Ministry of Business Innovation and Employment Building and Housing Codewords 53

steel construction

Issue 44 – November 2012

Working hard in Canterbury

Providing technical guidance for the rebuild

Much of the Ministry of Business, Innovation and Employment's (MBIE's) Building and Housing's work in Canterbury is technical. This includes producing technical guidance to help ensure that repairs and reconstruction work will withstand future earthquakes and will keep people safe. The three foundation technical categories – TC1, TC2, and TC3 – are part of this guidance.

A comprehensive sector education programme is under way to help practitioners apply this guidance. The training programme is designed to help the sector deliver efficient, quality repairs and rebuilds for Canterbury homeowners.

Initial training for Canterbury building consent authorities was held in August, with further training for staff scheduled for October. Workshops for engineers and architects are planned for November and December 2012.

→ Read MBIE's Canterbury earthquake: Technical guidance (http://www.dbh.govt.nz/canterbury-earthquake-technical-guidance)

Supporting the Royal Commission with technical advice

MBIE's Building and Housing Group has provided expert technical advice to the Canterbury Earthquakes Royal Commission of Inquiry, which recently held hearings into the performance of central city buildings, including the CTV building.

The findings of the Royal Commission, along with Building and Housing's recent technical investigation into the structural performance of key Christchurch CBD buildings, will inform future building design and construction. This significant work will underpin changes to the Building Act, the Building Code, and also professional standards – and will ensure NewZealand learns from the earthquakes.

→ Read about MBIE's Canterbury earthquake: Technical investigation (http://www.dbh.govt.nz/canterbury-earthquake-technical-investigation)

MBIE's Building and Housing's review of the Building Act's earthquake-prone building policy provisions and provisions will help the Government respond to the recommendations of the Royal Commission – and will help local government and the sector to implement the Commission's recommendations.

'Grand Designs' comes to Christchurch

Design professionals and property developers are invited to enter an exciting design competition for a housing development to be built in central Christchurch. Christchurch City Council and the Ministry of Business, Innovation and Employment are collaborating on the Housing Showcase Design Competition launched on 15 October. This competition is part of 'Breathe – The new urban village project'.

→ Register your interest at http://www.futurechristchurch.co.nz/breathe

MBIE update

This article briefly outlines progress with finalising the new Ministry's structure. The high-level structure for the new (MBIE) was confirmed on 31 July 2012, and the detailed operational structure was confirmed on 17 October 2012.

MBIE will have eight groups – two corporate and governance, three policy-related (including Building and Housing functions), and three for service delivery/operations.

Technical update

Lap lengths for reinforcing steel mesh fabric

This article advises how to make sure that sheets of reinforcing steel mesh are properly lapped to prevent a crack from forming and the slab (or other concrete member) from failing, at the lap joints.

Following the series of Canterbury earthquakes and the damage to concrete slabs-on-ground, measures were introduced to the Building Code's supporting document B1/AS1, to increase the resilience of concrete slabs on good ground against earthquake shaking. One of these measures was the requirement for Grade 500E reinforcing mesh fabric.

Reinforcing steel mesh fabric is used (with and without other reinforcing steel) in concrete members such as floor slabs and walls. It resists forces and displacements induced by gravity loads, earthquake shaking, ground movement, and shrinkage strains. Reinforcing steel mesh fabric is made from wires that are welded together to form sheets.

To ensure good performance, it is necessary to make sure that the sheets of reinforcing mesh are properly lapped to prevent a crack from forming and the slab (or other concrete member) from failing at the lap joints.

Laps need to be able to develop more than the elastic yield strength of the reinforcement, otherwise sudden failure can occur at the lap.

This may be achieved either by following the Building Code supporting documents, or by an alternative solution proposal.

Building Code supporting document methods

All reinforcement must meet the requirements of steel reinforcing Standard AS/NZS 4671:2001 Steel reinforcing materials.

Verification Method B1/VM1

The Verification Method B1/VM1 cites the NZS 3101.1&2:2006 Concrete structures Standard, which provides two methods for determining the lap length. Both methods will produce a lap that is at least as strong as the characteristic elastic yield strength of the longitudinal wire that is being lapped.

Using two cross wires for anchorage: Clause 8.7.6(a) of NZS 3101:2006 requires a lap of one mesh space plus 50mm.The minimum is 150mm for plain round wire mesh. This relies on a cross wire weld developing in shear half the elastic yield strength of the largest mesh wire at the joint.This method may be used for both plain and deformed wire mesh. The Building and Housing Group's guidance on reinforcing mesh is based on this method (see Guidance on reinforcement for concrete slabs-on-ground). (http://www.dbh.govt.nz/earthquake-concrete-slabs-guidance)

Straight lapping of mesh wires: This method can only be used to lap deformed wire mesh. The Building and Housing Group's guidance on reinforcing mesh specifies a lap length of 40 deformed wire diameters. A more optimal lap length may be calculated from NZS 3101. See Clause 8.7.6(b).

NZS 3101 does allow anchorage of plain round wire using hooks or bends (Clause 8.6.4) or mechanical anchorage (Clause 8.6.11). However, these methods are normally not practical or economic.

Acceptable Solution B1/AS1

This modifies NZS 3604:2011 Timber-framed buildings by requiring a lap of 225mm, or the welded wire manufacturer's requirements – whichever is greater.

Alternative solution proposal methods

Lap lengths can be varied from the above, if sufficient testing is carried out.

Testing must include lap length tests and cross bar weld strength tests, and must specify all the relevant test parameters, such as the minimum 28-day concrete compressive strength and wire mesh fabric properties (verified as meeting or exceeding AS/NZS 4671).

The assessment should also look at the potential impact on concrete crack formation at high stress levels. In other words, even if the lap length test performance is adequate, additional lap length allowance is still required, to limit crack formation resulting from the loss of reinforcement bond.

Such testing and assessment should be accompanied by a technical opinion from a building professional such as a chartered professional structural engineer.

For these Alternative Solutions, the information should be presented in such a way that building control officials can easily see how side and end laps are made, and that the basis for them is different from those laps specified in the Building Code supporting documents.

Related Standards

Concrete tile roofs amendment

In August 2011, E2/AS1 was amended to accommodate an increase in 'design wind speed' and 'design wind pressure'.This article describes how these amendments affect concrete tile roof underlays and underlay support.

In August 2011, Amendment 5 to E2/AS1 was published. This included several changes to accommodate an increase in the maximum design wind speed.

This change aligned E2/AS1 with the increased wind speeds in NZS 3604:2011 (Timber-framed buildings), increasing the maximum design wind speed by 10%, and design wind pressure by 20%.

These are significant increases. They resulted in the creation of a new wind category of Extra High (EH) for design wind speeds above 50m/s, but not exceeding 55m/s.

Most detailed information throughout E2/AS1 remained unchanged for wind categories up to Very High (VH). However, some special wall and roof cladding requirements were added for cladding materials and installations in the EH wind category.

The amendment affects concrete tile roof underlays and underlay support.

Concrete tile underlays

The majority of roof installations are Type I (double pan) tiles on roofs steeper than 20 degrees, for buildings with Low, Medium or High site wind speeds.

These do not require roof underlays.

However, the tile manufacturer's literature may recommend the inclusion of underlays as good practice, regardless of E2/AS1 minimum requirements.

Note that roof underlays are required for all concrete tile roofs in sites with Very High and Extra High wind conditions, irrespective of roof pitch. This is a new requirement in Amendment 5.

All Type II (single pan) tiles and Type III (flat profile) tiles require roof underlays, irrespective of roof pitch or site wind speed. This is unchanged from the previous amendment. (Refer to E2/AS1 paragraph 8.1.5 for roof underlay type and installation details.)

Roof underlay support

Type R1 underlays require full support, including anti-ponding boards. (Refer to E2/AS1 paragraph 8.1.5.1 for underlay support and paragraph 8.2.5 for anti-ponding boards.)

Type R2 self-supporting roof underlays do not require support. Although E2/AS1 makes no distinction about the type of underlay and the use of anti-ponding boards, Type R2 self-supporting underlays, if well installed by being pulled taut over the fascia and secured prior to installation of first tiles, will achieve drainage without anti-ponding boards.

Homeowners (or their designers), choosing underlays that exceed the minimum requirements of E2/AS1 are not bound by the details in E2/AS1, and your specific manufacturer's installation details may provide discretion over the use of anti-ponding boards in those situations.

However, it is highly recommended that for concrete tile roofs below 17 degrees, anti-ponding boards should always be used.

Related Standard

Summarised from MBIE's Codewords 53, October 2012.

 

Published in building.