The basic building blocks of engineering design should be applied from project inception for there to be good building performance.
The purpose of this Practice Advisory is to remind building designers to pay attention during the design process to the basic engineering design building blocks.
This information was confirmed as current in December 2016.
This Practice Advisory is issued as guidance information in accordance with section 175 of the Building Act 2004 and, if used, does not relieve any person of the obligation to consider any matter to which the information relates according to the circumstances of the particular case. This document is not a compliance document in terms of the Act and not a substitute for professional advice.
The purpose of this Practice Advisory is to remind building designers to pay attention during the design process to the basic engineering design building blocks, including:
- collaboration within the project team for integrated design, all fully understanding owner requirements
- having a shared understanding of assumptions being made within the various disciplines
- understanding ground conditions, load paths and deformation compatibility.
The Canterbury Earthquakes Royal Commission (CERC) highlighted structural design deficiencies in a number of Christchurch CBD buildings. With other high profile failures such as the Southland stadium, there has never been a more critical time to pay close attention to the basics of engineering design principles to have buildings meet or exceed the minimum requirements of the Building Code.
Issues of concern
This practice advisory has been prepared as a result of incidents demonstrating:
- A lack of collaboration between building designers from the different disciplines resulting in lack of coordination, misunderstandings on the assumptions made by each during the design process and parties being unaware of design changes being made. This gives rise to the potential for inefficient design, possible rework, or the final built structure being overly conservative or, worse, under designed, and
- Inadequate attention to basic engineering design principles to consider both the overall building’s gravity and lateral load resisting systems and detailing to tie all parts and components together in the horizontal and vertical planes.
Collaboration between building designers
Building design is multi-disciplinary and requires a collaborative approach between all building professions to be successful. The design team is responsible for delivering a client’s brief, which is often led by a project manager or an architect.
Early engagement and interaction across the wider design team can greatly assist coordination and benefit the client and the design team financially, logistically and technically. For example, early collaboration can assist in refining design objectives, setting appropriate performance levels and identifying ‘best’ design solutions early in the design process. This is especially important when designing complex, non-conventional buildings.
Further to the need for teamwork, special attention should be paid to the following:
An integrated design approach enables synergies between all design disciplines to be realised. This design approach brings key project members, including owners, builders, architects and engineers, together early in the design process, resulting in greater collaboration and better solutions across the various design disciplines. The New Zealand Construction Industry Council’s Design Documentation Guidelines provides a good template to kick-start an integrated design approach. Sometimes the procurement process can hinder integration of design parties. All building design professionals should attempt to influence decision makers to enhance integration throughout the whole design, build and commission process.
Collaboration between architects and engineers
The Canterbury Earthquakes Royal Commission recommended that a structural Chartered Professional Engineer (CPEng) should be engaged at the same time as the architect for the design of a complex building. A joint agency Guidance Note – Improving Collaboration between Architects and Engineers – provides useful insights and is a recommended reference. It calls for a change to long-held attitudes and perceptions concerning roles and responsibilities of design professionals to realise the value of collaboration.
Collaboration between structural and geotechnical engineers
The Canterbury Earthquakes Royal Commission also recommend that there should be greater cooperation and dialogue between geotechnical and structural engineers. The Canterbury earthquake sequence highlights the importance of understanding the geotechnical effects on building performance in a seismic event and the inclusion of soil modelling in the structural analysis of a building.
|Involve all the designers earlier rather than later.|
|Embrace collaboration between the different professionals with open minds.|
|Work in isolation when areas of work overlap each other|
Engineering problems are often complex and can only be solved by making assumptions and applying judgement. Assumptions are inherent in engineering software and in design standards and methods.
Designers make assumptions in analytical models to predict structural behaviour. Assumptions vary in complexity from material properties to human behaviour. It is important that designers realise and state what assumptions are made, including those inherent in analysis or design tools.
More often than not, assumptions are not verified in design analyses. It is vital that designers test and verify the basic assumptions used in seismic design. In some cases, derivation from first principles may be needed to verify the assumptions used in assessing building response.
|Confirm validity of design assumptions against design standards or software assumptions.|
|Carry out the ‘what if this does not work?’ test and go to first principles if needed.|
|Assume that ‘she’ll be right’.|
Foundation performance is critical to the overall building performance and the following geotechnical basics must be emphasised:
- It is important to understand a site’s ground conditions, including its surroundings and its interaction with the superstructure. It is important that an appropriate level of geotechnical investigation is carried out. We strongly encourage the use of the Geotechnical Database to both upload and download data.
- Liquefaction of near surface soils during earthquakes can result in differential movement of foundations, both vertically and horizontally, tilting of buildings and bearing failures. Designers need to consider the impact of liquefaction.
- Ground behaviour will influence the structural response of the super structure. Inadequate knowledge of ground behaviour may invalidate structural design assumptions.
|Establish that underlying ground is suitable to be built on.|
|Investigate properties of the ground to optimise the design and deliver adequate performance.|
|Take into account the interaction between the building and its supporting ground|
Complete load paths are critical to good building performance. Clear and simple load paths will enable structural integrity to be easily checked. This is especially important when assessing existing buildings for robustness and determining retrofit solutions. The following fundamentals must be emphasised:
- Avoid complex or unreliable load paths.
- Alternative load paths with ductile detailing are encouraged. This will provide buildings with the robustness to sustain loading in excess of those estimated in design calculations.
- Reliable load paths accounting for overstrength actions are needed for both the global load resisting system and for details designed to tie all parts and components together, both in the horizontal and vertical planes.
Robustness and resilience are vital in a seismic environment.
Refer to SESOC Interim Design Guidance and AS/NZS 1170, Part 0: General Principles (refer particularly to Section 6 Structural Robustness) for further guidance.
|Confirm that designs have complete load paths and robustness to accommodate overstrength actions.|
|Ask the ‘what if this fails?’ question and provide alternative load paths for robustness.|
|Detail or design complex or unreliable load paths.
Evidence from the Canterbury earthquakes suggests that insufficient consideration was given to the displacement compatibility of different structural elements in a number of buildings. As a result, these buildings performed unacceptably in the earthquake sequence.
Assessing and retrofitting an existing building is complex and deformation compatibility between existing structural system and strengthening system is critical. In assessing seismic performance of an existing building, construction drawings should be obtained whenever possible. However, consideration must be given to identifying its true structural form recognising that actual construction details or materials may differ from plans or facades. Intrusive testing may be necessary in some cases.
When an existing building is retrofitted to improve its seismic performance, it is crucial to identify the failure mechanisms of the existing building, and the ability of the strengthening element(s) to deform and accept load in a way that is compatible with the retrofit performance objectives.
For more information on improvement of existing buildings, refer to:
MBIE (formerly Department of Building and Housing) has previously published Practice Advisory 2 on the Basics of Structural Engineering and Practice Advisory 6 on General Design and Construction Practice. Please also refer to these for further guidance on good engineering practice.
|Establish deformation compatibility of structural elements taking into account elongation to avoid loss of strength.|
|Design structural systems with incompatible deformation capacity.|
- Department of Building and Housing (2005). Pay attention to the basics - structural concepts and load paths (Practice Advisory 2).
- Department of Building and Housing (2005). Achieve best practice - every step of the way: General design and construction practice (Practice Advisory 6).
- Institution of Professional Engineers New Zealand (IPENZ), New Zealand Institute of Architects Incorporated, New Zealand Registered Architects Board, Ministry of Business, Innovation and Employment, Structural Engineering Society New Zealand & New Zealand Society for Earthquake Engineering, (2014). Improving Collaboration between Architects and Engineers. (Practice Note 1).
- Ministry of Business, Innovation and Employment (2014). Assessment, repair and rebuild of earthquake-affected industrial buildings in Canterbury.
- Standards New Zealand, AS/NZS 1170, Structural Design Actions, Part 0: 2002 General Principles.
- New Zealand Construction Industry Council (NZCIC), (2004). Design: Documentation Guidelines.
- New Zealand Society for Earthquake Engineering. (NZSEE), (2006). Assessment and Improvement of the Structural Performance of Buildings in Earthquakes.
- Structural Engineering Society New Zealand (SESOC), (2013). Interim Design Guidance: Design of Conventional Structural Systems Following the Canterbury Earthquakes.