Help needed with a New Deck

[Burnsy]

Sorry, seems I was miss reading the table in regard to FLW.
Here are the correct figures.

At your current spacing of posts at 1500mm you will need to use 2 120x45mm sticks for each bearer (this is based on FLW of 2400), and buy them as single lengths of 4.8m, don't join them.

With a joist spacing of 1800mm or less, you can use 90 x 45 at 450mm centres. This is based on continuous spans. With single span this would be reduced down to 1500mm.

[freeplay]

Have a read of Allan Staines book and figure out different combinations. At present you are digging 28 holes. 28 loads of concrete, 28 stirrups etc.

I have a copy of the book...... however I'm a novice.

The building directions are straight forward.... I sure I can build it with no problems, but the stumps have me stumped.

The tables are double dutch to me.....and I sure 28 post must be overkill.

So I looking for help. on a better setup than 28 stumps.

[bpj1968]

To give an idea of how few posts you can have

The first three photos are of my deck.
Photo 1 Steel posts and bearers. 100x100 posts and bearers 6 m long. they were 3.2, 3.6 and 4.2 apart Height was about 3 metres above ground

Photo 2 Joists were 190x 45 TP for the main part.
Photo 3 I also had 240 x 45 for the 4.2 metre span.

photo 4 and 5 are of my old deck. (The brown fence in the foreground was not connected

4 posts 100x100. ledger bolted to wall of house

Can't remeber the exact sizes but were simialr to
Beareer were 2/240x45
Joists 190x45

That deck was 3.6 x 9.0m and 3 metres off the ground. Bracing was by metal strap across the top of the joists from corner to corner. I was suprised how rock solid it was with that and no decking.

[brynk]

gday freeplay

the first thing to consider in your design is where are you? this is required because it will affect what code & span tables you need to use if you are designing to 1684. this aside, you can use cantilevers to cut down the number posts required. from my (slightly out of date) span tables, working from your pool to the outer edge of your deck...

(using unseasoned f14 hardwood for bearers, 125x75)
pool -> 500 -> post -> 1750 -> post -> 1750 -> post -> 500 canti -> edge of deck

(using seasoned f7 treated pine for bearers, 2x 140x45)
pool -> 500 -> post -> 1750 -> post -> 1750 -> post -> 500 canti -> edge of deck

(now the joists - using unseasoned f14 hardwood 150x50, or 140x45 f7 treated pine)
edge -> 700 canti -> bearer -> 2300 -> bearer -> 2300 -> bearer -> 2300 -> bearer -> 2300 -> bearer -> 700 canti -> edge of deck

for the posts you have many options until they get over 1800 high. your worst case, the 2.4-3m high ones, require one of the following cast in a footing 600 × 600, 900 deep:

• reinforced concrete 250 × 250 column
• reinforced masonry 200×400 or 300x300 column
• 250mm dia. timber post

or if you use cross-bracing on your posts (eg, from the bottom of one post to the top of an adjacent post) this option is a little more complex because you need to work out what wind-forces you have on the deck - the worst case calls for 150mm dia. timber posts with 170x45 bracing at least 400 deep

food for thought - r's brynk

[freeplay]

Thanks Brynk...

Bearers and joists are all clear.

The iinfo on the posts is a bit hazy.

I guess for me the 150mm timber posts on stirups would be easiest.

Although... metal posts would be a lot easier in concrete..... but I don't have the tools or skills for welding brackets on the top to hold the bearers. If i did go metal... what size post would you suggest?

[brynk]

as you can see & probably have a gut-feeling that the columns/piers are overkill. this is because in this instance you are looking at the 'no bracing' option, and therefore racking (or raking) forces must be dealt with accordingly. hold a pencil in your fist to help you envisage - burrow it in a lot (say, 30% of overall length) and push from the side against the top - this represents a deep footing with a wind (or other lateral) force acting upon it;

now repeat the process with the pencil only buried 10%. it doesn't matter how strong your column is, if there is nothing other than the footing to resist that racking force then the structure will be insufficient (the force will be transferred through the column acting as a long lever, onto the footing). this is where the bracing comes in - the goal is to add members to form an X-shape from the base of the column to the top of the adjacent column - this resists that force before it reaches your footings. that way your footings only need to hold the combined structural weight up, not stop it from tipping over

when working out post sizes & bracing requirements you have two options -
1) build the footings, columns & bracing specified in the worst-case scenario (that was the 150mm dia., 400 deep one i mentioned) or
2) use the code to determine your wind category based on where you are; then work out the aspect of your new deck & the surface area the structure exposes to the prevailing winds (ie, which way the high end faces); now with this combined knowledge look up the strength requirements in the table based on the area in the country, aspect, type of timber & joints being used; finally, work out a footing & post design that meets the strength requirements - if your soil classification is not covered by the timber framing code (ie, you are in sand or highly reactive clay) then the footings will need to be determined from the footings & foundation code, as2870...

i agree that in this situation a steel-reinforced concrete footing is the best option, especially because there is a 3m high wall with water splashing over all the time. it's gunna be wet & muddy at the base of that wall & around the base of your columns - it's also sloping down hill - a good hunk of conc. buried deeply will do wonders for your stress levels - timber cast directly into concrete will be a durability issue especially with so much water around. for that timber you would need to be looking at something that can resist the wet-rot, structural treated pine or a hardwood with natural resistance. or concrete, steel or masonry - whatever you use water will get there & you will need to drain it

standard post-stirrups will be insufficient unless your columns are heavily braced. it may pay to look at the cyclone-rated stirrups (which are U-shaped pieces of folded flat-steel, not the ones that are a tack welded tube to the piece of tin sheet). you can buy one which is set into the concrete 300, with a 300 long attachment area sticking up out of the footing to fix off your posts. http://www.pryda.com.au/catalog.php?...Post%20Anchors

the code allows you to use steel to a certain height but beyond that, if you don't have a structural engineer to counter-sign then your approval won't even get looked at!

r's brynk

[pawnhead]

Some good information there brynk.

[oohsam]

28 stumps too much????
I had 40 and it was ok!!....Just hire a post hole digger for a day. It will do your 28 holes with no trouble, will cost ya 90 bux..then all u gotta do is cut post, put in brick, fill with cement and back fill...done and done!
bolt some bearers, nail some joists and you've got your subframe in 2 days work, maybe 3.

[freeplay]

brynk...wow... thanks for all the great info.
The "High Wind Post Anchors" might be the way to go i think. I agree that putting timber posts straight into concrete in my situation would only cause more work later on in life.
The anchors posts come in a 125mm size...do you think that will be ok?
oohsam - Hole digging and concrete mixing are not an issue for me. ( I have access to a mixer and a Dingo with an auger )

Some bracing is also not a problem... I had always intended to slat the sides anyway... so I will brace the 10m side and brace 1/2 the 5m side, and put a gate/door in the other 1/2. ( all pool equipment and garden tools to be stored underneath )

So the next question is: Would the 125mm hardwood posts on High Wind Post Anchors set in say 500mm or 600mm of concrete be sufficent?

[freeplay]

Would the 125mm hardwood posts on High Wind Post Anchors set in say 500mm or 600mm of concrete be sufficent?<!-- / message -->

[brynk]

gday freeplay

if you want to use the option of square posts on high-wind anchors you will probably still need column bracing. as to whether they will suffice or not, type of & how deep your footings need to be, the size of the bracing required, you still need to do the following: (imagining your deck is a giant umbrella on a windy day)

• use the code (as 1170.2) to determine your wind category based on where you are

now switching back to the timber framing code (as 1684.2 or as1684.3 if you are in a high-wind area)

• then work out the aspect of your new deck & the surface area the structure exposes to the prevailing winds (ie, which way the open ends face)
• now with this combined knowledge look up the bracing and uplift capacity requirements in the table
• work out a footing & post design that meets the bracing strength requirements
• finally, check the footing design to see if it meets the minimum bearing capacity - ie, how much downward force is flowing into this footing from the weight of the deck & the people using it (as opposed to the uplift restraint capacity described above) if the bearing capacity is insufficient then up the design until both the bearing, uplift and bracing capacities are met

if your soil classification is not covered by the timber framing code (ie, you are in sand or highly reactive clay) then the footings will need to be determined from the footings & foundation code, as2870... find out your soil classification from your house plans;

as to whether the high-wind anchors in the pryda catalogue are sufficient? given enough posts they would be! whether the minimum number of posts identified in the earlier span-design is sufficient or not will depend on the result of the above wind calculations, bearing calculations, bracing restraint calculations

can i suggest you obtain a copy of the code - if yer lucky you could buy one from a tafe or technical institute bookstore for cheaper than you could at standards australia; before buying there you might even be able to look it up in the library at said institute - that way you could have a look at the code itself and decide if you will spend the money & design it yourself or spend the money on an engineer or a builder instead! you can still engage either of these to do you a design without having them have anything to do with the construction, granted your project does not require development approval ofcourse

r's brynk