Design of Steel Structure Due to Bending-Beam Steel Structure

8/2/2019 Design of Steel Structure Due to Bending-Beam Steel Structure

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STEEL & TIMBERSTEEL & TIMBERSTEEL & TIMBERSTEEL & TIMBER

DESIGNDESIGNDESIGNDESIGN

DAA 3222DAA 3222DAA 3222DAA 3222

----BEAMSBEAMSBEAMSBEAMS----


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TOPIC OUTLINE

Lateral Restraint Shear & Moment Capacity

Local Buckling & Bearing

Deflection

Design Procedures

General

Not Full Restrained

Unrestrained


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LEARNING OUTCOME

Student would be able:

To identify the degree of lateral restraint To identify the shear & moment capacity of beam

structure

structure To identify the local buckling, bearing and deflection

of beam To calculate buckling resistance, bearing capacity &

buckling moment resistance To identify the degree of lateral restraint To design of beam subjected to Lateral-Torsional

Buckling


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DEFINITION

In building construction, a beam is a

horizontal member spanning an openingand carrying a load that may be a brick orstone wall above the opening

A beam is a structural member which issubject to transverse loads

MUST be designed to withstand shear &moment


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BEAMS

Purlin carries the roof load to the trusses

Raftera slopping beam carrying the roof load to thepurlins

Lintelcarries the brick or other masonry across theopening made by a door or a window

Joist one of the closel s aced beams su ortin the

flooring of a buildingStringer one of the closely spaced beams running

parallel to the roadway and supporting the flooring of abridge( also called as secondary beam)

Floor Beam the larger beam which are perpendicularto the roadway of the bridge or perpendicular to the joistof the building


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1. Jack Rafter

2. Wall Header

3. Common Rafter

4. Hip Rafter

5. Fascia

1. Floor Joist

2. Solid Blocking

3. Beam


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Figure 2: Fascia


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Figure 3: Roof System


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Figure 4: Bridge System


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TYPES of BEAM

Simple beamsupported w/out restrained

Overhanging beamfreely supported BUT extended beyond one or bothof its supports

Continuous beam

freely supported BUT extended over three or moresupports

Fixed-end beamhaving its ends fixed against rotation

Restrained beampartially fixed at one or both ends

Cantilever beam


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Figure 7: Beam Types With References to The Method of Support


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Types of beams load1) Concentrated loads from secondary beams

& columns

2) Distributed loads from selfweight & floor slab

Classified load

BEAM LOADS

1) Dead loads from self weight, slabs, finishesetc

2) Imposed loads from people, fittings, snow on

roof3) Wind loads mainly on purlins and sheeting

rails


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Depending on distribution loading from

slab1) Two way slab-trapezoidal & triangular loads

BEAM LOADS (contd)

-


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LATERAL RESTRAINT

WithoutRestraint

PartialRestraint

Full LateralRestraint

Degree of lateral restraint

-Lateral torsional stability isassumed adequate

-Maybe provided by the

concrete floor whichsufficiently connected tothe beam/bracing member

-D compression by loadingmake the flange is susceptible to fail by

buckling sideway-Overall moment capacity will not be reached


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Figure 8: Deform Shape of a Full Lateral-Restrained Beam


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Figure 9: Configurations of Full-Lateral-Restrained Beam


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Figure 10: Lateral-Torsional Buckling


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Figure 11: Lateral Restraint


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Figure 12: Deform Shape of A Beam W/out Full Lateral Restraint


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DESIGN FACTORS THAT

INFLUENCE THE LATERAL

STABILITY

-The length of the member between adequate lateral restraints

-The shape of cross-section-The variation of moment along the beam-The form of end restraint provided-The manner in which the load is applied, i.e. to tension or

compression


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DESIGN PROCEDURE

(General)For Ultimate Limit State:

-shear capacity(Clause 4.4.5)shear force due the design loading must not exceed

the shear capacity, the buckling due to shear action also

- moment capacity(Clause 4.2.5.2)bending moments due to the design loading must notexceed the moment capacity. The reduction moment due

to high shear and lateral torsional buckling due toinsufficient restrained also required.


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- local buckling & bearing(Clause 4.5.2 & 4.5.3)

when loads or reactions are applied through the flange to

t e we , t e oca res stance o t e we s ou not eexceeded.


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For Serviceability Limit State:

-deflection

the deflection due to the design loading should not

exceed the limits given in Table 8, BS 5950-1:2000


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FIGURE 13: FLOW CHART OF BEAM DESIGN (GENERAL)


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SHEAR CAPACITY

REMEMBER

v

be greater than shear capacity Pv

tD


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MOMENT CAPACITY

REMEMBER

The Moment due to design load M SHOULD NOT

be greater than the moment capacity Mc


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LOCAL BUCKLING &

BEARING

Figure 14: Cases where The checking of Local Buckling & Bearing Could Be

Avoided & Could Not Be Avoided


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STIFF BEARING LENGTH

- D stiff bearing length, b1 should be taken as thelength of support that cannot deform appreciably

- Assume that the load disperses at 450 through dsection elements which are firmly fixed together.


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Figure 15: Stiff Bearing Length


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BUCKLING CAPACITY

If the flange through which the load or reaction is

applied is effectively restrained against botha) rotational relative to the web

Then, provided that d distance ae from d load or

reaction to the nearer end of the member is atleast 0.7d


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Figure 16: Buckling Capacity of An Unstiffended Web


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Figure 17: ae & Appropriate Formula For Buckling Resistance


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EXAMPLE

BEAM SUBJECTS T


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BEAM SUBJECTS To

LATERAL-TORSIONALBUCKLING

The beam w/out full lateral restrain aresusceptible to lateral torsional buckling

Generally, it need not be checked separately The ultimate moment capacity & shear capacity

should be checked before the lateral torsional

buckling D bending strength pb is dependent on the

design strength py


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FIGURE 14: FLOW CHART OF DESIGN FOR UNRESTRAINED BEAM


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General Principles of Lateral TorsionalBuckling

- Is governed by its unrestrained length and itsslenderness.

- Occurs when the bending moment M reaches theelastic critical moment Mcr.

- Idealized case, failure only occur by buckling when

orcrMM 0.1/ crMM


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Effective Length LE & Equivalent Segment

Length LLT

- Th l n h l n h h m r i l h

beam is to lateral torsional buckling.- But the susceptibility to lateral torsional buckling

may be limited by the end restraints & the

intermediate restraints


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Figure 17: A Simple Beam Without Intermediate Lateral Restraint


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Equivalent Slenderness

- Generally, for I & H section (cl. 4.3.6.7)

Buckling Moment Resistance

- Generally, for I & H section (cl. 4.3.6.4)

LT

bM

qu va en n orm omen

- For normal loading, the equivalent uniform moment factorfor lateral-torsional buckling mLT should be obtained fromTable 18 for the pattern of major axis moments over the

segment length LLT.- Purpose- to indicate one of the major parameter in

determining elastic critical moment Mcr

LTm


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GJL

EHGJEI

Lm

M ycr 2

2

11

+=

m is mLTL is either LE or LLTEIy is flexural rigidity about y-axis

G is shear constantJ is torsional constantE is modulus of elasticH is warpin section constant H=I h2/4

-The criterion of the beam to pass the lateral-torsional

buckling checking is:

LTbx mMM / cxx MM &

Mcx is the major axis moment capacity of the cross sectionMx is the maximum major axis moment in the segment


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EXAMPLE


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HAPPY ENDING

Design of Steel Structure Due to Bending-Beam Steel Structure

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