Structural Steel in Flexure

8/12/2019 Structural Steel in Flexure

1/26

Structural Steel Desi

in Flexure


2/26

Topics to be covered

General Introduction

Design of laterally restrained steel beams

Plate Girders

Design of Gantry Girders


3/26


Beams are structural members frequently used to carry loads thatare transverse to their longitudinal axis.

They transfer loads primarily by bending and shear.


4/26

Beams in bridges

Beams in


5/26


Some of the commonly used sections in industry.


6/26

Stress and Strain in the Cross-secti

Strain

Stress

N.A.

small =y plastic

Working stress design Limit state d

small =y plastic

N.A.

small F Fy Fy Fy


7/26

Design considerations

For a beam (loaded predominantly by flexure) two essentialrequirements must be met to develop its full moment capacity:

1. The beam as a whole should not buckle laterally.

2. The elements of the beam (i.e. flange and web) should notbuckle locally.


8/26

Design of laterally restrained beam

The beam is designed for the following:

1. Maximum bending moment

2. Maximum shear

3. Maximum deflection

4. Local bucklingweb buckling and web crippling

Before proceeding to the design checks, the beam cross section isclassified as per the IS code and the section is preferably selected tobe plastic or compact to utilize the full strength of the cross section.


9/26

Design for bending

M


10/26

Design for shear

Note: V < 0.6Vd

Shear influence on bneglected else beamdesigned for combin


11/26

Deflection check

Deflection check describes the limit state

of serviceability for the beam.

Actual deflection must be less then

the allowable deflection.

Actual deflection is calculated for

working loads and not for factored loads.

Table 6 of IS 8002007 gives the deflection limits

for the design.


12/26

Web buckling and Web crippling

The application of heavy concentrated loads produces a region ohigh compressive stresses in the web either at the support or underthe load. This may cause either the web to buckle as shown in theFig (a) or the web to cripple as shown in Fig (b).


13/26

Web Buckling

Dispersion of concentrated loads at 45oto evaluate web bucklin


14/26

As per section 8.7.3 of IS 8002007,

The web acts as a compression member locally.

Iyy= B.(tw)3/12

= (b1+n1) .(tw)3/12

Area = B. tw

Rmin= tw/23

effective length = 0.7D

buckling classC

Using the above values allowable stress is found the design is safe if actualshear stress is below this value.


15/26

Web crippling


16/26

As per section 8.7.4 of IS 8002007,

Fwis greater than the factored shear force, the section is safe against web


17/26

The critical local buckling stress of the constituent plate element of beam, for a given material and boundary conditions is inverselyproportional to its breadth to thickness ratio. Hence by suitablyreducing the slenderness of the plate elements, its resistance tolocal buckling could be enhanced. Once the local buckling isprevented, the beam can develop its full flexural moment capacity

or the limit state in flexure.


18/26

PLATE GIRDERS

A plate girders consist of a vertical plate called web, and twoflanges each consisting of horizontal plate.

As the span increases, greater depth is required to resist the bendinmoment.

For spans less than 15 m the rolled beams or plated beams are useBut above that (15 m) and till spans to 30 -35 m, the plate girders ar

economic. The weight of the plate girder is greater than that of truss of the

same span but the fabrication costs and maintenance are small.


19/26

PLATE GIRDERS

Welded plate girder Plate girderwith coverplates

Bolted plate girder


20/26

Plate girders are used in both buildings and bridges. In buildings, whlarge column-free spaces are designed to be used as an assembly

for example, the plate girder is often the economical solution. In sucases, the designer must choose between a plate girder and a trus

Plate girders, in general, have the following advantages over trusse

1. Connections are less critical for plate girders than for trusses,

particularly statically determinate trusses. In a statically

determinate truss, one poor connection may cause the collapse of

the truss.

2. Fabrication cost of plate girders is less than that of trusses.

3. Plate girders can be erected more rapidly and more cheaply thantrusses.


21/26

4. Depth of a plate girder is less than the height of a comparable truss

Consequently, plate girders need less vertical clearance than

trusses. This makes them very attractive for multilevel highway

bridges.

5. Plate girders generally vibrate less than trusses under moving

loads.

6. Painting of plate girders is easier than painting of trusses. This

means less maintenance cost for plate girders.


22/26

Stiffeners

It is assumed all the bending load is taken by the flange and all the

shear is taken by the web.The web of the plate girder is a slender section as the depth tothickness ratio is very large. This makes the web more susceptible tocompressive buckling.

Hence we provide stiffeners. Stiffeners are classified into the followingtypes:

1. Longitudinal stiffeners

2. Transverse stiffeners

1. Intermediate stiffeners

2. End bearing stiffeners


23/26

INTERMEDIATE STIFFENERS

Intermediate stiffeners are provided to stiffen the web plate againstbuckling and to resist compressive forces transmitted from the web

during tension-field action.

BEARING STIFFENERS

Bearing stiffeners are provided in pairs at the ends of plate girders and,if required, at points of application of concentrated loads. Thesebearing stiffeners should extend roughly to the edges of the flangeplates, and their length should be closeto the depth of the web platein order to have close bearing with the flange plates. They aredesigned as columns with a cross-sectional area which includes acentrally located strip of the web.


24/26

Design of plate girder - WSD

Economic depth of plate girder d= 1.35 (M/b.tw)0.5

Area of flange: M - Aw

b * d 6

Stiffener requirement:

d/t < 85, no stiffener required

85 < d/t < 200, vertical stiffeners provided

200 < d/t < 250, vertical stiffeners provided along with longitudinalstiffener at 2/5 d from top flange

250 < d/t < 400, vertical stiffeners provided along with longitudinalstiffener at 2/5 d from top flange and at neutral axis


25/26

Design of Gantry Girder

Over to the board.!!


26/26

Questions welcome

if not..

Documents

Structural Steel in Flexure