Reply To: Revisions to deck post-tender

Holman’s Hanymen

Thank you for the responses and comments.  Very useful to focus the mind and find my way around. 

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.

The conclusion of the above was that 4 x 2 joists with 6 x 2 beams should be capable of carrying the loads without any additional beams or posts.  I agree with Karl that the cost of downsizing the joists will not be substantial  (around £100 ?).  Alternatively, it is possible that the number of supports could be reduced.  Based on the TDA guidance (using the higher loading and double beams) I reckon 10 supports would be able to support the main deck area (excluding the stairs and any intermediate supports that may be required due to the curved front).

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 !.

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. 

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.

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.

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.

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.

Karl Harrison said:

Great comments Clive, you answered my query with respect to the 100mm joists, any dimension can be used in this instance although I always refer to the Span tables. I usually stick with the bigger the better and for the cost savings from 150 to 100 dimension joists wouldn’t really make a saving worth worrying about due to the increased amount of the beams that would then be required and also posts…

I can’t comment upon the TDCA’s want for a 3kN Loading, this doesn’t match with TRADA, I would rather use the qualified direction via the span tables promulgated by the eminent Dr’s of wood science in house at TRADA after all they wrote the regulations to include EC5.

I shall ask Janet from the TDCA for her comments.