Limit state design

Eurocode 5 represents a significant change for designers in timber and timber products as it is based on limit state design, rather than permissible stress. This brings timber into line with steel and concrete for which this approach has already been taken. There are generally two limit states to be considered: first, the ultimate limit state beyond which parts of the structure may fail or collapse; and secondly the serviceability limit state beyond which excessive deformation, deflection or vibration would render the structure unfit for its purpose. The ultimate limit states are determined from the characteristic values of the loads or actions and the material properties, to which partial safety factors are applied. Generally, the characteristic values of the material properties in BS EN 338: 2003 are higher than in the BS 5268-2: 2002 grade stresses, as they are derived from laboratory tests without reductions for long-term loading or safety factors which become the responsibility of the designer. The characteristic values are used in limit state design (Eurocode 5) and the grade stresses apply to the BS 5268-2: 2002 permissible stress design code.

Table 4.7 Softwood species/grade combinations which satisfy the requirements of BS EN 338: 1995 and BS 5268 Part 2: 2002 strength classes as listed in Eurocode 5 Part 1:1

European Standard BS EN 338: 1995 Strength Classes

Species

Origin

Grading Standard

C14

C16

C18

C22

C24

C27

C3O

British grown softwoods

Douglas fir

UK

BS EN 519 BS 4978

GS

SS

Pine

UK

BS EN 519 BS 4978

GS

SS

Spruce

UK

BS EN 519 BS 4978

SS

Larch

UK

BS EN 519 BS 4978

GS

SS

Imported softwoods

Redwood

Europe

BS EN 519 BS 4978

GS

SS

Whitewood

Europe

BS EN 519 BS 4978

GS

SS

Sitka spruce

Canada

BS 4978 US/Can

GS 1,2

SS Sel

Hem-fir

Canada

BS EN 519

Spruce pine-fir

and US

BS 4978

GS

SS

Douglas fir-larch

US/Can

1,2

Sel

Southern pine

US

BS EN 519 BS 4978 US/Can

3

GS

1,2

SS

Sel

Western white woods

US

BS 4978 US/Can

GS 1,2

SS Sel

Pitch pine

Caribbean

BS 4978

GS

SS

Parana pine

BS 4978

GS

SS

Radiata pine

New Zealand Chile

BS EN 519

Zimbabwian pine

Zimbabwi

BS EN 519

S. African pine

S. Africa

BS EN 519

Western red cedar

BS 4978

GS

SS

Grading: BS EN 519 refers to machine grading, BS 4978 refers to visual grading,

S indicates available machine graded to the class indicated.

GS and SS are General Structural and Special Structural visual grades respectively.

US/Can refers to the US and Canadian visual grading standards for lumber.

The Canadian and US grades 1, 2, 3, and Sel refer to No.1, No.2, No.3 and Select respectively.

Grading: BS EN 519 refers to machine grading, BS 4978 refers to visual grading,

S indicates available machine graded to the class indicated.

GS and SS are General Structural and Special Structural visual grades respectively.

US/Can refers to the US and Canadian visual grading standards for lumber.

The Canadian and US grades 1, 2, 3, and Sel refer to No.1, No.2, No.3 and Select respectively.

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