[Karl Harrison Landscapes Ltd]

unfortunately I could not find the TRADA tables online but tables quoted in manufacturers leaflets (Q-Deck) confirm that 4 x 2’s at 600 cts will span 1.75m using C16 timber and 6 x 2’s will span up to 3.2 metres if used at 400 cts.  I was not able to find tables for single beams (bearers) and hence a “back of the envelope” calculation to confirm that 6 x 2’s at 1.5m centres would be sufficient.  Q-deck only recommend the use of twin beams and TDA only provide details for a single (187 x 69) beam.

Trada tables are available direct from them, http://www.trada.co.uk/

Firstly, related to Ivan’s query, is that an experienced contactor worth his salt would be aware of this and would have suggested a cheaper alternative when he tendered – as opposed to querying notching and eliminating the shoes after submitting a tender !.

I would be careful with “experienced Contractor” statements; if you are referring to experienced decking companies then I couldn’t agree more. If, however, you refer to Carpenters, Landscapers, Builders and Contractors then I would suggest this is not the case. I have seen hundreds of projects (if not much more) where money is so much of a driving factor that just about every corner is cut and no one seems to care until it’s too late.  I have yet to meet a contractor that has even heard about the span tables and in a high number of cases simply apply what knowledge they think is right at the time… I don’t argue anymore I simply walk away.

Secondly, and less related to Ivan’s query, is the use of tables.  Tables (by their nature) take a simplified approach and consider the worst case loading / design scenario which leads to conservative designs.

I agree, Span Tables save time and the result of using them is fit for purpose without over engineering a project.

Designers have a responsibility to understand what they are doing and prepare a safe design.  Use of “approved” tables simplifies the design and minimises design costs but consequently member sections tend to be overestimated because they have to consider all design scenarios.  There is a balance of using less economic sections against the additional cost of undertaking structural calculations / employing a structural engineer to validate the design.

I think this is best for a separate discussion maybe “where are the boundaries for Decking Design”. What are the limitations of a garden designer, should they specify the job of a structural engineer or have to understand span tables at all? One could argue that as there are no official codes for deck construction so is there two levels of design, one being a beautiful garden design that has deck as part of the scheme and then, two where the structure is drawn out in accordance with…

 

Whilst I cant comment on the TRADA tables, those that I was able to view only provided limited information and were not particularly appropriate for use as a design tool.  They tend to concentrate on design solutions with maximum spans as opposed to providing a range of section sizes for different spans / loading scenarios / site constraints etc.  I assume the use of a 1200 post grid in Ivan’s design is to coordinate with the 400 joist spacing for the decking boards.  If this is a common design scenario then tables that provide section sizes for 1200, 1500, and 1800 module designs, in addition to the maximum spans, may provide more flexibility for designers to select more efficient designs.

The reason for limited open information is that this field of structural design is far more complicated than at first glance, of course there are basic elements to decking structures and they are common knowledge, if you look at the codes under EC5 for timber structures they will blow your socks off…

 

Ideally, tables should be prepared to meet the needs of the users as opposed to those preparing the Codes whose primary aim is to ensure safety is not compromised.  Unfortunately, the codes are more likely to be influenced by the interests of the larger organisations in the industry (who tend to be represented on the committees responsible for drafting the codes) as opposed to the smaller practises that (I suspect) undertake the majority of the work in the industry.

As far as I am aware they are. If you want to get deeper into timber structure and its design, join TRADA. Timber structure design has been over looked by too many for too long, now it’s a specialist field and at Napier in Edinburgh you can study a masters in Wood siences where the entrance level is a post grad in structural engineering…

Perhaps the above is food for thought and apologies if I have wandered away from Ivan’s query and will be pleased to hear TRADA’s feedback  regarding the higher loading now being put forward by TDA.

There is always more than meets the eye in the world of timber.