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 Design of Steel Structures

Design of Compression

Members

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Class 4 - Compression Members

BS 5950  – 1: Cl.4.7.4

b) Compression Resistance for class 4 slender cross sections

Pc = A eff Pcs

Pc  - Compression Resistance

 A eff -  Effective cross  – sectional area from Cl.3.6 Pcs -   Value of pc from 4.7.5 for a reduced slenderness

  of λ (Aeff /Ag)0.5 in which λ is based on the radius

  of gyration r of the gross cross - section 

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 A eff - Effective cross –sectional area

Cl.3.6.2.2

The effective area is calculated bydisregarding those parts of the crosssection that are more susceptible to localbuckling, i.e. those parts that will beineffective when highly stresses.

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 A eff - Effective cross –sectional area (cont.)

Figure 8

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 A eff - Effective cross –sectional area (cont.)

Figure 8

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Design of Column Base Plates

Columns which areassumed to benominally pinned attheir bases areprovided with a slabbase comprising asingle plate filletwelded to the end of

the column andbolted to thefoundation.

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Design of Column Base Plates (Cont.)

The dimensions of theplate must be sufficient

to distribute the axialcompressive load to thefoundations and toaccommodate the

holding down bolts

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Design of Column Base Plates (Cont.)

BS 5950-1:2000 Clause 4.13.1.2

The Bearing Strength for Concretefoundations is given as 0.6f cu, where f cu isthe characteristic concrete cube strength

at 28 days

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Design of Column Base Plates (Cont.)

BS 5950-1:2000 Clause 4.13.2.2

Formula for determining the minimum thickness of arectangular base plate supporting a concentrically loadedcolumn is given by:

tp  = c[3w/pyp]0.5  ≥  Twhere:

c - is the largest perpendicular distance from the edge ofthe effective portion of the base plate to the face of the

column cross sectionPyp - is the design strength of the base plate

T - is the flange thickness of the column

W - is the pressure under the base plate, based on anassumed uniform distribution of pressure throughout the

effective portion

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Design of Column Base Plates (Cont.)

BS 5950-1:2000 Figure 15

The effective areas indicated for various cases in the code are