduction
• Steel structures are the structure made of steel components
connected together to sustain & share applied load with
adequate safety and serviceability.(alloy of iron & carbon)
• Structure member may be subjected to axial forces
(compression & tension), bending, shear, torsion or a
combination of these.
• Its strength is approximately 10 times than that of concrete.
• Larger strength /weight ratio.
• More economical than concrete structure for tall buildings
• Eco friendly material can be constructed very fast.
• Can be easily repaired and retrofitted to carry higher loads.
 1.1 TYPES
• (i) On the basis of strength
(a) Structural mild steel - it is most commonly used steel for
general construction purposes of building, bridges,
transmission tower, and industrial structures.
(b) High tensile structural steels - Enhanced mechanical properties
and increased resistance to atmospheric corrosion. Saving in
weight due to greater strength & atmospheric corrosion
resistance.
• (ii) On the basis of weight – Size of member may be same, but thickness
may vary. The weights of member depend on wall thickness of
steel member. Light weight (L), Medium weight (M), Heavy
weight (H).
• (iii) On the basis of sections (Rolled steel sections)
(a) ɪ – sections
> Used as beam & column.
> Best for B.M & Shear force. (B.M. 80% by flange &
other web) (S.F. 95% by web & other by flange)
• Standard ɪ sections
(i) ɪSJB ………………Indian standard joist beam
(ii) ɪSLB……………… Indian standard light beam
(iii) ɪSMB…………… Indian standard medium beam
(iv) ɪSWB ………………Indian standard wide beam
(V) ISSC ………………… Indian standard column section
(v) ɪSHB …………………Indian standard heavy beam
(b) Channel (С- sections)
> Used for beam & columns
> built up channels are used more than single C- section.
> Column, bridge truss member, single – C in suspension
bridge.
 Standard C- sections
i) ɪSJC …………………Indian standard joist channels
ii) ɪSLC………………… Indian standard light channels
iii) ɪSMC……………… Indian standard medium channels
> MC - Indian standard medium wt. channels with sloping flange
> MCP - Indian standard medium wt. channels with parallel
flange
C Angles- (L- section)
> Great application in fabrications.
> used as connecting elements to connect structural
elements.
> Compression & Tension member.
Standard L- sections
i) ɪSEA…………………Indian standard equal angles.
ii) ɪSUA…………………Indian standard unequal angles.
ISA AXBXT
d. T- sections
> Used for built up sections
Standard T- sections
i) ɪSNT ………………Indian standard normal tee bars
ii) ɪSST…………………Indian standard long-legged bars
iii) ɪSWT……………… Indian standard wide - legged bars
iv) ɪSLT…………………Indian standard light - legged bars
v) ɪSJT…………………Indian standard junior -legged bars
e. Tubular section (hollow section)
> Circular
> Square
-> Rectangular
1.2 Properties of structural steel (stress
strain characteristics, allowable stress &
other mechanical properties)
• The figure is obtained from the plotting of stress in y-axis &
strain in x-axis for the mild steel.
• The plot lone OA is a straight line & point A is called the “limit of
proportionality” .within the OA the specimen follows the hook
laws i.e. stress applied is proportional to the strain produced.
• If the specimen is stressed beyond the limit of proportionality
up to the condition shown at B, the material still remains elastic.
But in the range B to C the relation between stress & strain is
nonlinear. The stress at B is called “elastic limit” & the region
from o to B is called elastic zone.
• If the specimen is stressed beyond the elastic limit plastic
deformation takes place.at the condition shown at C, there is
considerable extension corresponding to decrease in load. The
stress at C is called the “upper yield point “.as the material is
subjected to further strain the corresponding stress decrease &
at the condition shown at D the materials offers greater
resistance to greater to greater strain. The stress at D is called
the “lower yield point”.
• The lower yield stress is only marginally less than the upper yield
stress and not noticeable in a controlled test. The stress
corresponding to the yield point (UYP) is called yield stress. But the
lower yield point is usually taken as the yield point for design
purpose.
• As the strain is increased, the stress increases and at the condition
shown at E, a ”neck” is developed. The stress corresponding to E is
called the “ultimate tensile strength”.
• AS the strain is further increased the stress decreases and the
specimen breaks at the condition shown at F. the stress at F is called
the “stress at fracture”. The failure point F may occur at any point
after point E.
Some mechanical properties:
I) Ultimate strength/stress:-
 > also called tensile strength
> ultimate strength is maximum stress at which a tensile
specimen fails by fracture and is given by
𝑈𝑙𝑡𝑖𝑚𝑎𝑡𝑒 𝑡𝑒𝑠𝑖𝑙𝑒 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ
Ultimate strength=
𝑜𝑟𝑖𝑔𝑖𝑛𝑎𝑙 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑐/𝑠
ii) Ductility:
> is the property of material or a structure by which it can deform
before failure or fracture. quantitively it is represented by the %
of elongation at failure of a uniaxial test specimen.
iii) Yield stress:
> the stress corresponding to the yield point i.e. upper yield
point is called yield stress.it is starting point of plastic zone.
iv) Toughness:
> ability of material to Absorb energy in large amount due to
strain undergoing plastic deformation is called toughness.
 > Steel member can still withstand large forces even it has
large deformation.
v) Modulus of elasticity:
> the stress up to which strain is proportional to stress is
called proportional limit .the ratio of stress to strain is
constant up to proportional limit & the ratio is called modulus
of elasticity.
𝑆𝑡𝑟𝑒𝑠𝑠 𝑤𝑖𝑡ℎ𝑖𝑛 𝑝𝑟𝑜𝑝𝑜𝑟𝑡𝑖𝑜𝑛𝑎𝑙 𝑙𝑖𝑚𝑖𝑡
> Modulus of elasticity =
𝑆𝑡𝑟𝑎𝑖𝑛
𝜎𝑥
E=
𝜖
Where, σx = uniaxial stress below the proportional limit
ε = strain corresponding to stress σ
E= modulus of elasticity also called young’s modulus of
elasticity
E = 2.0 x 105 MPa
σy = 250 MPa
 yield strain, εy = 0.00125
strain hardening , εst = 0.015 (for Mild steel)
Maximum strain , εmax = 0.25
vi) Allowable stress:
> Also known as permissible stress.
> The stress developed in structure due to load do not
exceed the elastic limit. this limit is determined by
ensuring that stress remain within the limits through the
use of factor of safety.
𝑌𝑖𝑒𝑙𝑑 𝑠𝑡𝑟𝑒𝑠𝑠
> Allowable stress =
𝐹𝑎𝑐𝑡𝑜𝑟 𝑜𝑓 𝑆𝑎𝑓𝑒𝑡𝑦

1.3 Use of steel as structural members

i) Beam, e.g. - Normal beam, gantry beam, plate girder used in
bridge
ii) Columns
iii) Tower & building e.g. - suspension & cable stay bridge tower,
transmission line tower, mobile/telecom tower, high rise
building, skyscrapers
iv) Roof truss member e.g. - Industrial sheds, general roof truss
v) Bridge truss member, e.g. - foot bridge, highway bridge
vi) Arch e.g. - bird nest stadium bridge
vii) Large cantilever member
viii) Portal frames
ix) Thin plate structure e.g.- dome, shell roofs, water tanks, storage
bin or silos
1.4 Standards, codes and specification for design of steel
structures.
> The design of structural steel is controlled & governed by code
& specification
> Codes provides general guidelines of the minimum
requirements for the design of structural component or a
system.
> The codes which are actually law and regulations, specify
minimum : designs load, design stresses, construction types,
material quality and other factors.
> These codes are written specifically for certain areas and
disciplines of an engineering practices.
> the important things to remember about specifications &
building codes is that they are written, not for the purpose of
restricting engineers but for the purposes of protecting the
public.no matter which code or specification is or is not being
used, the ultimate responsibility for the design of safe structure
lies with the structural design engineer.

Code practices:
> guidelines-method-formulas
> limiting values
> Description/ explanations
> Permissible values
 > Various other structure values
> does & don'ts
> Chart & values ( relation between terms)
> Figures, example, drawing for typical one.
1.5 Advantages & Disadvantages
ADVANTAGES
> high strength/ weight ratio
> Better quality control
> Faster to erect
> Reduced site time-fast track construction
> can be built up with high quality relationship & narrow tolerance
> less material handling at site
> less percentage of floor area occupied by structural members
> has better ductility & hence superior lateral load behavior,
better earthquake resistant
> ease for repair
> Repetition use . > expanding existing structure.
Disadvantage
>Skilled labour is required
> Higher cost of construction.
> Maintenance cost is high
> Electricity may be required
> Poor fire proofing as at 5380 65% & at 8710 15%of strength
remains
> Susceptibility to buckling
1.6 Method of Design
Working stress method (WSM)
Ultimate load design (ULD)
Limit state Method(LSM)
Working stress method (WSM)
Also known as elastic design method
1900 , conventional & old approach method
based on allowable stress & elastic behavior
FOS=1.8
Very high factor of safety
Used for analysis of service load.
Used in design of tank chimney, leak proof structure
Ultimate load method (ULM)
➢ 1956 AD, Known as plastic method, load factor
method.
➢ The service loads on the structure are multiplied by a
factor of safety to obtain ultimate loads. Ultimate
stress are real value.
➢ Deflection , crack effects under working load were
not considered.
➢ Method is used to check the ultimate loads on the
structure after designing with the help of working
stress method , to give an idea of real factor of safety
of structure.
 Limit state method
➢ 1960, Limit state method is the ultimate stage at which
structure remains unfit for use.( limit state of collapse &
limit state of serviceability)
➢ It deals with ultimate load.
➢ Partial factor of safety is used for load as well as material
strength.
➢ FOS 1.15 .
Thanks
Silo
This Photo by Unknown Author is licensed under CC BY-SA

Cable stayed bridge

 bird nest stadium bridge

This Photo by Unknown Author is licensed under CC BY-SA