Basic requirements as noted by building control officers Architects’ floor plans at all relevant levels of the building should be provided showing direction of span of the floor(s) and roof. Relevant section and elevation drawings will also be required. The Engineer will then be able to prepare relevant plans, section and elevation drawings to include information that will justify design loadings and sizing calculations for the proposed structural element(s).
All proposed structural elements will be clearly cross-referenced between design drawings and calculations. A notation system should be used and agreed with the Architect.
The design drawings will show the size (and centres, in the case of timber joists) of all proposed structural elements.
Architects’ existing and proposed layout drawings will be required. The proposed layout drawing should clearly show the area(s) of proposed works.
General details of connections between structural elements (including end-bearing details) will be provided. Design calculations to justify the adequacy of these connections and end-bearings will also be provided. In the case of piled foundations, steel frames, and the like, sub Contractors’ calculations will be required.
Where ties are to be provided, these should be clearly shown.
It is strongly recommended that all design calculations are to be in accordance with the latest codes of practice.
Structural Engineer’s (or designer’s) contact details including telephone number, facsimile number and e-mail address if available.
Revisions should be clearly highlighted in the drawings and calculations.
Good practice guide Chimney breasts should not be used for the support of new structure.
It is unlikely that the use of Gallows brackets would be approved where chimney sections are removed.
The allowable end bearing stress of a beam on old brickwork should be limited to 0.42 N/mm2.
In the absence of laboratory test results, the following typical value of allowable bearing pressure on virgin soil will be acceptable. Brick earth/clay 80 – 100 kN/m2. Sand/ballast 150 – 200 kN/m2. Other values should be agreed with the Building Control Officer where appropriate. Existing foundations may need to be exposed and checked to support new loadings.
When load bearing walls are removed, especially at the lowest floor level to create clear openings such as through-lounges, rear extensions etc, steel box frame(s) with adequate joint rigidity for the purpose of maintenance lateral sway stability/robustness to the structure. It is usually assumed that any stability contribution of adjacent properties are not to be considered in the assessment of the stability of the proposed structure.
For the purpose of controlling cracking in old/existing brick buildings, the total dead and live load deflection for beams supporting existing roofs and existing walls over new openings is to be limited to span/360 or 15mm, whichever is the lesser.
Timber floors offer limited restraint to steel beams. Positive fixings at say quarter points along the span of a beam can be provided to resist 1% of compression force in the flange as lateral load. As timber floors have limited diaphragm action, total force resulting from the restraint should be limited to 6kN.
Where beams are loaded eccentrically, design for torsion will be required or alternatively torsion restraint would be provided.
Retaining walls to light wells adjacent to pavements may need to be checked for the effect of wheel loads and consent obtained from the Highways Authority for design loading requirements.
Basement construction: Basement retaining walls should be designed in accordance with the appropriate design code of practice. Mass concrete storey height basement walls are not recommended. Shorter mass concrete underpins should be justified by calculations or sized in accordance with The Building Regulations 2000 Approved Document A, clause 2C24b on page 24. Sequence of excavation and construction work will be required.
Loft extension: Provide adequate decking to the flat roof and ties to the party walls. The flat roof is to be constructed in such a manner as to provide adequate diaphragm action capable of transferring shear from one end of the roof to the other. Any alternative design must shown adequate justification of roof stability.
Please note that the new Approved Document A 2004 Edition has come into force from 01.12.04. This has a considerable impact on the design of residential buildings of 5 storeys or commercial buildings of 3 storeys, where ties will be required both horizontally and vertically throughout the building.
Guidance notes for roof extensions and conversions. If you are considering converting an existing roof space or creating a new roof storey (for example a mansard roof or similar) it is necessary to consider a number of issues relating to the structural viability of the scheme before progressing with the design. At the most basic level it will be important to demonstrate that the new loadings on load bearing walls do not cause damage to the underlying structure, including the foundations. It should be recognised that in most cases, original buildings were designed only to carry the loads as considered by the original Designers. It cannot be assumed that loadings can be increased just by visual inspection alone. Wall thicknesses and foundation sizes will need to be established and in some cases, trial holes may be required to provide adequate information for design and analysis purposes.
In every structural scheme it will be necessary to satisfy the Local Authority Building Control Officer of a viability of the structural design. In cases where loadings are carried into party walls on each side of a building or indeed party supporting structures within the same buildings (for example in the case of individual flats) then questions will need to be answered as they arise. Every refurbishment project can be considerably different to each other and therefore it is difficult to anticipate all of the possible design questions which may occur.
Since 1985 it has been necessary to consider the effects of adding additional storeys above four storeys (including ground floor and basement levels) and additional strengthening for the new structure at high level has been required. Usually this takes the form of a strengthened upper floor level, to carry any ‘collapse loads’ from additional walls and roof above together with restraint ties to ensure that structural elements remain intact in the event of accidental damage. During the last year, there has been a revision to the Code of Practice indicating that all buildings from single storey up to four storey require horizontal tie restraints at each level under all circumstances. Buildings above four storeys require horizontal and vertical ties to be installed throughout the structure.
Residential buildings have now been classified dependent on the number of storeys. Up to four storeys classification is 2A in which horizontal ties only are required. Above four storeys classification is now 2B requiring the horizontal and vertical ties as noted above. In practice this ensures that the addition of a new storey either within the roof space or by replacing the existing roof on a four storey building would require special measures to ensure that all floors are attached to the party walls or other major structural load bearing elements. In effect this means that many buildings would need to be framed with columns and beams to ensure that damage caused to lower levels will not have an adverse effect on the structure causing a disproportionate collapse resulting from accidental damage situations.
Martin Redston Associates have discussed this matter with a number of Building Control Officers and Independent Checking Authorities to obtain clarification of these rules. There appears to be a general consensus that there is no possibility to waive these requirements. There may be however a possibility of mitigation if it can be proved that key structural elements are not damaged under accidental loading conditions. However this would mean that a pressure of 3.4 tonnes/ square metre could be applied.