[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.

Karl

[Holman’s Hanymen]

Hello Ivan

I’m a bit of a novice with decks but a nice design.

I am just in the process of researching details for decks to cost out some ideas for a customer and the issues are very relevant.  Unfortunately I don’t have the guides and codes available so am trying to put together details from scratch so that I can also apply to other jobs if they arise.  Whilst I am a jack of all trades (and consider myself to be very practical) I also have an engineering background.

I generally support the points Karl has made and if the contractor wishes to change the design he should provide justification (structural and durability) and any cost saving of his alternative design.  Any changes can then be considered against the cost savings with your client.  

• •  I spec’d the deck with 100x100mm posts at maximum 1.2m centres, with notched tops to posts. Contractor wants to do without notched tops and just use 2 No. bolts to screw bearers to side of upright posts – any thoughts?

  Notches provide an ideal way for the weight of the joists to bear directly on the support without the load being transferred through the fixings.  Notches are a fag to cut but the joists could be supported on a flat topped post using timber plate connectors.  If bolts are going to be used I would use toothed timber connectors – possible belt & braces but ensures mechanical resistance.  Also, if bolts are used (and not recessed) they will obstruct any vertical boards applied as skirtings.

• •  I specified galvanised decking post shoes at the base of each post rather than set the posts into concrete by upto 600mm. This was partly with the view that if the deck ever needs replacing, the same foundations could be used, rather than having to re-dig. Contractor wants to set posts into foundations and avoid using galvanised shoes.

  Fixing posts into the ground provides lateral stability but maintenance will be more difficult in the future.  If wood is in contact with the ground ensure it is treated to Class 4 to provide a 15 year life.  Use of shoes is a good sustainable feature that is likely to work in practise but for good measure you could add a couple of concrete posts to provide the lateral stability of the deck if it is not tied back to a building

• • I spec’d deck with both joists at 400mm centres and bearers beneath the joists at 1.2m centres, the contractor says he never uses bearers beneath joists, I want to leave them in. All are specified as 150x50mm and the maximum spans are 3.5m for joists and 4.8m for bearers

           Should provide a very solid deck.  If bearers are used as you have shown I reckon the joists could be reduced to 4 x 2 – say the bearers notched  / supported on the posts and the 4 x 2 laid on the bearers.  A potential saving for the client !.  (I reckon a 1.5m grid with 6 x 2 bearers and 4 x 2 joists at up to 600 centres will sustain the residential loading of 1.5Kn/m2 with C16 stress graded timber – not sure whether Karl would agree.  However, design is choice and one doesn’t have to adopt the minimum in design!.

Lastly, it is good practice for the cuts on preserved timber to be treated with preservative – there may be a note on your drgs (or on a separate spec?) but I didn’t notice it.

Hope the points are helpful.

Clive

PS  I was intending to post the above on Friday but was thwarted by technical difficulties.  Having undertaken some more research the notching of posts seems to be well covered by TRADA and TDA but I am sure a robust detail could be developed using timber plate connectors to avoid notching.  I also note that the TDA recommends a design load for decks of 3KN/m2 which is different to the loadings currently used for buildings in the UK (residential 1.5, office 2.5 and commercial 4.0).  I doubt whether the smaller joists I outlined above would meet the 3KN/m2 loading but does anybody know the background to the 3KN/mm2 loading adopted by TDA.

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