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
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.
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).
Pre-design soils investigations are essential for all projects. You pay for a soils investigation, whether or not you conduct one.
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.
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.
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.
The topography is a consideration for wind speeds and, therefore, wind forces.
The nearest highway, intended as the main communication route for the property, should be identified.
The water distribution network from the public mains to the house and garden should be shown.
Details of sewage treatment and disposal should be given.
The electric distribution network from the public supply to the house should be shown.
The telephone distribution network from the public supply to the house should be shown.
A statement should be made about the intentions (eg total reliance on the public collection system).
Access roads, driveways and parking areas should be shown on submitted site plans. Construction details should also be given.
Details of drainage patterns and eventual disposal of water should be given.
These should be located and detailed.
"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
Unfavourable shapes raise warning flags. Such building designs need more-careful scrutiny.
Check the distances from corners and the distances between adjacent openings. Check protection against hurricane winds and flying debris.
Are the roofs continuous with the main roof? Are the roofs demonstrably stronger than non-projecting roofs?
Flat is worst, hipped is best. Therefore flat roofs should be much stronger than hipped roofs in order to resist the same winds.
The steeper the better, up to about 35°.
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.
These experience higher wind forces than enclosed roofs.
These reduce uplift wind forces at the eaves. However they need to be reinforced to resist horizontal wind and earthquake forces.
"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)
This should be consistent with results of the soils investigation. See item 3.
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.
In addition to sizes, the expected save bearing capacity of the founding stratum should be stated.
This should not be treated as an optional extra. Omission should be the exception.
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).
The details should be shown.
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).
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.
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.
Where the main structure is the frame, the columns should be stronger than the connected beams for good performance in earthquakes.
Minimum sizes are prescribed in the standards. These relate principally to earthquake resistance and fire protection.
These should be completely detailed or there should be a statement as to the constructor’s responsibility for these matters.
Laps should be defined to achieve the desired continuity.
These are often complex and difficult to execute. Larger-scale details are desirable.
This is the main determinant of durability. Strength of concrete is also very important in this matter.
The choice must be explicit since the two shapes do not give the same numerical results.
The acceptable method(s) should be stated.
Types and frequencies should be stated.
Where the main structure is the frame, the columns should be stronger than the connected beams for good performance in earthquakes.
Nominal dimensions and mass/length should be stated.
Holding-down bolts may be detailed by steel fabricator. In which case the forces should be supplied by the designer.
It is usual for these details to be determined by the steel fabricator. The forces should be supplied by the designer.
See 9.3.2.
These should be clearly articulated.
The materials, thicknesses and application methods should be specified.
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.
This should be specified for the blocks and for the mortar.
Two-core blocks should be specified to facilitate reinforcement.
This should be specified as a minimum dimension.
The acceptable geometries should be stated.
Amount and location should be specified. Method of placement may usefully be addressed.
Special block-work reinforcement should be specified. This reinforcement needs to be specially protected against rusting (eg by galvanising).
These should be detailed.
This may need to be specified, including the use of special cements or additives.
Grout is preferable to mortar for filling the vertical cores.
It may well be desirable to describe this. The USA method is recommended.
See 7.9.
This should be given as a minimum dimension.
These should be detailed.
The former should be detailed. The latter should be specified in general terms (eg maximum distance apart).
Mesh shipped from the factory as rolls should not be permitted.
These may be used either as main reinforcement or as supplementary reinforcement.
These should be detailed.
This should be specified as to the amount and the acceptable methods of achieving it.
Poker vibrators are ineffective in thin slabs. Vibrating screed should be specified.
See 8.4.2.
The overall scheme should be presented on layout drawings.
This should be dimensioned.
If the floor must act as a diaphragm, the orientation of the boards may be important. In which case it should be specified.
These act to spread loads from one joist to adjacent joists. If this is required by the design, the struts should be detailed.
The types and grades of lumber should be specified.
....are determined so as to satisfy the requirements of:
and
These should be detailed.
These should be detailed.
....should be considered.
See 12.1.
See 12.2.
See 12.3, except that deflexions are not usually critical in roof structures when subjected to wind uplift.
See 12.4.
Quality (including strength) should be specified.
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).
This should be described.
Independent test results supplied through the manufacturer should provide the necessary comfort level. The test should replicate the multi-cyclical nature of hurricane loading.
To be considered.
To be described.
Does the lever arm of the resisting fixing come close to the lever arm of the wind force?
See 14.4.1.
The adequacy can only be established by laboratory tests.
This is largely a matter of detailing. When will the shop drawings be submitted?
The fixing (with structural silicone) to the frames is important.
Examine for deflexions, crazing and discolouration.
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?
Is this defined on the drawings?
Examine the hinges, bolts and latches.
The rougher the surface the steeper the slope that is required. Deflexions of the structure are to be considered.
These should be carefully and fully detailed.
Should be shown.
By rule of thumb or calculation?
Inlets often interfere with structure, or vice versa. Outlets often reduce (critically) the strength of columns.
Details should show the transition from builders works to external works.
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