Structural Engineering FAQ

Here’s our answers to some frequently asked questions about structural engineering:

Who should I talk to if I need to find out whether the structure of a house is fine?

You should engage a building and pest inspector to check if the structure of the house is fine first. Building and pest inspectors specialise in finding problems with houses and most inspectors are more than capable of finding structural problems with houses.

if you want to know whether the structure is ok in a house you are buying then a building and pest inspection is an excellent investment (and a service that even we, as structural engineers, use when we buy properties).

Once a problem has been found with the structure of a house, then you are ready to talk to a structural engineer to help determine how the structure can be fixed or upgraded.

After that, you’ll probably need a building contractor to give you a price for the rectification work.

What is the best type of slab for cyclone areas?

Great question.

The thing about cyclone areas is that during a cyclone, very fast winds create large wind forces that try to lift your roof off.

The house superstructure options are timber frame and concrete masonry. The main house slab options are waffle slab and raft slab.

It is our opinion that waffle slabs and raft slabs can both be designed for large uplift forces – but the engineers that specify waffle slabs in cyclone areas sometimes ‘forget’ to add extra resistance to resist these loads.

The reason is that waffle slabs are a cheaper slab option than raft slabs. The builders that prefer waffle slabs don’t want the extra effort of tying a house down for large wind loads – having to do that undoes the cost saving effect of choosing a waffle slab.

Raft slabs are built with some of the footing below ground so they come with more concrete and more skin resistance with the ground. Almost by accident, they are a better choice for cyclone areas. Unless, of course, the waffle slab engineers remember to add footings for the large uplift loads. The trick is to ensure that the engineer that designs your footing and slab system has a copy of the truss plan and reaction sheet. If anything goes wrong you can prove the engineer knew where the large tie-down forces are.

Do wall studs need to line up with floor joists?

Wall studs do not need to line up with floor joists unless the wall studs are carrying a heavy, concentrated load.

If a wall stud is carrying a heavy load then either align the stud over a joist, provide a double or triple bottom plate for that wall or provide blocking between the joists.

Check with your structural engineer on which method is best for your situation.

How do access my plans from Brisbane Council?

That’s easy. Contact Brisbane City Council! This web page has details about ordering a copy of your building records and documents.

How far apart should trusses be?

Trusses and rafters in Australia are either 600mm (2 feet) apart or 900mm (3 feet) apart.

The spacing of the trusses affects the way the roof and ceiling are supported so check with your truss designer and ensure your battens and direct stick ceilings are appropriately supported.

How do you nail studs to the bottom plate?

In Australia, nail studs to the bottom plate with 2/75 × 3.05 mm nails skewed through the stud into the bottom plate. This is the minimum nominal fixing for connecting a stud to a bottom plate.

What is blocking in framing?

Blocking in timber framing is additional pieces of timber used to span between major structural timbers like joists, studs, and trusses. Blocking is used to support and spread concentrated loads. For example, you might find the carpenter installs blocking at the garage door opener to spread the loads from the garage door motor to the rest of the roof frame.

Can you use screws for framing?

Yes, you can use screws for framing. Most carpenters find that nails are quicker and easier to install because nails can be placed with nail guns but screws have the advantage over nails because they are thicker pieces of steel and so are stronger connectors. The most common type of screw we specify is a No. 14 Type 17 batten screw.

How far apart are wall studs?

Studs are the vertical members in a wall frame. In Australia, studs in residential construction are 450mm (1.5′) apart. Sometimes, if the wall does not carry any heavy loads the studs can be spaced at 600mm (2′) apart but the wall cladding has to be strong enough to span between the studs.

How serious is a hairline crack in your house slab?

Let’s get this straight. A hairline crack in your concrete slab is not a big issue.

A hairline crack is not big enough to let a termite in. It isn’t big enough to be a structural issue.

A hairline crack in a house slab is a slight aesthetic impediment – that’s all.

Unless….. unless you specified to your builder that the portion of the slab has to have no noticeable cracks. Yes. You can do that. However, once the slab has cracked it isn’t easy to ‘repair the crack.

What is the function of a wall plate?

A wall plate is the piece of timber along the top (top plate) or bottom (bottom plate) of a wall frame. The purpose of a wall plate is to transfer the concentrated pint loads from the roof frame into the individual wall studs. So a top plate is actually a timber beam that supports the roof framing and transfers loads into the wall frame. The bottom plate is actually a beam that takes loads from the wall studs and transfers that load into the floor frame or concrete slab.

How does Agi Pipe Work?

Agi pipe is an in-ground drainage pipe that collects water that is already in the ground. It is normally placed into the bottom of a hand-dug or excavator dug trench and then the whole trench is backfilled with gravel.

Water that is moving sideways in the ground comes to the trench and passes through the gravel to the bottom of the tench. The agi pipe collects the water and allows it to drain freely down the hill.

This whole system is often called an agi drain or agricultural drain.

What mesh size should I use in residential construction?

This is a question that should be answered by your structural design engineer for your particular project because the slab mesh is controlled by three things:

  • The size of the concrete slab pour and the spacing of control joints. Smaller slabs need less mesh reinforcement.
  • The reactivity of the site. Clayey soils need stronger slabs.
  • How critical it is that the slab doesn’t crack – the aesthetics and purpose of the slab. For example control of slab cracks is more critical in a polished concrete slab than a standard house slab.

So ask your structural engineer about what slab mesh you need in residential construction.

What is AS1684.2?

AS1684.2 is the Australian standard for residential timber-framed construction in non-cyclonic areas.

It is a reference standard of good practice and standardised framing details for the construction of timber-framed houses.

Builders, designers, and engineers use the standard to specify their requirements for timber framing for houses.

However, the standard itself is rather expensive. So at Cornell Engineers, we use AS1684.2 to generate details and sizes for timber houses and put all that information directly on the plan.

What is AS2870?

AS2870 is the Australian standard for residential footings and slabs. It’s a manual for how to design footings and slabs for residential dwellings to suit the soil conditions that exist on that site.

AS2870 can be purchased from SAI Global on their Australian standards website. https://infostore.saiglobal.com/

AS2870 is mostly used by structural engineers.

Click here for an extract from AS2870.

What are the guidelines for building near a sewer main in Brisbane?

Building near sewers in Brisbane and the rest of Queensland is now controlled by the Queensland Development Code Mandatory Part 1.4 – all known as QDC MP1.4.

https://www.hpw.qld.gov.au/SiteCollectionDocuments/QDCMP1.4BuildingOverOrNearRelevantInfrastructure131213.pdf

It’s a tricky document because it isn’t well written. So we made a video that explains why building near a sewer main could affect your house. https://www.cornellengineers.com.au/building-over-sewers-2/

All it means is that there ARE rules for building over and near sewers. Your structural engineer can explain what you need to do for your particular situation.

What are sub-base specifications for industrial slabs?

Sub-base is the ground under an industrial slab.

You already know that when you drive on dry ground you’re fine but when you drive on muddy soil your car or truck could sink.

The ground under an industrial slab reacts to moisture the same way so engineers specify what sort of ground has to be put right under an industrial slab so that the ground and slab on top don’t fail.

The thicker a concrete industrial slab the worse the ground underneath can be. But concrete is expensive. So engineers weigh up the cost of replacing and improving the ground under a slab vs just having a thicker industrial slab. It also costs money to improve the ground before a slab is poured so there is a careful balance between sub-base cost and cost of the industrial slab.

Is core filled blockwork waterproof?

No. Corefilled blockwork is not waterproof. Water can seep between the blocks (through the mortar), through the walls of the block and even through the cores (even though they are core filled).

Concrete masonry blockwork needs to be sealed to make it waterproof.

When concrete masonry blocks are used to build retaining walls, the sealant is normally placed on the dirt side and then is protected with corflute or some other tough plastic.

How do I attach aggline to downpipe?

Don’t do this. Do not attach agg lines to a downpipe.

Read this: How agricultural drains wreck houses.

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