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Structural adjustment

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Two revised Building Regulations Approved Documents have crept up again - this time on how the building stands up

Part A

Surely the whole point of Building Regulations Part A was that if you were designing standard domestic dwellings using traditional construction methods, you could flick through the rule-of-thumb structural span tables and choose the best dimensioned joist, rafter, purlin and so on for the job. Nobody reads much more than that. The Approved Document's merit was to provide an easy-to-read structural solution so architects could get on with more important issues.

The revised Part A, which comes into force on 1 December, has removed the structural check-off tables altogether. You are now advised to get a copy of TRADA's Timber Tables* and keep them handy. This tendency of Building Regulations to refer to additional material is at best irritating; at worst it means that architects' practices need to update their library and referencing systems.

Although the span tables are pretty much similar, the TRADA tables have been recalibrated to take account of timber grades C16 and C24 in place of strength classes SC3 and SC4. As noted here before (AJ 7.9.00, pp4042), the new grades and old strength classes are not, in fact, interchangeable. Even though Part A:1992 span tables will still ensure that a building is structurally sound and building control will undoubtedly still accept reference to strength-class timber, and timber merchants will give you C16 if you ask for SC3, it is still advisable to get the new TRADA document for full compliance.

Typically, there are the little changes that you have to scour to find. The maximum allowable floor area 'enclosed' by structural walls on three sides has increased from 30m 2 to 36m 2; the section on plain concrete foundations now states that there should not be 'non-engineered fill (as described in BRE Digest 427)', whereas previously this was simply referred to as 'made ground'. Furthermore, trench-fill stepped foundations now get a mention and require that the overlap be twice the height of the step or 1m, whichever is the greater.

Curiously, the soil field test to assess the type of ground has been changed: Type VI soil used to be described as soil that was 'fairly easily moulded in the fingers and readily excavated'; now it is described as that able to accommodate a 'finger pushed in up to 10mm'. Presumably this is more quantifiably scientific, although Type VII is that which accommodates a 'finger easily pushed in up to 25mm'.

Depends on the pointiness of the finger, I suppose, but consequently the previous data on the minimum foundation widths allowable on Type VII soil has been removed, as have strip foundations with footings.

Wind loading now refers back to BS 6399: Part 2:2001, instead of the more modest CP3. Initial comparison of the diagrams of maximum wind speeds seems to suggests that the wind has died down over the 12 years since ADA:1992 came out, with contour lines appearing to show wind speeds at around half those of the original 1992 document. The new diagram is still called 'Map showing wind speeds in m/s' but now has a postfix that reads 'for maximum height of buildings'. Turning over the page you discover that you now have to calibrate the required answer taking into account topographic data, ground cover, altitude and location to work out the maximum allowable building height.

As a result of recent world events, section A3, one of the more interesting sections in the original document, has been bolstered substantially. The wording 'the building shall not be constructed so that in the event of an accident the building will not suffer collapse to an extent disproportionate to the cause' remains. Using a different wording, the clause still states that 'the area at risk of collapse of the structure (should) not exceed 15 per cent of the floor area of that storey or 70m 2'; the primary change is that the clause recommends that, in such an eventuality, the building remains stable. Removal of any element that cannot guarantee this is deemed to be a 'key element' requiring special attention.

Finally, the reference sections and 'further guidance' pages are extensive, although slightly over the top.

Maybe this is not something to be bemoaned because it seems to take into account every possible avenue of enquiry and will undoubtedly be useful to some.

In general, this document appears to be a useful advance on the ADA:1992, notwithstanding the ridiculous decision to remove the span tables. They have managed to keep the text to similar proportions to the prior document (although in some instances they have changed the order, which means that those of you well acquainted with the 1992 version will find yourself getting lost for a while), but first-principle explanations such as 'a significant change is when the loading upon the roof is increased by 15 per cent', are generally helpful.

* TRADA 'Span tables for solid timber members in floors, ceilings and roofs', available from the Timber Research And Development Association, £17.50 (non-members) £10 (members). Order via: www. trada. co. uk Part C The new Approved Document Part C, 'Site preparation and resistance to contaminants and moisture', comes into force on 1 December. Traditionally one of the more boring of the Approved Documents, it been given a makeover to include a range of issues not usually within its ambit. Even though sections have halved, the document has doubled in size from 23 pages to a hefty 49, with many more diagrams, although the generic detailing leaves something to be desired.

For example, diagram 9 on page 30 shows, for some reason, a cavity wall construction with the external dpc higher than the internal dpc. I was always taught that if the inner and outer leaf courses aren't level, then the best option would be that the outer leaf's coursing be slightly lower so that the wall ties would at least fall to the external leaf and that, at dpc level, it would be more difficult to bridge the cavity.

No reason is given for this detail.

The disabled-access threshold looks as if it has been conceived by a first year student. It now requires a circa 1.5-2.5 ¦ landing and a 15 ¦ sill, with, it seems, a gratuitously useless 'drainage slot' of indeterminate size and location thrown in for good measure.

The opening clause C1: 'Site Preparation' states that buildings will be deemed to satisfy if they guard the building (and 'persons in and about said building') against the adverse effects of 'contaminants on or in the ground covered, or to be covered, by the building and any land associated with the building'. This extends the coverage of this clause massively, potentially requiring extensive site investigations and remedial treatment. Indeed, clause 0.11 states that 'the environmental health department of the district council should be informed if contaminants are found on a site where the presence of contamination has not been formally recognised through the planning process'.

Although this might sound like a reasonably natural thing to do anyway, it also sounds as if property developers will now be required to do the investigation, assess the risk and pay the penalty (and add to the local authority's survey data for free).

Once again (see Part A above) the list of supplementary reading is significant in its scope and expense.

Page 12 alone has ten BRE Digests, one British Standard and one reference to the Foundation for the Built Environment (recently renamed the BRE Trust). Similarly, Section 6:

'Roofs' is trite, lasting for one page and relying on deemed-to-satisfy references to British Standards, Codes and Approved Documents. Acting as a central location for the dissemination of references is helpful, provided that this is not at the expense of clear and concise advice.

There are many handy hints dotted through the document, such as the examples of what contaminants might be left from certain types of industrial activity, but then there's the 'conceptual model for a site showing source-pathway-receptor'. This diagram shows the possible ways in which hazardous stuff can get at us, from accidental ingestion of contaminants in the soil to airborne contact with contaminated vapours.

The Protect and Survive feel of this section is enhanced by advice on contaminant attack on building services or structures. To allay fears, it suggests that developers/architects and others undertake a risk assessment to 'review all site data to decide whether estimated risks are unacceptable, taking into account the nature and scale of any uncertainties associated with the risk-estimation process'. There's nothing like being decisive, is there?

It advises us to be wary of vegetation (or its absence), surface materials (especially of unusual colours) and fumes and odours, all of which might be indicative of contamination.

All in all, this Approved Document looks like an advance on the previous one, but only time will tell how userfriendly it is.

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