Plate Girder 2

8/3/2019 Plate Girder 2

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Plate girder: Designand check procedure

Asst. Prof. Hang Thu Vu

[email protected]


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Overview

Select a suitable size and shape for steelbeams is a process of making an intelligentestimate and then checking the beam for

possible modes of failure. Because steel beams usually consist of thin

elements they are susceptible to certainspecial modes of failure.

The check list of 8 items in below governs thedesign for both medium size steel beam anddeep steel beam


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1. Applied BM exceeds ultimatemoment capacity of cross-

section. Procedure:Check M* Sfy

This describes the maximum capacity of thebeam against bending

Any additional effect will reduce this full

capacity For the beam, local buckling and lateral

buckling are both critical effects to consider


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2. Applied BM causes localbuckling of the compression

flange and parts of the web incompression.Procedure: The full section modulus S may be reduced due to

local buckling. Define an effective section modulus Ze

where Ze S.

Determine Ze. The moment capacity of cross sectionbecomes Ms = Ze fy.

Check M* Ms [In AS4100 Check in (1) is covered by Check in (2)]


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Procedure:

Way 1:

The moment capacity of cross section Ms may

be reduced due to lateral buckling.

Determine moment Mb at which the beam willlaterally buckle, for which Mb Ms

Check M* Mb

3. Slenderness of beam resultsin lateral buckling (sideways

movement and twisting of entirebeam) before M

sis reached.


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Mode 3 (cont.)

Or Way 2: Provide full lateral restraints for the beam either by

continuous lateral restraints

Or provide lateral supports at intervals close enough

to ensure that Mb = Ms . For example, with I-beamswith equal flanges we ensure sub segment length L

Check M* Ms

( ) ymy frl

2505080

+


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4. Shear force is carried mainlyby web. Shear force causes web

to yieldProcedure:

Confirm if

Determine shear force Vw which causes webto yield.

Check V* Vw


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5. Shear force causes web to

buckleProcedure: Confirm

Determine shear force Vb which causes webto buckle. Where Vb Vw

Check V* Vb


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Mode 5 (cont.)

If check in (5) does not satisfy The web buckles.

It must be strengthened with stiffeners which

brace the web against buckling. Design and check for intermediate transverse

web stiffeners (and if the web height is big)

longitudinal stiffeners.

AS4100 does not give the cut off limit for web

height, a reference limit from Japan bridge

code d1 > 152 tw


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Mode 5 (cont.)

After including the system of stiffeners againstbuckling, check if end post is required for the endpanel.

For the end panel, if the width too big s > 3dp orthere is interaction shear/bending, design andcheck for end post

Note for (4) and (5): For section with non-uniform

shear distribution, for example I beam with un-equal flanges, the web shear capacity must bereduced (Clause 5.11.3)


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6. Compressive bearing loadcauses the web to yield

Procedure:

Form the effective web strut for yielding

Determine the force Rby which causes thestrut to yield

Check R* Rby


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7. Compressive bearing loadcauses the web to buckle

Procedure:

Form the effective web strut for buckling

Determine the force Rbb

which causes the

strut to buckle

Check R* Rbb

Note for (6) and (7): for RHS and SHS,require a check for interaction of

bending/bearing (Clause 5.13.5)


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Mode 6 and mode 7 (cont.)

If check in either (6) or (7) does not satisfy The web fails under bearing load. The effective web strut must be increased for

cross sectional area (i.e. overcome yielding)and stiffness (i.e. overcome buckling) byincluding load-bearing stiffeners.

Design and check for load-bearing stiffenersvia form up and assess capacity of effectiveweb/load-bearing stiffener strut for yieldingand buckling subsequently.


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8. Interaction of shear/bendingdoes not satisfy

Procedure: Locate where high shear force and high

bending moment occurs simultaneously

Confirm if the location is in critical range,M* 0.75 Ms or

V* 0.6 Vv.

If it is, use the shear and bending interactionmethod

Check V* Vvm


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Mode 8 (cont.)

Cut off limits for moment and shear force

If interaction happens at end panel, designand check for end post


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9. Deflection of beam issufficiently large to cause

concernProcedure: Determine deflection and if necessary,

natural frequency and amplitude.

Determine deflection limit and/or frequency

of machinery.

Ensure natural frequency of beam is not

close or coincident with natural frequency ofmachine

Check


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Observation

Do you recognize the modes that mostly govern a

small/medium size beam and those for a deep

beam?

The question for flexural beam in mid-term exam istypically designed for checking a standard section,

small/medium size beam. It is there to wrap up what

need to consider for the small beam.

We will go through the solution procedure for it intutorial to and observe how it is very different from

the deep beam.

Plate Girder 2

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