model paper design of steel structure...

MODEL PAPER

DESIGN OF STEEL STRUCTURE (302) Q. 1) (a) Write down different types of structural steel sections with diagram ?

(b) What do you understand by limit state design method ? Explain limit state of strength ?

Ans. (A) .

The steel sections manufactured in rolling mills and used as structural members are known as rolled

structural steel sections. The steel sections are named according to their cross sectional shapes. The

shapes of sections selected depend on the types of members which are fabricated and to some

extent on the process of erection.

ROLLED STEEL BEAM SECTIONS

The rolled steel beams are classified into following four series as per BIS : (IS : 808-1989)

a. Indian Standard Joist/junior Beams ISJB

b. Indian Standard Light Beams ISLB

c. Indian Standard Medium Weight Beams ISMB

d. Indian Standard Wide Flange Beams ISWB

ROLLED STEEL CHANNEL SECTIONS

The rolled steel Channel sections are classified into four categories as per ISI, namely,

a. Indian Standard Joist/Junior Channels ISJC

b. Indian Standard Light Channels ISLC

c. Indian Standard Medium Weight Channels ISMC

d. Indian Standard Medium Weight Parallel Flange Channels ISMCP

ROLLED STEEL ANGLE SECTIONS

The rolled steel angle sections are classified in to the following three series.

1. Indian Standard Equal Angles ISA

2. Indian Standard Unequal Angles ISA

ROLLED STEEL TEE SECTIONS

The rolled steel tee sections are classified into the following five series as per ISI:

1. Indian Standard Normal Tee Bars ISNT

2. Indian Standard Wide flange Tee Bars ISHT

3. Indian Standard Long Legged Tee Bars ISST

4. Indian Standard Light Tee Bars ISLT

ROLLED STEEL BARS

The rolled steel bars are classified in to the following two series:

1. Indian Standard Round Bars ISRO

2. Indian Standard Square Bars ISSQ

ROLLED STEEL FLATS

ROLLED STEEL PLATES

(B) . Limit state method : Limit state refers to the acceptable limit for safety and serviceability of structure before the failure of structure.

This method is considered as as ideal method for design as it includes, the structural safety of structure against collapse as well as serviceability. The design of members is initially done by considering the limit state of collapse and the members is checked for serviceability requirement after the design .

LIMIT STATE OF STRENGTH :

o Strength (yielding, buckling)

o Stability against overturning and sway of structure

o Fracture of structural element due to fatigue

o Plastic collapse of structure

o Brittle fracture of structural element

Q.2) Write down different types of bolted and welded connections with diagram ?

Ans.) Bolted connections :- There are two types of bolted joint like 1. Lap joint 2. Butt joint

Lap joint: When the two members to be connected together by overlapping then the joint is called lap joint.A single bolted lap joint and a double bolted lap joint are shown in figure respectively.

Butt joint: When the two members are to be connected by placing the members end to end and additional plate/plates provided on either one or both sides to attach them properly,then this joint is called butt joint.And the additional plate is called cover plate. If the cover plate is provided on one side then it is called a single cover butt joint,But if the cover plates are provided on both sides of the main plates then it is called double cover butt joint.A single cover butt joint and double cover double bolted butt joint shown in figure below.

Welded connections:- Based on type of weld, welds can be classified in to fillet weld, groove weld

(or butt weld), plug weld, slot weld, spot weld etc.

1.1. Groove welds (butt welds) - Groove welds (butt welds) and fillet welds are provided when

the members to be joined are lined up. Groove welds are costlier since it requires edge preparation.

Groove welds can be employed safely in heavily stressed members. Square butt welds are provided

up to a plate thickness of 8mm only.

1.2. Fillet welds - Fillet welds are provided when two members to be jointed are in different planes.

Since this situation occurs more frequently, fillet welds are more common than butt welds. Fillet welds

are easier to make as it requires less surface preparation. Nevertheless, they are not as strong as the

groove welds and cause concentration of stress. Fillet welds are preferred in lightly stressed members

where stiffness rather than strength governs the design.

1.3. Slot and plug welds - Slot and plug welds are used to supplement fillet welds where the required

length of fillet weld cannot be achieved.

Q.3) Calculate the strength of a 20 mm diameter bolt of grade 4.6the main plates to be 12 mm thick

for the following cases :-

(A) Lap joint

(B) Double cover butt joint each cover plate being 8 mm thick.

Ans.)

Q. No. 1 Explain characteristic strength of Concrete.

Ans. Characteristic Strength of Concrete is defined as the strength of concrete cube size

(150mmX150mmX150mm) after 28 days curing below which not more than 5 percent of the test results

are expected to fall.

Q. No. 2 Difference between Limit State Method and Working State Method.

Ans. LIMIT STATE METHOD: It’s a method of designing structures based on concept of safety and its probability of safety. Here in addition to ultimate load approach addition serviceability is also given importance. Limit state is a condition just before collapse. A structure designed by limit state should give proper strength and serviceability throughout its life

ASSUMPTIONS:

1. Plane section normal to the axis of the member remains plane after bending. 2. The tensile strength of concrete is ignored. 3. The maximum strain in concrete at the outer most compression fiber is 0.0035 4. The compressive stress strain curve may be assumed to be rectangular, trapezoidal , parabola

, or any other shapes results in the prediction of strength in substantial agreements with results of tests

PRINCIPLE LIMIT STATES:

Limit state of collapse——— FLEXURE, COMPRESSION, SHEAR, TORSION

Limit state of serviceability——— DEFLECTION, CRACKING, VIBRATION.

WORKING STRESS METHOD: It’s a traditional method used for reinforced concrete design where concrete is assumed as elastic, steel and concrete act together elastically where the relationship between loads and stresses is linear.

ASSUMPTIONS:

1. A Section which is plane before bending remain plane after bending (based on theory of bending, Bernoulli’s equation).

2. Bond between steel and concrete is perfect with in elastic limit of steel. 3. All tensile stresses are taken by reinforcement and none by concrete, except as otherwise

pacifically permitted. 4. The stress‐strain relationship of steel and concrete under working loads is a straight line i.e.

modulus of elasticity is constant. 5. The modular ration ‘m’ has the value (280/sigmacbc), where (sigmacbc) is the permissible

compressive stress due to bending in concrete in N/mm2.

Q. No. 3 Explain balanced section, under reinforced section and over reinforced section.

Ans. (I) Balanced Section

The steel provided in the beam is such that both concrete and steel reach the limiting values of strain simultaneously.

(ii) Under Reinforced Section

The steel provided in the beam is such that steel reach the limiting values of strain prior to concrete. These results in yielding of the steel and the steel could yield till it attains the ultimate strain at which point it breaks. It is important to notice that it is the yielding stain that is considered as failure strain in steel and not the ultimate fracture strain. The steel still has to undergo strain hardening, softening and then fracture.

(iii) Over Reinforced Section

The steel provided in the beam is such that concrete reach the limiting values of strain prior to steel. This results in breaking of concrete and since now there is no concrete present to take the compression the beam fails suddenly.

The additional margin that we get in under reinforced section helps in prevention of a sudden failure and provides the necessary warning to the inhabitants of the building.

Q. No. 4. Calculate Moment of Resistance (Mu) of a section 400mm X 600mm , When area of steel is 4bars of 16@mm dia. Use M25 grade of Concrete & Fe415 grade of steel. Effective Cover is 50mm.

Ans. 1. Given Data :‐

Width of Beam b=400mm Depth of Beam D=600mm Effective Cover =50mm Effective Depth of Beam=550mm Area of Reinforcement =4‐16@=804.24mm2 Grade of Concrete Fck=25N/mm2 Grade of Steel Fy = 415 N/mm2

2. Limiting Depth of Neutral Axis-

Xulim. =0.48xd (for Fe415)

=.048x550

=264mm

3.Actual Depth of Neutral Axis(Xu) Xu=0.87xFyxAst/0.36xFckxb =.087x415x804.24/.036x25x400 =80.65mm

Note:- Xu<Xulim. So section is under reinforced section

4.Moment of Resistance (Mu)=

Mu= 0.36 x Fck x b x Xu (d-.42 x Xu) or 0.87 x Fy x Ast (d-0.42 x Xu)

Mu= .036 x 25 x 400 x 80.65 (550-0.42 x 80.65)

= 149.85 Kn-m

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model paper design of steel structure...

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