Residential Slabs and Footings

For the most part residential slabs and footings shouldn’t require any “engineering”. AS2870 Residential slabs and footings is mostly a prescriptive code like volume 2 of the National Construction Code (NCC or BCA {Building Code of Australia} ), and the residential timber framing code AS1684.2. As such building designers, builders and concrete contractors should be able to use the code.

Need for Specialist, or Not

If your house is on the side of a hill, or you otherwise have a problematic site then I suggest that whilst you may need an engineer, you don’t want an ordinary civil engineer, you want someone specialised in geotechnical engineering (and I don’t mean 20 years stuffing numbers through CORD {Code Oriented Raft Design} and/or SLOG {Slab on Ground} software). I mean someone who is going to give proper consideration to the likelihood that soils can slip down the side of a hill, and that the house resting on the soils will go along for the ride. And a whole multitude of other considerations, which I cannot identify, because I’m not specialised in the geotechnical sciences and associated technologies. {NB: Merely setting trenched piers into firm natural won’t ensure that the soil below will not slip if the conditions are right.}

Code Compliance

It should be noted that prescriptive codes simplify the design process, and provide a compromise between the time required for detailed design, the economic use of materials, and the speed of construction.

There is no such thing as over engineered. Either something uses more materials than necessary due to a lack of engineering, or it lacks fitness-for-function due to lack of engineering.

There is no such thing as over engineered. There can only be a lack of engineering.

Conrad Harrison

Prescriptive codes try to be conservative, and consequently may use more materials than detailed design may provide. However there is little value spending $1000 on detailed design to merely save $1000 worth of concrete: far better to have the $1000 worth of concrete, than a pile of scrap paper saying not necessary. For a market builder however, the saving for each house, may add up to something more significant over a year.

However whilst the code may try to be conservative, it may not be so in some situations. Prescriptive codes tend to be based on simplified design rules, which can be applied rapidly to a large range of situations, to generate lookup tables and/or design curves. So simplified rules are used to generate the code and then further simplified rules are used to apply the code. So if approaching the limits of a code, then maybe advisable to seek more detailed design.

More importantly codes do not cover all the characteristics of a system, they only cover those characteristics which have proven to be critical in the past. Assuming a pareto principle applies, then the codes only cover those 20% of characteristics which give rise to the system being considered 80% defective. The remaining characteristics are ignored by the code, but they are not expected to be ignored by designers.

WARNING: Just because something is code compliant doesn’t make it fit-for-function.

If the other characteristics are ignored then they may become the primary source of defects, and most apparent and noticeable defects. So just because something is declared code compliant doesn’t mean it represents quality. Unlike the building industry, other industries tend to go out of their way to indicate their products surpass the standards, not merely comply with, they may even go as far as identifying the defect in the codes.

So remember there is more to good design than mere code compliance.

Preliminary Design: Water And Vegetation

Water and vegetation are important considerations. Whilst vegetation is an important consideration in terms of the hazards posed by vegetation to the building, the main issue of concern is that related to soil water content.

The roots of vegetation can damage water supply and sewer pipes, and paving. Trees if uprooted by a wind storm can topple onto buildings and damage the buildings. It is therefore advisable that trees are kept away from buildings by a distance at least equal to their maximum height, and that includes buildings on neighbours property. So when a tree falls it should remain on your property. So if for example the rear fence is 5m from the house, the maximum height tree suitable is less than 2.5m high, and located midway between the house and fence, so as to clear both the fence and house when it falls. That is not much of a tree, therefore landscaping is an important part of designing the property and siting the house.

Landscaping design also influences irrigation and stormwater drainage design. The soils mostly concerned with in South Australia, expand and contract with moisture content. It is therefore important to try and maintain a stable moisture content in the soils. Trees can remove water from the soils, causing the soils to dry out and shrink. If the soils shrink away from the building, then it loses support and cracking may appear.

Irrigation and stormwater on the other hand add water to the soils, causing the soils to expand, which can apply upwards load to the building, resulting in the appearances of cracks.

In brick veneer construction the brick veneer is primarily a decorative cladding. Cracks in such brick work as thus mostly an aesthetic issue, rather than a safety issue. Though if the wall has potential to fall down, then it is a hazard.

Since AS2870 is primarily concerned with soil heave, and making the footings large and stiff enough such that cracks in the masonry are not visible. The most economical footings can be achieved by avoiding the use of masonry construction.

If going to use masonry then make sure the landscaping is also appropriately designed to maintain soil moisture levels, and keep irrigation water and stormwater away from the house.

These are thus issues which need to be considered before looking at the structural design of the footings.

Site Survey and Soils

Survey

The starting point to determining if need a specialist, is to collect some data from the site and describe the site. The data may include a land survey by a surveyor, the survey should be suitable for producing contours. Contours are important for designing the stormwater drainage system, and otherwise determining how to fit the house to the site.

Levelling a sloping site is of questionable value. Typically cut and fill, cuts from the high side and fills the low side. If too much soil on the site, then it has to be removed. If not enough soil on the site then soil has to be imported. If the soil is not placed according to an engineering specification, then it is not of much use in supporting the building {eg. the soil has increased potential to slide down the slope of the hill}. Consequently if the site is filled then trenched piers are installed and founded in firm natural ground. The slab is basically suspended between the piers, and not otherwise considered supported by the soil.

The soil just makes it easier to provide the mould for the concrete. But having expended effort installing the fill, effort then has to be expended removing it to provide trenches for the beams and piers of the footing.

It maybe a lot easier and less expensive, to put the house on exposed piers and construct a suspended slab (eg. Bondek). The space below the floor maybe usable as storage space: without fully levelling the ground, it can be stepped (terraced). Access to plumbing services under the slab may also be beneficial.

If going to carry out significant earthworks to level the site, also need to give extra attention to stormwater drainage, will also need to consider retaining walls as part of the landscape design.

If opt for split level construction to reduce the extent of earth works, then it should be noted that there need to be a retaining wall to support the soil from the upper level. This retaining wall will be inside the building, it needs to be properly water proofed. Also water needs draining from behind the wall.

Whether split level or underground basement cellar, it is important to know where the subsurface water table is. The construction needs proper waterproofing at all joints. The walls and floor surface also need protection from water and water vapour.

Soils

So you have weighed the other factors and decided to go with concrete slab on ground. We now need to determine if we can use the prescriptive part of the code (AS2870) or need the services of an engineer. To decide this we need to get the site classified to AS2870, and to do this we need some soil bore logs.

The bore logs describe a soil core sample, taken to a depth of 3m, and identifying the different soil strata within the core, and assigning a plasticity index to each stratum. The plasticity index is a subjective judgement, and thus dependent on the person making that judgement. For that very reason, we have always used J & J soil testing to produce the bore logs, even after they merged with Geodrill Australia. In South Australia the main soil drilling and logging businesses are:

  1. Geodrill Australia.
  2. Copper Triangle Consultants
  3. Coffey (not typically employed on residential)

Typically a project will require 3 bore logs, though for some projects it may be permissible to use fewer. Some projects my require more, especially if the bore logs from a given property are significantly different from the neighbouring sites. Which is something which will be determined when council compares against their records.

Getting the Site Classified to AS2870

Now whilst the plasticity index of the soil is a subjective judgement as to the expansive nature of the soil, the people who produce the bore logs don’t necessarily go the next step and classify the site to AS2870.

To classify the site, the plasticity index has to be used to calculate an estimate of the soil heave to be expected on the site. This calculation is not overly complex, but due to its iterative nature is best done by computer. Prior to moving to windows and adopting MS Excel for our calculations, we used the following software I wrote: Calculation of Soil Heave, and site Classification (MSDOS).

Once have the site classified to AS2870, can then determine if the footing can be taken directly from the tables or whether the footing size needs to be calculated using “engineering” principles. If can get the sizes from the tables then a building designer should be able to specify the requirements for the footing without need of an engineer. However, in South Australia, you may need an engineer to certify such footing specification prior to submitting to council. {NB: Not the usual requirement when using prescriptive codes.}

If the soil loggers won’t classify the site, then may have to use the services of an engineer. If the soil loggers don’t assign plasticity index may also have to use services of an engineer. Not all civil engineers are able to assign plasticity index.

If in a remote area then getting soil bore logs may be difficult or expensive. typically may get someone to dig a hole, and place soil samples in bags. Compare this to a soil core, about 50mm diameter, placed in a box about 1m long, and divided into about 3 segments. This soil sample then needs to be delivered to someone who can log its description.

The soil loggers when making the subjective judgement about the plasticity of the soil are already assuming risk. The risk to classify the site is not that much greater. So I don’t see why they cannot produce a typed report which provides the AS2870 site classification.

More importantly the soil loggers, could maintain a database with all past site data and classifications, which combined with a geographical information system (GIS), can be used to compare each new soil sample, and otherwise produce a contour map of expected soil heave and or site classes.

Drafting and Design-Drafting

As I have wrote in earlier posts, there are the roles of:

  1. Tracer
  2. Copy-Drafter
  3. Drafter
  4. Design-Drafter
  5. Designer

Simply put, a tracer traces an existing drawing. A copy-drafter reads an existing drawing and attempts to produce a copy: if the drawing contains flawed information then the copy-drafter will fail to produce a copy. It is a good check that a drawing is complete, and properly represents the information it displays. For example a floor plan may show a corridor as being 1200 mm wide, whilst it is drawn 900 mm wide. This may mean the floor plan doesn’t fit together as shown. It may also mean that walls are not above footing beams, and thus not properly supported.

A drafter produces an original drawing based on a collection of freehand sketches, verbal descriptions, design drawings and catalogue information. A designer may produce a design sketch looking at an object from an unusual direction, the drafter doesn’t copy such drawing but uses the information, to produce a drawing more suitable to communicate the proposed object to others. As another example a floor plan and typical section are not full description of a building. A drafter visualises every section of the building and draws each different section. If they cannot draw the section, then it is likely that the design-concept is incomplete, that information is lacking. {NB: I am not referring to them being unable to draw the section or other detail, because they lack skills in technical drawing. Only referring to inability due to lack of project specific information}

A design-drafter, conducts design which is concerned with dimension and geometry and is not concerned with assessment of other functional considerations. A design-drafter can integrate other functional considerations into their design by reference to prescriptive codes and suitable manufacturers catalogues. For example they can comply with the set out requirements of the timber framing code, and those of the residential slab and footing code.

A designer makes use of the applied sciences (often called engineering sciences or technical sciences), to assess critical functions of a system so as to make it fit-for-function. Some kind of designer is required, if there are no suitable prescriptive solutions available.

Drafting the Slab and Footing

Residential slabs and footings to AS2870 are highly standardised: this is partly controlled by the limitation of the lookup tables, and partly due to nature of the technology and the equipment used to construct. Therefore drawing of slabs and footing are within the capabilities of building designers, who typically operate at the level of design-drafters rather than fully fledged designers (eg. they lack the science to independently defend the design decisions they make with respect to fitness-for-function. So they are designers in the sense that graphic artists are designers, not the sense that “engineers” are designers. ).

To dig a trench in the ground needs a trenching machine of some description and therefore the width of the footing beams is controlled by the width of such equipment. So beams are typically 300 mm wide, and the depth of the beam needs to be determined.

The concrete needs to be vibrated, and therefore the machine which vibrates the concrete needs to fit between the steel reinforcing. Concrete also needs to get between the steel reinforcing to fill the trench. Hence there is a limit to the amount of steel which will fit into the 300 mm width. So it is mostly the depth of the beam which can be adjusted to modify the strength and stiffness of the beam.

Footing Layout

The footing layout starts with an outline of the floor plan. If the floor plan is provided as a paper drawing, and the footing designer uses CAD then time has to expended copy-drafting the floor plan into CAD. This can take upto 5 hours if not longer. If the footing designer uses manual instruments then they can use tracing paper, drafting film or a lightbox to match the footing layout to the floor plan. It is therefore preferable, for efficiency, that the building designer and footing designer both use the same drafting technology. If they are one and the same person, then we have the best efficiency: as they will also be using the same CAD standards for one thing.

So we have the outer envelope of the floor plan. Then where there are brick walls there will be set downs (rebates), and where there are openings or doorways there will be no set down. There maybe other special detailing requirements at doorways. For example at a garage door, the slab needs to slope away from the building, to ensure water drains away from the building rather than into the building.

The floor slab should be elevated above the pavements surrounding the building. The pavements should drain water away from the building. The concrete should be visible between the top of pavement and the bottom of the brick work. If not then two features have been compromised. The facility to drain water away from the house, and the ability to observe and detect termites entering the house, placing the timber frame at risk. The timber frame being at risk from termites and also getting damp.

The walls at the perimeter of the building should all be sat on beams. these beams are illustrated by hidden detail lines. Hidden detail lines are typically dashed lines. Those qualified in technical drawing should know such, but those producing DIY drawings may not.

All walls in the house should have a beam centred underneath them. As a start all of these beams should extend to the perimeter beams. None of the beams should be placed more than 4m apart. If beams are more than 4m apart, then divide the space up between, so that each new beam is less than 4m apart.

Check for torsion. Consider the floor slab a plate, the bits which are sticking out can be folded down. A beam mounted on the face of another beam, will produce a torsional moment in the supporting beam. This is undesirable and needs to be avoided. It can be avoided by providing a back span to the beam, preferably by extending the beam across the full width of the building, from perimeter beam to perimeter beam. This typically occurs at verandahs and bay windows.

Some tidying up maybe required where beams intersect and where beams are in parallel and close proximity to one another. There are some rules which allow beams to be offset from a wall. By following these rules it maybe possible to eliminate some beams which are in too close a proximity to one another.

If the site is on fill, then trenched piers will be required to support the beams. As a starting point draw them about 900 mm long on plan, and 300 mm wide to match width of beams. Place them along the beams no more than 4m apart. You will need structural design so this may change.

If drawing an house extension connected to the existing house, then show footing dowels along the joint line. Show them about 600 mm long, and at no more than 600 mm centres, between beams. All beams to have at least one dowel, preferably two, so adjust the spacing to suit. Structural design will be required, therefore it may change.

Identify the steel reinforcement fabric in the top of the slab. This can be a simple note, accompanying a note for the thickness of the slab. If using loose bars or fabric with a rectangular pattern then draw the reinforcement on the plan illustrating the direction and extents.

Label of the beams (eg. B1, B2, or FB1, FB2 etc…). Provide a schedule for the dimensions and reinforcement requirements of the beams. This information should be in AS2870 unless need structural design.

Check for re-entrant corners. Show reinforcement bars across the corners, typically show 1800 mm long. A re-entrant corner is the corner on the inside of a L-shape.

Brick Control Joints

Brick control joints can be drawn on a separate floor plan or incorporated with the building design floor plan. It basically consists of placing a symbol on the floor plan to locate the position of the joint. Joints should be no more than 4m apart, preferably located near openings in the brick work. So set out locations by starting at door openings and move to the ends of the walls. Such locations are dependent on the masonry code (AS3700). Structural design maybe required.

Sections and Details

Once have the footing layout, can now start to visualise sections through the floor slab and footing beams. Basically start at the left hand end, and move to the right visualising the section millimetre by millimetre. If the section changes then draw the section, or check if standard details provide a suitable section. If no standard detail then draw the section, it may need its own unique specification and structural design

Irrespective of whether using standard details or drawing project specific details the footing plan should provide reference markers to the appropriate details. The reference marks are either section markers showing where the section is taken, or detail markers showing the location of the detail.

Document Style

The documents (drawings) should have two information blocks. One block describes and identifies the project, the other identifies the individual document. Each view on the document should also be identified by title or reference number. Each project should preferably have its own unique identification number, and each drawing its own identification number. Additional codes may be provided for revisions and issues. An issue is not a revision. An issue occurs when a copy is sent to someone. A revision occurs when a change occurs to the drawing. A person certifying the design needs to be able to uniquely identify the document they are referencing.

Since the assumption here is that formal design and assessment of suitability is outsourced, and the people doing such have no need to produce drawings. It is also helpful if the drawing information block contains space for the designers or assessors approval stamp. The approval stamp usually contains some disclaimer about the limits of the check, and a date as to when the check was made and a signature.

In a traditional design consultancy office, drawings have signatures of drafters, designers, drafting checker, design checker, and chief designer (architect, engineer). That is it goes through a process of production, checking and approval: in-house before released to regulatory authorities. For sole practitioners these tasks are all carried out by one and the same person. If the design is critical then it is recommended to get an independent review carried out, before seeking regulatory approval.

Note here the first review is in place of formal design process. The independent review is additional to this, its purpose is to ensure that the design assessment reached the correct conclusion.

Conclusion

The specification of residential footings to AS2870 should be within the capabilities of building designers, as in the main it is just a design-drafting exercise. A large portion of structural design is a design-drafting exercise and there is no reason why architects and building designers don’t actually design (qualitatively) and document the whole building.

Whilst there maybe need for specialist designers, there is not necessarily a need for these designers to employ their own drafters. The building design and architectural practices could produce all the drawings. Especially these days given the use of advanced building information modelling (BIM) software.

The specialist designers can mark up the paper drawings as they usually do, the drafting company can modify the computer model and documents accordingly.

Some building projects do not need separate structural drawings and it is more productive to add the extra structural specification to the building design drawings.

Building design and drafting services businesses can thus increase the extent of their services by producing structural drawings as well as building drawings. Fees and delays for engineering can then be reduced, if not eliminated.

If as a building designer you can provide the footing layout plans to the engineer, then the engineer doesn’t have to wait for their own drafter to draw up the footing before they start checking the numbers.

Similarly if as a building designer, you can organise a surveyor and soil bore logs, then time will be saved.

If the soil bore log companies can type up their bore logs so that they are consistently readable, and don’t have to get familiar with an individual’s handwriting then that will save time. If typing the data into a computer, then could equally well produce a report which calculates soil heave and classifies the site to AS2870. If using a GIS, can also provide a soil heave contour map, as well as classification of adjacent sites: which would help determine if additional bore logs are required for the current project.

In short the design and/or specification of residential slabs and footings could be a lot faster.

Owner-builders

Also for owner-builders, don’t forget that, your house doesn’t have to have a concrete slab on ground, nor does it have to have brick walls.

The bricks are typically decorative cladding on timber or steel framing. Other cladding systems can be used to keep the wind and rain out. If other cladding systems are used then can typically use smaller concrete footing beams.

Alternatively can suspend the floor 600 mm above the ground, maintain access to plumbing below the floor, and provide ramps up to the floor. Noting that finished floor levels (FFL) are typically required to be at least 100 mm to 200 mm above kerb level: however such is a prescriptive guide it isn’t always sensible nor required: so don’t go modifying (filling) the site until had some proper stormwater design carried out.

In short there should always be a step between the outside and inside floor levels, preferably a minimum of 100 mm to avoid it being a tripping hazard. If no step then hopefully the pavement is properly ramped away from the building and door openings.

Traditional houses were constructed with timber floors. The 600 mm clearance from the ground, provides a crawl space to inspect the timbers, and otherwise provide adequate ventilation for the timbers and keep it clear of surface storm water. Since the clearance is to provide crawl space, it should be measured from the bottom edge of the lowest structural member. Traditional houses had steps to reach the floor, today it is preferable to have ramps. The ramps are more suitable for access/egress by: wheelchairs, baby prams, and the elderly using wheeled walking frames, persons with hip and knee problems will also find the ramps better. Don’t think of it as design for the disabled, think of it as human factors design.


NB: If you are a drafter, or building designer interested in producing own slab and footing plans, then we can provide drafting review and guidance (for a fee). Contact Conrad

Also whilst Roy is still available for final review, we can also do the engineering if needed. However, in the long term, advising people to go elsewhere for geotechnical engineering, as I personally don’t do calculations for dirt and concrete. I don’t mind getting up to speed on concrete design if there is a demand, that’s just another material (albeit its a complex composite material). Soil mechanics however is another extensive science. Though if there is adequate demand here on the Yorke Peninsula, I may consider becoming an employing business and employ someone on staff. Preferably I would expect them to have geotechnical knowledge relevant to coastal engineering, agricultural engineering, and mining engineering, as well as residential footings.


Revisions:

  1. [29/03/2019] : Original
  2. [30/03/2019] : Expanded