Although buildings usually have set requirements, there are some conditions in which special recommendations should be followed so that these structures can withstand the forces dealt by the weather.
This first assignment for Building technology - Building within 1 km from the ocean, will deal with the various aspects in designing buildings that are to be built near coastal zones. In this assignment the requirements (Mostly from the Australian Standards) for Wind, Concrete, Masonry, Anchors and Fasteners Structural Steel, Timber work fasteners and other protective coatings will be looked at and documented.
The information used for this
assignment mainly came from the Australian Standards, while some textbooks
also had some relevant recommendations. The City of Wanneroo was also asked
if it had some requirements for building near the coast. The only two were
that the wall ties be either stainless steel or a plastic that is approved
by the Australian Standards and that any exposed Steel be hot-dipped galvanised.
The lack of requirements and information provided by the local government
authorities shows why so many parts and systems of buildings fail when
exposed to the weather conditions that are experienced near the ocean.
As expected buildings near the ocean will be the first ones to bear the brunt of the winds. Besides the force of the wind hitting the structure, other problems such as soil erosion and the abrasion of materials will also occur.
Terrain categories are a way that we can assess what weather conditions will occur in a given area. When building near the ocean and coast a housing site or area is most probably Category 1. In the Australian Standard (AS 1170.2), a Category 1 rating is described as being an exposed open terrain with few or no obstructions and water surfaces at serviceability wind speeds only.
First of all the strength of the building must withstand the pressure of the wind and the roof structure of the building should withstand enough pressure so it will not separate from the main structure. For this to happen the recommendations of a hoop iron strap going around the top wall plate and going 1200mm (14c) down again into the brickwork must be followed. Unfortunately this method of tying down the roof structure is seldomly used.
Another consequence of the stronger winds is that the glazing will have to withstand the wind pressure. The Australian Standard for Glazing (AS 1288) provides tables which show the thickness of glass when compared against the area of glass (M2) and then compared with the wind pressure(kPa). See figure 19.6.
During and after the construction phases, sand and dust provide a major problem firstly with abrasion of materials and secondly the erosion of the foundation material can, in some areas be a nuisance as some extra fill will have to be brought in to replace it. To overcome this problem some vegetation or paving should be established so that the soil has a means of being settled.
The flashing and sealing of
joints in buildings near the coast also must be looked at, as rain and
sea spray can enter places that wouldn’t under normal circumstances. In
this case a sealant such as silicone or similar material should be used
to seal were expansion can be expected and metal flashing should be galvanized
or treated in a way were it cannot be corroded or rust.
Finally all winds will cause materials to rattle so any items that are known to make noise should be tied down by anchors or other approved fasteners or fragile materials such as glass have a soft foam / silicone material to absorb some of the shock therefore reducing the noise factor.
Concrete can be one of the most important materials in construction, as it is the item that transfers the weight and pressure of the building down into the foundation. Therefore when building within 1km of the ocean special recommendations must be considered so the slabs or footings do not fail.
The Australian Standards (AS 3600) dealing with concrete, point out what type and strength of concrete is needed in particular areas of Australia. Under these standards the concrete listed is expected to stay from 40 to 60 years. In table 4.3 the exposure classifications are listed, and say that if the surface or exposure environment is Coastal (up to 1km from coastline but excluding tidal and splash zones) A1 standard concrete be used for plain concrete members which means with no reinforcing and reinforced members use concrete required at a B2 standard. Clauses 4.4 and 4.5 specify what concrete is needed for these classifications.
Þ 4.4 REQUIREMENTS FOR CONCRETE FOR EXPOSURE CLASSIFICATIONS A1 AND A2 Members subject to exposure classifications A1 or A2 shall be initially cured continuously for at least three (3) days under ambient conditions, or cured by accelerated methods, so that the average compressive strength of the concrete at the completion is not less than 15 Mpa.
Concrete in the member shall have an fc not less than -
(a) 20 Mpa for exposure classification A1 ; or
(b) 25 Mpa for exposure
Þ 4.4 REQUIREMENTS FOR CONCRETE FOR EXPOSURE CLASSIFICATIONS B1, B2 AND C Members subject to exposure classifications B1, B2 or C shall be initially cured continuously for at least seven days under ambient conditions, or cured by accelerated methods so that the average compressive strength of the concrete at the completion of curing is not less than 20 Mpa for exposure classification B1, 25 Mpa for exposure classifications B2 and 32 Mpa for exposure classification C.
Concrete in the member shall have an Ÿc not less than -
(a) 32 Mpa for exposure classification B1;
(b) 40 Mpa for exposure classification B2; or
(c) 50 Mpa for exposure classification
A quick summary of clauses 4.4 and 4.5 would be that the plain concrete be at a strength of 15 Mpa and that the reinforced concrete members such as the slab and footings be at a strength of 25 Mpa.
Another factor to consider when building within 1km of a coastal zone is the amount of cover on reinforcing steel in the concrete. If corrosion gets to the steel, the concrete would lose its strength and therefore be much weaker. In clauses 4.10.3 (Cover for corrosion protection - AS 3600) the cover for reinforcing, is given according to the exposure classification and the characteristic strength of the concrete.
The minimum amount of cover required for the exposure classification that we are in (B2) is 45mm were we are using concrete of 40 Mpa or 65 mm if the 32 Mpa concrete is used. These two cover lengths are to be used if standard formwork and compaction are used.
Alternatively if rigid formwork and intense vibration is used, the cover on 40 Mpa concrete decreases to 35 mm and if the 32 Mpa concrete is used the cover will only need be 50mm.
When building within 1km of the ocean the masonry of the construction is not the real problem, but the accessories such as mortar, wire ties etc. are the parts of the masonry system that can fail if they are not chosen correctly.
First of all the clay masonry bricks must comply with AS 1225. Secondly the mortar must comply with the general standards set out in AS 1315, 1316, 1317 and 1672. In summary these say that the mortar should be a mixture of a cementitious material, sand with little impurities in it and an adequate amount of water and other chemicals or additives so that the mixture is workable enough to place the masonry units in the right place, and be durable enough to withstand the local conditions. In the case of building near the coast, M3 mortar should be used.
As well as the masonry units and mortar being of the right standard, there must be at least 2 courses of damp-proof material to stop the moisture from penetrating through the walls in between the cavities. The mix for the damp-proof course is 1 part cement; 0 parts lime and 3 parts of sand (1:0:3). Flashings should also meet the necessary corrosion standards and steel in masonry should be of an E3 exposure classification or R3 rating for corrosion resistance. The following metals should be used in masonry construction;
(a) Galvanised steel that is hot dipped zinc or aluminium/zinc in accordance with AS 1397 or AS 1650.
(b) Cadmium coated steel complying with AS 1790.
(c) Grade 316 stainless steel which complies with AS 1449.
Weep holes should also be allowed for so that the excess moisture can drain from the masonry construction. These weep holes should be placed in the course above the damp-proofing and at centres not exceeding 1200mm.
Wall ties should comply with the standard required for steel in masonry as well as AS 2699, and should either be stainless steel, hot-dipped galvanized, plastic or of an other approved material. In addition they should be no more than at 600mm centres
The mortar joints shall be no more than 10mm in thickness in either the bedjoint or the perpends. All other steel in the masonry work should comply with AS 3700 (Clause 2.9).
The steel work in a building whether near the ocean or not needs to have some sort of corrosion protection to stop it from rusting. Any site will have moisture but being on the coast will accelerate the problem of the rust acting on the steel. AS 4100 deals with structural steel and for this topic the main issue is corrosion. Clause 3.5.6 on corrosion protection recommends that when a structure is to be exposed to a corrosive environment, the steel work should be given protection against it. The extent of protection shall be considered when compared to its use and the local weather conditions.
If the corrosion protection gets damaged, then the area that will be protected must be dry, clean and free from grease, before it is re-coated with a suitable material.
Timber work fasteners
Timber work fasteners should firstly be protected from corrosion depending on their use. Nails minimum diameter should be 3.15mm and be galvanized (hot-dipped). Other than the nail, more expensive fasteners such as stainless steel M10 or M12 bolts can be used as well as galvanized straps.
Any timber work must be protected from the rust especially near the ocean, as if the fixings in the timber work fails the building is just a stack of timber which can be easily destroyed with a strong gust of wind. As the environment near the sea is an environment conducive to corrosion, care must be taken when choosing the fasteners.
Anchors & Fasteners
Other anchors and fasteners follow the general standards of that for all other fasteners. In this category masonry nails could be used to fasten items to masonry. Dynabolts or Tigerbolts can also be used to fix and fasten an item to masonry or concrete. As these metal anchors are free from corrosion or have adequate corrosion protection they are the most logical choice for a metal anchorage system.
As well as these, Ramset guns and explosive power tools can be used to fasten steel to masonry in most cases. Other chemical anchor systems can be used but there are limitations on some of these near the ocean as the atmosphere can react with the chemicals inside the anchor.
Other protective measures
Most items that one would consider when building near the ocean are practical and common sense. All metal work no matter how large or small should have the adequate corrosion protection and all items that are subject to weathering be sealed with a material that can contract and expand with the seasonal movements.
While researching for this report I found it very hard to collate material as it was all put down in different standards that needed to be looked at. Secondly the Local government authority that was approached (City of Wanneroo) did not have any literature about building near the ocean for builders, and the requirements set by them, did not stop the failure of structures due to corrosion. Finally most of the requirements set by the Australian Standards were actually not practised in the building industry due to the fact of poor education by the sub-contractor, government authorities and most builders.
Þ Australian Standard 1170.2 (AS 1170.2) - (Wind)
Þ Australian Standard 3600 (AS 3600) - (Concrete)
Þ Australian Standard 3700 (AS 3700) - (Masonry)
Þ Australian Standard 4100 (AS 4100) - (Structural Steel)
Þ Australian Standard 1684 (AS 1684) - (National Timber framing Code)
Þ Building Construction, Volume 1 - Trust Publications