Why are span tables so useful in building design and construction?
What are span tables?
BM TRADA’s Span tables have been in existence for decades now, helping engineers, designers and builders quickly find the right-sized softwood members for domestic projects and make the most efficient use of the timber available. Easy to look up tables allow the user to choose the size of solid softwood timber members necessary to give adequate support to floors, ceilings and roofs in dwellings
In 1991 an earlier iteration of these tables was funded by government and included in the Building Regulations. While this is no longer the case, the span tables are still referenced in Approved Document A and - following several revised editions in book form - the latest version prepared by BM TRADA is in line with the Eurocode 5 design code. Now with wiro-binding and in its 4th edition, it can be purchased directly from this bookshop.
The BM TRADA Eurocode 5 Span Tables book is a must for any designer or builder.
Using Eurocode 5 span tables
The objective of the span tables is to indicate the maximum span of a softwood member subject to:
• its application
• its spacing
• the dead and imposed loads it must support
• the moisture environment
• the strength grade of the timber
The application is defined in the table caption.
The dead load is usually a variable within each table. The futures given for the uniformly distributed dead load are forces which correspond to the following approximate weights of the building element being supported.
• 0.25 kN/m2 is approximately 25 kg/m2
• 0.50 kN/m2 is approximately 50 kg/m2
• 0.75 kN/m2 is approximately 75 kg/m2
• 1.0 kN/m2 is approximately 100 kg/m2
The imposed load is that for a dwelling and is set for the whole table. The moisture environment is defined as the service class. Throughout these tables, the spans are suitable for service classes 1 and 2. Service class 2 is the more severe and corresponds to ‘external uses’ where member is protected from direct wetting’. EC5 gives the same results for service classes 1 and 2.
These tables are offered for strength grade C16 softwoods (a common grade for softwoods grown in Britain) and C24 softwoods (which are more expensive and are usually imported from northern Europe).
If the span figure is shaded, it means that additional bearing length is needed at the supports.
An asterisk means special circumstances may apply, such as additional joists to support partitions.