Notes to Guide the Review of the Design of Small Buildings

by Tony Gibbs, Director, CEP

Note: This paper was presented at the USAID/OAS PGDM building inspector training workshop, held in Antigua in January 2001

1 General

It is important that a minimum of design and detailing decisions be left for the construction period. Ideally the project should be fully detailed before construction starts. It is better to sort out details in the comfort of an air-conditioned office rather than on top of scaffolding in the hot sun. Therefore the documents submitted for design approval should be complete. This does pose logistical problems which must be addressed in the organisation of the regulatory process.

2 Statement of Design Criteria and Standards

The design submission should be accompanied by statements on design criteria (eg basic wind speed, earthquake zone, rainfall intensity, gravity loads) as well as design standards (eg CUBiC, ACI, BSI). Also, the exceptions to the design standards should be stated (eg concrete strengths are less than the minimum requirement in BS8110).

3 Description of Subsoils

Pre-design soils investigations are essential for all projects. You pay for a soils investigation, whether or not you conduct one.

3.1 Test Pits

The minimum investigation would be by test pits. Even when a more detailed or deeper investigation is intended, it is often desirable to start with test pits. The results should be presented as logs indicating the depths of different strata. Test pits provide mainly qualitative information.

3.2 Sloping Ground

In steeply sloping ground the land developer (for multi-lot sites) should be required to present a geological report addressing the potential for land slippage. Single-lot projects may not be able to afford geological surveys. In such circumstances conservatism is advisable.

4 Siting

4.1 Boundary Survey

There are many cases of buildings being constructed accidentally on (or too close to) the neighbour’s property. Boundaries should be shown on site plans. Distances of the building from adjacent boundaries should be dimensioned on submitted site plans.

4.2 Location of Building

The topography is a consideration for wind speeds and, therefore, wind forces.

4.3 Utilities

4.3.1 Roads

The nearest highway, intended as the main communication route for the property, should be identified.

4.3.2 Water

The water distribution network from the public mains to the house and garden should be shown.

4.3.3 Sewerage

Details of sewage treatment and disposal should be given.

4.3.4 Electricity

The electric distribution network from the public supply to the house should be shown.

4.3.5 Telephone

The telephone distribution network from the public supply to the house should be shown.

4.3.6 Solid Waste

A statement should be made about the intentions (eg total reliance on the public collection system).

5 External Works

5.1 Access Road and Parking

Access roads, driveways and parking areas should be shown on submitted site plans. Construction details should also be given.

5.2 Stormwater Drainage

Details of drainage patterns and eventual disposal of water should be given.

5.3 Gates and Fences

These should be located and detailed.

6 Building Configuration

"I do not now intend by beauty of shapes what most people would expect, such as that of living creatures or pictures, but for the purpose of my argument I mean straight lines and curves and the surfaces or solid forms produced out of these by lathes, rulers and squares. For I mean that these things are not beautiful relatively, like other things, but always and naturally and absolutely, and they have their proper pleasure no way depending on the itch of desire." – Plato

6.1 Plan Shape

Unfavourable shapes raise warning flags. Such building designs need more-careful scrutiny.

6.2 Window and Door Openings

Check the distances from corners and the distances between adjacent openings. Check protection against hurricane winds and flying debris.

6.3 Balconies

Are the roofs continuous with the main roof? Are the roofs demonstrably stronger than non-projecting roofs?

6.4 Roof Geometry

6.4.1 Flat, Monopitch, Gable, Hipped

Flat is worst, hipped is best. Therefore flat roofs should be much stronger than hipped roofs in order to resist the same winds.

6.4.2 Slope(s)

The steeper the better, up to about 35°.

6.4.3 Ventilators

If they are located at the ridge they would help. In other locations their effect could be positive or negative, depending on geometry and wind direction. The detailing of any ventilator is critical if water ingress is to be avoided.

6.5 Overhangs

These experience higher wind forces than enclosed roofs.

6.6 Parapets

These reduce uplift wind forces at the eaves. However they need to be reinforced to resist horizontal wind and earthquake forces.

7 Foundations for the Building

"On account of the fact that there is no glory in the foundations, and that the sources of success or failure are hidden deep in the ground, building foundations have always been treated as stepchildren; and their acts of revenge for lack of attention can be very embarrassing." – Prof Karl Terzaghi (birthday: 02 October)

7.1 Type of Foundation

This should be consistent with results of the soils investigation. See item 3.

7.2 Depth(s) of Foundations

These should be shown provisionally on the pre-construction drawings. Depths may well be adjusted during construction, since the soils encountered are rarely identical to those found in the soils investigation. This does not invalidate soils investigations.

7.3 Sizes of Foundations

In addition to sizes, the expected save bearing capacity of the founding stratum should be stated.

7.4 Blinding

This should not be treated as an optional extra. Omission should be the exception.

7.5 Foundation Stays

These are required in earthquake zones where the bearing capacity of the founding stratum is less than (say) 2 tons per square foot (200 kPa).

7.6 Reinforcement

The details should be shown.

7.7 Strength of Concrete

In addition to strength, the durability should be addressed. In soils with significant sulphur content, special precautions are warranted (eg the use of sulphate-resistant cement).

7.8 Cover to Reinforcement

It is conventional for greater cover to be provided underground. This is not necessarily a well-founded practise (pardon the pun). However, there is no harm in doing this.

7.9 Excavation and Fill within the Building Perimeter

This has an influence on the performance of slabs on grade. Care should be exercised in the removal of all unsuitable soils and in the compaction of fill. These should be specified at the design stage.

8 Reinforced-concrete Frames

8.1 Relative Strengths of Columns and Beams

Where the main structure is the frame, the columns should be stronger than the connected beams for good performance in earthquakes.

8.2 Sizes of Columns and Beams

Minimum sizes are prescribed in the standards. These relate principally to earthquake resistance and fire protection.

8.3 Reinforcement

8.3.1 Amount and Distribution

These should be completely detailed or there should be a statement as to the constructor’s responsibility for these matters.

8.3.2 Laps and Continuity

Laps should be defined to achieve the desired continuity.

8.3.3 Ductile Details

These are often complex and difficult to execute. Larger-scale details are desirable.

8.3.4 Cover

This is the main determinant of durability. Strength of concrete is also very important in this matter.

8.4 Concrete Strength

8.4.1 Cubes or Cylinders?

The choice must be explicit since the two shapes do not give the same numerical results.

8.4.2 Curing

The acceptable method(s) should be stated.

8.5 Testing of Materials

Types and frequencies should be stated.

9 Structural Steel Frames

9.1 Relative Strengths of Columns and Beams

Where the main structure is the frame, the columns should be stronger than the connected beams for good performance in earthquakes.

9.2 Sizes of Columns and Beams

Nominal dimensions and mass/length should be stated.

9.3 Connection Forces or Details

9.3.1 Columns to Foundations

Holding-down bolts may be detailed by steel fabricator. In which case the forces should be supplied by the designer.

9.3.2 Beams to Columns

It is usual for these details to be determined by the steel fabricator. The forces should be supplied by the designer.

9.3.3 Welds

See 9.3.2.

9.4 Responsibilities of Fabricators and Suppliers

These should be clearly articulated.

9.5 Corrosion Protection

The materials, thicknesses and application methods should be specified.

9.6 Fire Protection

The materials, thicknesses and application methods should be specified. Fire protection may not be required for small buildings where evacuation within half an hour is easily achieved.

10 Concrete Block Walls

10.1 Strength

This should be specified for the blocks and for the mortar.

10.2 Geometry

10.2.1 Cores

Two-core blocks should be specified to facilitate reinforcement.

10.2.2 Face Shell

This should be specified as a minimum dimension.

10.2.3 End Zone

The acceptable geometries should be stated.

10.3 Reinforcement

10.3.1 Vertical

Amount and location should be specified. Method of placement may usefully be addressed.

10.3.2 Horizontal

Special block-work reinforcement should be specified. This reinforcement needs to be specially protected against rusting (eg by galvanising).

10.3.3 Connections to Adjacent Members

These should be detailed.

10.4 Mortar

This may need to be specified, including the use of special cements or additives.

10.5 Grout

Grout is preferable to mortar for filling the vertical cores.

10.6 Method of Construction

It may well be desirable to describe this. The USA method is recommended.

11 Concrete Slabs

11.1 Base Preparation for Slabs on Grade

See 7.9.

11.2 Geometry

11.2.1 Thickness

This should be given as a minimum dimension.

11.2.2 Edge and Local Details

These should be detailed.

11.2.3 Separation and Construction Joints

The former should be detailed. The latter should be specified in general terms (eg maximum distance apart).

11.3 Reinforcement

11.3.1 Mesh in Sheets

Mesh shipped from the factory as rolls should not be permitted.

11.3.2 Rebars

These may be used either as main reinforcement or as supplementary reinforcement.

11.3.3 Strengthened Areas

These should be detailed.

11.3.4 Cover

This should be specified as to the amount and the acceptable methods of achieving it.

11.4 Method of Compaction

Poker vibrators are ineffective in thin slabs. Vibrating screed should be specified.

11.5 Curing

See 8.4.2.

12 Timber Floors

12.1 Arrangement of Members

The overall scheme should be presented on layout drawings.

12.1.1 Spacing of Joists

This should be dimensioned.

12.1.2 Orientation of Floor Boards

If the floor must act as a diaphragm, the orientation of the boards may be important. In which case it should be specified.

12.1.3 Herringbone Struts

These act to spread loads from one joist to adjacent joists. If this is required by the design, the struts should be detailed.

12.2 Materials

The types and grades of lumber should be specified.

12.3 Sizes of Members

....are determined so as to satisfy the requirements of:

12.3.1 Strength


12.3.2 Deflexion

12.4 Connections

12.4.1 Timber to Timber

These should be detailed.

12.4.2 Timber to Other Materials

These should be detailed.

12.4.3 Durability

....should be considered.

13 Timber Roof Frames

13.1 Geometry and Structural System

See 12.1.

13.2 Materials

See 12.2.

13.3 Sizes of Members

See 12.3, except that deflexions are not usually critical in roof structures when subjected to wind uplift.

13.4 Connections

See 12.4.

12.4.1 Timber to Timber

12.4.2 Timber to Other Materials

12.4.3 Durability

14 Lightweight Roof and Wall Cladding

14.1 Materials

Quality (including strength) should be specified.

14.2 Geometry

14.2.1 Thickness

This should be stated in decimal fractions of millimetres (or Imperial equivalent) net of protective coatings. Gauge measurements are imprecise since there are several "standard" gauges (eg SWG, AWG, BWG).

14.2.2 Profile

This should be described.

14.3 Connections

14.3.1 Strength

Independent test results supplied through the manufacturer should provide the necessary comfort level. The test should replicate the multi-cyclical nature of hurricane loading.

14.3.2 Durability

To be considered.

14.4 Tiles and Shingles

14.4.1 Type

To be described.

14.4.2 Connections

Does the lever arm of the resisting fixing come close to the lever arm of the wind force?

14.5 Membranes

14.5.1 Type

See 14.4.1.

14.5.2 Method of Fastening

The adequacy can only be established by laboratory tests.

15 Windows and External Doors

15.1 Protection against Wind and Rain

This is largely a matter of detailing. When will the shop drawings be submitted?

15.2 Protection against Flying Debris

15.2.1 Laminated Glass

The fixing (with structural silicone) to the frames is important.

15.2.2 Polycarbonate

Examine for deflexions, crazing and discolouration.

15.2.3 Fixed Shutters

These too should exhibit wind and debris resistance. Are they likely to be in place in advance of a hurricane 15 years after the construction of the building?

15.3 Fixing of Frames

Is this defined on the drawings?

15.4 Securing of Doors

Examine the hinges, bolts and latches.

16 Rainwater on the Roof

16.1 Slopes

The rougher the surface the steeper the slope that is required. Deflexions of the structure are to be considered.

16.2 Edge Details

These should be carefully and fully detailed.

16.3 Gutters

16.3.1 Locations

Should be shown.

16.3.2 Sizes

By rule of thumb or calculation?

16.3.3 Inlet and Outlet Details

Inlets often interfere with structure, or vice versa. Outlets often reduce (critically) the strength of columns.

16.4 Disposal at Perimeter of Building

Details should show the transition from builders works to external works.

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Page last updated on 05 Jun 2001