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How do you read span tables?

I never can get my head around these things.

Thinking of using Archibeam steel bearers for my deck but not sure on the spacing. (with wood bearers it's usually just a 1800 grid)

This is the span table in question.

I'm more of a visual person. Could someone make a picture on how the spacing would look based on the span table

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Building and Construction

Comments

MitchellMc

Hi @dowd_hardware,

I'm very much a visual person as well. I believe the load width in millimetres across the top is the centres at which the bearers are placed. So if we take the first set of 900mm centred bearers, your 150x50 at a single span can span 3300mm. If you increase the width between your bearers to 1200mm, your bearers can now only span 3000mm, and so on. The greater the distance between bearers, the less they can span as they'll be individually carrying more weight.

Let me mention one of our knowledgeable members, @ProjectPete, to see if he can confirm for us.

Mitchell

dowd_hardware

https://www.workshop.bunnings.com.au/discussion/comment/20777#Comment_20777

Thanks mate. That clears it up.

It's funny, one image, and I understand it in 2 sec

Toval

https://www.workshop.bunnings.com.au/discussion/comment/20777#Comment_20777

Hi Mitchell

I know this is an old thread but it's been the most helpful in learning how to read span tables.

My issue is that I am building a deck with little clearance. I have about 300mm to build with. As such I will probably use hardwood as I can get F17-F27, H4 equivalent timber, so I don't have to worry about water and can try to get smaller bearers and joists. The deck will be 3800mm wide, 3000mm long.

I want to have the joists above the bearers as this will improve airflow.

My issue is that spanning tables for hardwood seem to limit floor load width (which I assume is span load width) to 1200mm. I was going to have joists at about 450mm spacings.

You can see by the attached photos. The second attachment is the bearer F27 hardwood with the same span/floor load widths.

I can accept 2/170x45 bearers as these will run the lenght of the deck (2 bearers spaning 2400mm each, deck is 3800mm wide). But I would prefer 3 bearers with smaller wood but the span tables don't have, say, 1600mm span loads.

But the Joists needing to be 170x45 (1200mm span width) means I am beyond my 300mm limit for my deck. If I use 450mm spacing, how do I work out what joists are okay? Originally I found 70x45mm timber which I thought should okay for the joists given it's hardwood F27 but I can't find a span table to work it out.

Thanks for any help

Joel

Screenshot 2024-03-02 131138.png Screenshot 2024-03-02 131534.png

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/79991#Comment_79991

Welcome to the Bunnings Workshop community @Toval. It's marvellous to have you join us, and many thanks for your question about building a deck.

Don't worry; I'm now equally as confused. It's quite hard to work out these types of things in your head. I'd suggest you start by drawing some diagrams of your deck; I'll get you started with this rendering I've created for you below. For the bearers I've used single 170 x 45mm timber and for the joists 90 x 45mm. The bearers are supported every 1200mm by stumps, and the joists span 1000mm between the bearers. The joists are spaced 450mm from each other.

Now that you have the basic design, you can minimise the amount of footers if needed by doubling up the 170 x 45mm. That would allow you to span the 3000mm side, but for the 3800 width I'd still have one set of central supports so they are only spanning 1900mm.

Please let me know if you have any questions.

Mitchell

Toval

https://www.workshop.bunnings.com.au/discussion/comment/104455#Comment_104455

Thank you for the reply MitchellMC

I have rendered an image below with the joist and bearer measurements as you suggested. I guess my issue is that I don't full understand how you derived the joists based on the span tables. Everything I read says that 450mm spacings are standard, and I have done that in the rendering below. But the tables I found for hardwood state that 1200mm. On my reading 90x45mm joists at 1200mm span load means the joists can be 2400mm long. In my case they go across the deck (so that the decking boards align going away from the house, in line with the hardwood flooring inside the house).

But my deck is 3800mm wide, which, again on my reading of the span table, means I should be using 170x35mm joists. But again the table says that I would use this size for 1200mm spans, which is not what is recommended by everything else I have read (ie 450mm).

I am happy to go with what you have suggested, it's similar to what I had already thought. But I have to get a building permit from council and I really don't want it to get rejected.

For reference I will have two foundations/footings (six total), one at either end on the bearers. Not seen in this rendering. The counter weight lenght on the joists are 500mm. Decking boards 135x19mm, with and edging not shown. Orientation is looking at the house, hence joists running left to right.

Thanks agin

Screenshot 2024-03-03 095729.png

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/119312#Comment_119312

You're correct that joists are always spaced 450mm apart, as that's as far as 19mm decking can span before it gets bouncy @Toval.

The size of joist required is dependant on how far it needs to span between the bearers. Your table includes the floor load width, which is the total width of the floor that the individual bearer is supporting. I've indicated below a floor load width. It's not the overall width of the floor. So, your rendering shows a deck that far exceeds the requirements. Remember your table is for bearers not joists. Here is another table that might be easier to understand as it removes the floor load width. You can see that even a 90 x 45mm joist on a continuous span (is supported more than just at its ends) has a max span of 1800mm.

You're on the right track and have shown a great comprehension of how a deck is constructed. I think it's just this floor load width that is throwing a spanner in the works.

I do apologise if this confuses the situation even more. It's rather hard to decipher and explain through text and is why engineers get paid so much.

Mitchell

Toval

https://www.workshop.bunnings.com.au/discussion/comment/128253#Comment_128253

Thanks again Mitchell,

I see where I was getting stuck. I thought the span distance was the total deck, not the distance between the bearers. Even writing it I see that it should have been obvious.

The spacing between my bearers will be about 1280mm so as you have pointed out 90x45mm joists will cover 1800mm spans (continuous span) and therefore be more than adequate at 450mm spacings of joists.

For the bearers, my floor load width is 1280mm so by my table I use the 2400mm single span column, as I am only using footings at either end of the bearers (single span). The space between the footings will be 3000mm. So the timber I need is 2/170x35. I will use 2/170x45, despite it being more than I need I will be mounting them on 90mm stirrups. Overall the bearers are bigger than I need, as are the joists, but as I'm not a professional I am happy to over-engineer.

I think I might finally understand it. At least enough to do this deck.

Thanks again for you help.

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/134022#Comment_134022

You're on the money @Toval! It is always better to over-engineer rather than under.

Just remember that approval might be required for your deck, so before you start any work, it's best to contact your local council to check.

Keep us updated, and please reach out again if you need further assistance.

Mitchell

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