Steel structure design NOTES

Steel Structure design

Introduction

Structural engineering is the branch of civil engineering which deals with the design, analysis, and research of structures to achieve design load and ensure safety.

Various properties of steel

Following are the properties of steel

• Physical properties
• Chemical properties
• Mechanical properties

Physical properties - This includes strength, durability, ductility, etc.

Chemical properties -  Chemical properties of steel include the composition of various components present in steel like iron, carbon, etc. The physical properties of steel depend on the chemical properties of steel.

Mechanical properties - Mechanical property of steel defines its behavior when the load is applied to it.

Some of the mechanical properties of steel are listed below -

• Elasticity
• Plasticity
• Brittleness
• Ductility
• Malleability
• Hardness
• Toughness
• Elastic Toughness
• Fatigue
• Creep

Structural Steel

Structural steel is the type of steel which is used for manufacturing construction materials in a variety of shape and size.

Depending on the chemical composition of various components of steel, steel are of following types- 

• Mild steel
• Medium carbon steel
• High carbon steel
• Low alloy steel 
• High alloy steel

 Stress-Strain relationship of Mild Steel

When the tensile loads are subjected to mild steel bars it changes its shape and size. Due to tensile loads on steel, stress is produced on it which is load per unit area. Also, the resultant elongation per unit length is called strain.

Stress-strain curve for mild steel -

Stress-strain curve for Mild steel

Definition of terms shown of the above figure.
Limit of proportionality - The steel obeys the hook's law within the limit of proportionality i.e. within the limit of proportionality stresses in steel are directly proportional to strain produced in steel. Thus the limit of proportionality is the point beyond which steel doesn't obey Hook's law.
Elastic limit - Elastic limit is the maximum stress within which a material can regain its original length, shape, and size after the removal of applied loads. In practice, the limit of proportionality is the same as the elastic limit.
Upper yield point - This is the point from where the sudden decrease in the stress-strain curve occurs.
Lower yield point - This is the point where the decrease in the stress-strain curve stops.
Ultimate load - This is the maximum load that can be applied to the structure before failure or breaking.
Breaking point - This is the point beyond ultimate load and at this point, steel fails to bear a particular load.

Advantages and disadvantages of steel 

Advantages -

• Steel structures have high strength per unit weight and can bear high loads in less cross-sectional area.
• Steel requires less cross-sectional area and hence steel structures have less self-weight which decreases the dead load of the building.
• Steel structures are of small size and can be conveniently handled and transported.
• Steel is a ductile material therefore it doesn't fail easily but gives clear evidence of failure by large deflection.
• Among all the construction materials steel has the highest scrap value.
• Additions and alternations can be made easily in steel structures.
• Properties of steel do not change with time which makes it a perfect structural material.
• Steel structures have a long life if properly maintained.
• The strength of steel structures can be increased very easily by connecting additional members or plates.
• Because of the high density of steel, steel is water and airtight.
• Failure in steel is easily visible which makes inspection jobs very easy.
• Steel members can be added, replaced, assembled, and disassembled very easily.

Disadvantages -

• The initial cost of steel is high.
• Steel structures are not fireproof hence it needs fireproof treatment which makes it costly.
• Steel structures corrode easily when placed in exposed conditions. Therefore, they require frequent treatment.

Why steel is preferred over RCC? | # JUST FOR KNOWLEDGE

This question is very common because we know that steel is expensive and heavy as compared to RCC.

Well, the answer is simple and can be understood by the simple points given below -

(i) -  Strength of steel .>> Strength of RCC - Because the strength of steel is more than the strength of RCC, therefore, the cross-section required for gaining a particular strength is less in steel than RCC.

(ii) - Less time-consuming - Because steel requires less cross-sectional area than RCC. So obviously, it will take less time for doing work in a small area rather than in a large area.

(iii) - Overall economical - As we know that steel needs less cross-sectional area, therefore, the requirement of steel is less as compared to concrete i.e. we have to purchase less steel for gaining a particular strength which makes it economical.

Connections

A steel structure is formed by various steel members joined together.
The joints/connections between the structural members is known as structural steel connection.
The connections between the structural members can be made in the following ways -
(i) Riveted connection
(ii) Bolted connection
(iii) Welded connection
(iv) Pin connection

Riveted connection

The joint/connection in which rivets are used to connect the structural members is called a riveted connection.
Rivets
Rivet is a piece of the round ductile steel bar(mild or high tensile) used to connect two or more than two steel members.
Parts of Rivet

• Head → Acts as a lock.
• Shank/Body → Connects the two or more structural members.
• Tail → Extra length given to rivet for the formation of the second head.

Note - The size of the rivet is expressed by the diameter of the shank of the rivet.

Types of Rivets

Rivets can be classified in the following two types

1. According to the shape of the head
2. According to the driving force required to insert the rivet

1. According to the shape of the head

According to the shape of the head, rivets are of following types -

• Snap head rivets
• Pan head rivets
• Pan head with a tapered neck
• Flat-headed rivets
• Round countersunk headed rivets
• Flat countersunk head rivets

Types of rivets on the basis of the shape of their head

2. According to the driving force required to insert the rivet

According to the driving force required to insert the rivet, rivets are of following types -

• Hot driven rivets
• Cold driven rivets
• Shop rivets
• Field rivets
• Hand driven rivets
• Power-driven rivets

Hot driven rivets

The rivets which are heated to red hot before driving are called hot driven rivets.

Cold driven rivets

The rivets which are driven in normal temperature with the help of a large amount of pressure are called cold driven rivets.

Shop rivets

The rivets which are driven at factories are known as shop rivets.

Field rivets

The rivets which are driven at the site of construction work are known as field rivets.

Hand driven rivets

The rivets which are driven by hand are known as hand-driven rivets.

Power-driven rivets

The rivets which are driven with the help of mechanical equipment are known as power-driven rivets.

Testing of rivets

Following tests are performed for testing the rivets -

• Flattening test
• Bend test

Flattening test

In this test, the rivet is heated to red hot and then the head is flattened with the help of repeated blows of the hammer or any other mechanical equipment such that the edges of material do not show any cracks.

The head of the rivet should be hammered until the diameter of the flattened head is 2.5 times the diameter of the shank.

If no cracks are formed on the head of rivets the rivets pass the flattening test and vice versa.

Bend test

In this test, the shank of the rivet is bent so that the two parts of the shank touch each other.

After performing this test, visual inspection is done for finding any cracks.

Some technical terms used in riveted joints

Nominal diameter of rivet - The diameter of the shank (Body) of the rivet before driving is known as the nominal diameter of the rivet. This is symbolized as 'D'.

Gross diameter of rivet -  The diameter of the shank of rivet after driving is known as the gross diameter of the rivet. This is symbolized as 'd'. The strength of rivet is calculated on the basis of its gross diameter. 

Remember - If D <  25, then d = D + 1.5 & If D > 25, then d = D + 2  

Gauge line - Line parallel to the direction of the applied force is called the gauge line.

Gauge distance - Perpendicular distance between two consecutive gauge lines is called gauge distance, this is represented by 'g'.

Pitch of rivets - Center to center distance between two adjacent rivets measured parallel to the direction of the applied force is known as pitch of rivets.

As per IS: 800-1984 clause 8.10.1, values of the pitch of rivets are shown below -

-Minimum pitch: Pitch should not be less than 2.5 of the nominal diameter of the rivet i.e. p⊀ 2.5 D

-Maximum pitch : 

 *For parts in compression: The maximum pitch of rivet in the direction of compression should not be more than 12t or 200 mm | Out of these two values whichever is less is the maximum pitch of rivet in compression.

 *For parts in tension: The maximum pitch of rivet in the direction of tension should not be more than 16t or 200 mm | Out of these two values whichever is less is the maximum pitch of rivet in tension.

Edge distance (m) - The distance from the center of the last rivet to the edge of member/cover plate is called edge distance.

According to IS: 800-1984 Clause 8.10.1, Table 8.2 | Standard values for edge distance are -

Nominal diameter of Rivet (D) (mm)	Gross diameter of rivet (d)  (mm)	Minimum edge distance for rolled edges (mm)
12	13.5	17
14	15.5	22
16	17.5	25
18	19.5	29
20	21.5	29
22	23.5	32
24	25.5	38
27	29	44
30	32	51

Staggered pitch (s) - This is the distance between two consecutive rivets (arranged in a zig-zag manner) along the direction of force applied on a member.

Types of riveted joints

Riveted joints are of two types

Lap joint 

Butt joint

Lap joint

In this joint two members are joined by overlapping both members in a certain length.

Lap joint is further classified into following types -

Single riveted lap joint

Double riveted lap joint

Triple riveted lap joint

Butt joint

In this joint, the ends of two plates are joined against each other with the help of cover plates placed at the top or bottom.

Types of butt joint

Single riveted butt joint

Single riveted single cover butt joint

Single riveted double cover butt joint

Double riveted butt joint

Double riveted single cover butt joint

Double riveted double cover butt joint  

Strength of riveted joints

The strength of the joint is "the maximum force that can be applied to the joint without failing the joint".

Strength of the joint is taken least among the following -

Tearing strength of joint (Pt)

Bearing strength of joint(Pb)

Shearing strength of joint(Ps)

The strength of the joint can be calculated by two methods -

Considering per pitch length of the joint

Considering the full width of the joint

Considering per pitch length of the joint 

Tearing strength of the joint (Pt)

Tearing strength of the joint can be calculated by the expression given below -

Pt  =  σat x (p - d) x t

Where,

σat = Allowable tensile stress in the plate

p = Pitch of the rivet

d = Gross diameter of rivet/Diameter of the rivet hole

t =  Thickness of thinner plate (in case of lap joint) and thickness of the main plate (in case of butt joint)

Bearing strength of joint (Pb)

Bearing strength of the joint can be calculated by the expression given below -

Pb = n x σpf x d x t

Where,

n = no of rivets per pitch length ( Here n = 1, 2, 3 for single riveted, double riveted, triple-riveted lap or butt joint respectively)

= Allowable bearing stress in rivets

= Gross diameter of rivet/Diameter of the rivet hole

= Thickness of thinner plate (in case of lap joint) and least of the thickness of the main plate or combined thickness of two cover plates (in case of butt joint)

Shearing strength of joint(Ps)

Shearing strength of the joint is the resistance offered by the joint against shearing stresses applied to the joint.

Shearing strength of the joint can be calculated by the expressions given below -

Shearing strength of joint -

In single shear

In this case, only two plates are joined with the help of rivets.

= n x τ 

   

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