Role of engineers in seismic design and detailing of reinforced concrete buildings in Australia.
In Australia, there is a generation of engineers, contractors and clients who are not convinced that damaging earthquakes occur in Australia, despite the long history of earthquakes in Australia and the 1989 Newcastle Earthquake (IEAust 1990). It may be perception or simply not having experienced an earthquake event. With the pressures of modern design and construction, there is insufficient time to think about the issues, as well as the many requirements of the Building Code of Australia (ABCB 2016).
The fundamental principle of concrete design for the successful performance of a concrete structure under seismic actions is that design and the reinforcement detailing are inseparable. Engineers have the role of selecting appropriate reinforcement detailing which is crucial to ensure that the structure will respond under seismic loading in the manner for which it has been designed. This is also a delicate balance between life safety and the client's cash flow. Time and time again, earthquakes have shown that correct detailing of reinforced concrete structures can significantly improve the capacity of the building to resist seismic actions, even for a poorly designed structure or a structure subject to a much larger event.
The Steel Reinforcement Institute of Australia (SRIA) has spent the past 2 years researching and redeveloping the former 1995 brochure on seismic design. The new SRIA publication Guide to Seismic Design and Detailing of Reinforced Concrete Buildings in Australia provides guidance on a range of issues relating to seismic design, detailing and construction. The overarching goal of the new publication is to improve the robustness of the built form, by shaking up the understanding of seismic design and detailing in Australia.
To achieve this overarching goal, the new SRIA Guide is intended to provide designers with:
* A better appreciation that the risk from seismic hazard in Australia is 'real' and that it must be properly considered during design
* A clearer understanding of the key philosophical differences between designing for wind and earthquake and the design consequences that follow
* To highlight common mistakes that designers are currently making (often unknowingly) in their seismic designs
* A few simple cost-effective design and detailing improvements that will improve robustness
* Reliable reference material, including the new SRIA Guide, and relevant conference papers for further consideration and study
While there have been many significant earthquakes in Australia over the past 150 years, fortunately most have occurred in remote locations where the damage caused was minimal. The top five worst Australian onshore earthquakes in modern times ranked by cost, magnitude and damage, according to an Australian Geographic July 2012 article are considered to be:
* Newcastle NSW, 28 Dec 1989, M5.6
* Beachport SA, 10 May 1897, M6.5
* Meckering WA, 14 Oct 1968, M6.9
* Ellalong NSW, 6 Aug 1994, M5.4
* Adelaide SA, 1 March 1954, M5.5
The earthquakes at both Newcastle and Meckering occurred on public holidays, greatly reducing the death toll. Has Australia actually had a near miss? In terms of damage, in Newcastle there were 35,000 homes, 147 schools and 3000 buildings damaged over an area of 9000 square kilometres, nearly every building in Meckering was destroyed, and the earthquake in Adelaide (which was centred 12 kilometres away) resulted in a number of serious injuries, 30,000 insurance claims and damage to some 3000 buildings.
The reality is that earthquakes are a regular occurrence in Australia. The return periods can be long and they often occur in isolated areas, but when they occur in cities, as observed in Newcastle and Adelaide in Australia, and Christchurch in New Zealand, the effects can be devastating. According to Geosciences Australia (Latest News dated 16 August 2016), on average Australia will experience:
* One shallow earthquake of magnitude 6 or more every 10 years (equivalent to the 2011 Christchurch earthquake)
* Two shallow earthquakes of magnitude 5 or more every year (equivalent to Newcastle)
A major earthquake will generate the most severe structural demand ever experienced by a building. Given the rare and extreme nature of earthquakes, for economic reasons, designers are largely concerned about preserving life and preventing structural collapse. For most concrete structures, this will require the structural system to resist the imposed deformation inelastically over a number of load cycles.
AS 3600 (Standards Australia 2009) and AS 1170.4 (Standards Australia 2007) provide Australian designers with the minimum design rules for earthquake design for buildings to meet the low to moderate seismicity of Australia. Most commercial buildings in Australia are in situ reinforced concrete, designed and detailed in accordance with AS 3600. Complying with AS 3600 deems the structure to have sufficient ductility to provide an adequate level of life safety. However, this concept of life safety is often poorly understood or not properly articulated by designers.
For lower values of structural ductility factor, [mu] [less than or equal to] 2, detailing of the concrete is only required in accordance with the body of the Standard. For higher values of the structural ductility factor, 2 [mu] [less than or equal to] 3, additional reinforcement detailing is required in accordance with Appendix C of AS 3600. Levels of ductility [mu] > 3 are outside the scope of the Standard, and design and detailing to NZS 1170.5 (Standards New Zealand 2004), and NZS 3101 (Standards New Zealand 2006) is suggested. Design and detailing can also be carried out in accordance with ACI 318M-14 (American Concrete Institute 2014).
The SRIA Guide is not a complete document covering all design situations or requirements, but an assortment of basic seismic principles, design advice and fundamentals to assist and help designers of all experience levels. Over the past two decades, there have been significant advances in analysis software and our understanding of earthquake design has improved through advances in research and investigation of the actual performance of buildings under seismic loads. The SRIA Guide focuses on key, functional and practical aspects of seismic design and detailing of reinforcement with references to specialist information as technology and reduced design and construction times is shifting the focus away from the vital reinforcement detailing phase of the project.
2. Risk mitigation and low damage design for buildings
As demonstrated in the recent Christchurch event, client and society expectations may differ from the seismic performance achieved by complying with the minimum requirements of the Standards, which are only for life safety. Questions should be asked in the early phases of a project to establish an appropriate importance level for the building. For example, does a building such as a museum require protection of irreplaceable contents in the event of an earthquake, or is there a need to ensure buildings containing dangerous materials such as a biological laboratory dealing with dangerous viruses remain serviceable following an extreme event? The SRIA Guide contains checklists with questions to initiate these client/ designer discussions so these issues establish the scope from the project outset.
The highest level of protection for a building available is base isolation, which is common in high seismic regions such as Japan, but it has not been used in Australia. The next level of protection is to minimise the damage by using a more robust and regular structure with a higher level of ductility. This approach should ensure minimal damage to the primary structure even in rarer earthquake events, with many alternative load paths and backup systems designed and detailed for greater forces than the minimum required by the Standard. The great advantage of this approach is that by limiting the damage to the structure, not only should it remain operational, but it should also be repairable. Insurance may be less, and the mitigation of the risk of structural damage and loss of business continuity is achieved, but at an increased cost to the original construction of the reinforced concrete building. It is estimated that the increased cost would be as little as 1-3% of the total construction cost of a reinforced concrete building, over the lowest level of protection required by the Standard. This assumption is based on the structural cost being 25% of the total cost of the building, and the additional design and detailing representing an increased structural cost in the order of 5-10%, including professional fees.
3. Analysis and design
Design experience plays an important role in delivering a well-conceived quality design with adequate detailing including the earthquake design. This process of engineering analysis and design satisfies the clients brief by delivering a safe, serviceable, aesthetic, economical and sustainable structure. Designers should always strive for simplicity, clarity, excellence in their design and detailing and maintain a strong focus on the detailing of reinforcement for seismic loads, as part of the design process, not as an afterthought.
Traditionally, earthquake design has been based on a quasi-static forces approach where hypothetical static loads are applied to simulate the dynamic forces of an earthquake. This may in fact bear no resemblance to the forces in an actual earthquake event, as earthquakes do not know about Standards, methods of design or indeed the building being designed. For this reason, the earthquake actions will almost certainly not be those specified by the Standards and demonstrates why seismic reinforcement detailing delivers the actual building performance.
There are a number of fundamental problems with the force-based method of analysis. These include choosing the right model, the selection of appropriate member stiffnesses and determining the static forces that are appropriate for the design being considered. Member stiffnesses cannot be resolved until the design is complete, and yet they will change during a seismic event. The distribution of local forces is based on elastic estimates of stiffness. This tends to concentrate the strength in elements of the greatest potential of brittle failure, such as walls.
In addition, the static design lateral forces applied to the structure may not bear any relationship to the actual dynamic forces applied under earthquake cyclic action. For example, AS 1170.4 does not contain a Clause for vertical acceleration, as the predicted values were not high enough to include separate criteria (in excess of the normal load factors) for the seismic analysis of the building structure.
There have been large advances in the past 20 years in our understanding of how concrete performs under seismic loads, in the technology and design of concrete, changes to reinforcement and enormous advances in computers, software and analysis tools. The computing power and software now available to designers has led to far more elaborate and sophisticated analysis and design of buildings and indeed more refined design.
This technology can lull the designer into a false sense of security, believing they fully understand how the structure will act under the dynamic loads of an earthquake, when the actual effects of an earthquake may be far different from the computer model. This is because the structure may be sized on non-seismic load considerations; member stiffnesses will change during the earthquake and other factors such as local failures will affect the model. This may result in the model and sophisticated analysis being entirely inappropriate in a major earthquake event.
History has shown that in real structures, earthquakes exploit the weakest link. The behaviour under load of individual elements can be complex depending on the materials used and many other factors, which will change under earthquake actions. Idealised computer models of the frame or structure are used for the analysis of a structure to simulate how the real structure may behave, but we must not lose sight of the fact they can be very crude when assessing the structure under seismic loads.
Analysis is only part of the design process. Good designers know there is far more to design than just analysis. Designers must understand the behaviour of each member and how they are expected to resist all of the applied actions, and why these members need to be detailed for the seismic actions. This is a process of systems thinking combined with practical detailing, and it is imperative that designers ensure viable load paths exist.
The concept of ductility also appears to be poorly understood in Australia because of our low to moderate seismicity and lack of appreciation that the structure must carry the bulk of the earthquake load inelastically to allow economic structures that meet the ductility demand and deliver adequate life safety. Designers will often compare the wind load (designed for elastically) with the earthquake load from AS 1170.4 and fail to appreciate that this only represents a small portion of the actual earthquake load that needs to be designed for elastically, missing the primary requirement that the bulk of the earthquake load needs to be carried inelastically through appropriate seismic detailing. For example, if the ductility of the structural system delivers a structural ductility factor [mu] = 2, then only half of the actual earthquake load needs to be designed for elastically. However, the structural system will need to be adequately detailed to resist the imposed deformation from the remainder of the earthquake load inelastically over a number of load cycles. Given the rare and extreme nature of earthquakes, this is the only design methodology available to achieve economic structures that deliver adequate life safety in extreme events.
4. Reinforcement and concrete
For a structural ductility factor [mu] [less than or equal to] 2, AS 3600 Appendix C, allows the structure to be designed and detailed in accordance with the main body of the Standard and both Ductility Class L and N reinforcement can be adopted as flexural reinforcement (Ductility Class L only in the form of mesh) and both Ductility Classes as fitments. Although not covered by AS 3600, any chord members, collector reinforcement or drag bars used in diaphragm action should be Ductility Class N reinforcement, because of the anchorage requirements and ductility demands for this reinforcement.
Although Ductility Class L reinforcement is allowed in the body of AS 3600 for load bearing walls and suspended floors and slabs when acting as diaphragms, designers need to ensure the reinforcement is capable of meeting the increased ductility and drift demands.
For a structural ductility factor, 2 < [mu] [less than or equal to] 3, structures have to be designed and detailed in accordance with the main body of AS 3600 and the additional requirements of Appendix C, with only Ductility Class N permitted as flexural reinforcement. For structures designed and detailed in accordance with Appendic C of AS 3600, or for Importance Level IL4 buildings, Ductility Class L reinforcement is not recommended in structural elements except where used as fitments for beams and columns, shrinkage/temperature reinforcement, for reinforcement to steel metal decking or non-structural elements, because of the increased ductility demands.
The quality of procured reinforcement steels must be verified to support the earthquake design assumptions and deliver the required building performance. This is achieved by simply obtaining 3rd party certification of supplied reinforcing materials.
While structures will have concrete strengths typically in the range of 25-40 MPa, high strength concrete up to 100 MPa is allowed under AS 3600. High strength concrete is principally used in columns and walls where the size of such elements needs to be minimised.
Designers should be careful using high strength concrete (>50 MPa) for the design of columns and walls for buildings designed for a structural ductility factor [mu] > 2, or with a post disaster function, as high strength concrete is a brittle material requiring additional detailing of the reinforcement to ensure ductile behaviour and prevent brittle failure.
Some of the items requiring consideration for columns designed and detailed in accordance with Appendix C of AS 3600 include:
* The minimum spacing of the column fitments should be in accordance with Clause C4.2.2(c) of AS 3600 (refer Section 10 below) and extend for a minimum distance equal to the largest column dimension or one sixth of the clear height above the floor slab, and the same distance below the floor slab or beam soffit.
* All fitments should be closed fimtents having a 135[degrees] hook at both ends.
Depending on the design load, the area of fitments and their spacing in columns may need to be adjusted to provide confinement to the column core in accordance with Clause 10.7.3 of AS 3600.
Careful consideration is required when using concrete strengths > 50 MPa for walls, including providing boundary elements where required, rather than using a higher strength concrete to satisfy the requirements of Clause C5.3 of AS 3600.
AS 3600 refers back to AS/NZS 1170.0 (Australian Standard, 2002) for robustness design. Some provisions for robustness are contained in AS/NZS 1170.0 and briefly discussed in the commentary to the Standard, but they essentially involve a general statement about tying the structure together and ensuring it is capable of supporting a nominal lateral load. Also, while the Building Code of Australia (ABCB 2012) states that a building must perform adequately, it only provides one alternative verification method to satisfy this robustness requirement, which involves the removal of a supporting column, transfer beam or section of load bearing wall. It is left up to the designer to determine the governing robustness requirement. Despite the limited way that robustness is dealt with, because of overseas experience and failures, designers must still consider the robustness of reinforced concrete buildings, including reinforcement detailing.
In simple terms, a structure should be safe and Section 2.1 of Eurocode 0 (BS EN 1990 2002) provides the following definition of robustness: the structure shall be designed and executed in such a way that it will not be damaged by events such as explosion, impact, or the consequences of human error, to an extent disproportionate to the original cause.
Progressive and disproportionate collapse must be avoided at all times. This means that the failure of one member should not set off a chain of events where the structure progressively collapses as occurred in the failure of the columns of the Newcastle Workers Club in 1989 (Melchers 2011; Woodside 2012).
Robustness will require that all structures have a resistance to lateral loadings, and if none are specified, then a notional percentage of the vertical loads should be adopted. Redundancy is also an important issue as failure of any single load-bearing member must not lead to the collapse of the entire structure.
The building structural form will significantly affect its robustness and for this reason needs to be considered at the concept stage. An example of this might be a large transfer beam supporting a significant part of the building. Failure of this element would be catastrophic and should be avoided if possible, or the design robust enough to provide a considerable reserve of strength. These critical elements can be designed elastically for the full earthquake load to ensure robustness.
Columns and walls should not be heavily loaded and designed so that the design values are below the balance point and all reinforcement is well detailed (Wibowo et al. 2014). Compatibility of drift with other structural members must also be considered.
Precast and tilt-up structures are more susceptible to the effect of abnormal actions than some traditional forms of construction because of the presence of joints between the structural elements. However, experience has shown that it is possible to manage these issues by effectively tying together the various elements of the structure and providing correct detailing (New Zealand Concrete Society, 1991; Wilson, Robinson, and Balendra 2008).
Buildings should have sufficient robustness to survive with minimal risk of collapse if subjected to a ground motion in excess of that specified by Australian Standards. Well-proportioned and well-detailed in situ reinforced concrete structures are inherently robust if detailed to ensure that plastic hinges do not form in undesirable locations. To avoid undesirable plastic hinges in columns, a strong column/weak beam concept is the preferred option (refer Section 8). Systems thinking is also essential to ensure the structure is tied together; can resist some notional lateral load, and the failure of a particular element will not lead to progressive collapse. There are several overseas documents on structural robustness and progressive collapse that provide more information (The Institution of Structural Engineers 2010; Byfield et al. 2014).
Reinforcement detailing for robustness also needs to address some basic requirements as follows:
* Minimum reinforcement should be provided in the top and bottom faces of horizontal members such as beams and floor slabs even if the design does not require it or detailing is not required in the Standard.
* Detailing of reinforcement in accordance with Appendix C of AS 3600 will be required to minimise damage, for buildings with a post-disaster function and for buildings where the structural ductility factor, [mu] > 2.
* Critical members should be reviewed for their role in the structure, detailed as required, and alternative load paths considered.
* Reduce the collapse risk of punching shear failures at columns by providing additional bottom face reinforcement as shown in Figure 1.
6. Acceptable drift limits
AS 1170 .4 sets out the maximum drift requirements for buildings. However, the maximum inter-storey drift due to reduced stiffnesses must not exceed 1.5% of the storey height at each level at the ultimate limit state. These lateral displacements can be large (in the order of 30-50 mm). Many structures may not be able to accommodate such drifts without premature failure of structural elements. Also, calculations associated with drift are often poorly understood as stiffness assumptions have varying degrees of accuracy.
Even if a part of a structure is not designed specifically to withstand seismic forces, it must be designed for the full drift (deflection) of the whole structure calculated in accordance with Clause 5.4.2, Clause 5.5.4 or Clause 6.7.1 of AS 1170.4. Moment frame systems are much more flexible than shear wall systems and need careful review for drift, especially when used in association with shear walls.
7. Ductility demands
One of the issues when designing structures in an area of low to moderate seismicity such as Australia is that when a major earthquake occurs which exceeds the design return period (generally 1/500 years), then the increase in peak ground acceleration over the design event can be significant and therefore the increase in the lateral forces can be large. For a rare event with say a return period of 1/2500 years, this can be of an order of three or more. This increase is shown in Figure 2 and reflected in the probability factor, [k.sub.p], in AS 1170.4: [k.sub.p] = 1.0 for a return period of 1:500 years and 1.8 for a return period of 1:2500 years. However, for structures designed in areas of high seismicity, the increase in peak ground acceleration between a 1/500 year and 1/2,500 year event is not as significant, perhaps 30%.
8. Structural systems
Structural systems should be as simple as possible with readily understood gravity and lateral stability load paths. Some structural systems are more satisfactory than others in resisting earthquake-induced forces. One of the early tasks of the structural designer is to select a structural system that results in the best system for seismic performance of the building within the constraints dictated by the client, architect, the site and other conditions. Wherever practicable, alternative structural configurations should be considered at the concept stage to ensure that an undesirable geometry or structural form is not adopted before the detailed design of the building begins. In particular, structural irregularities both vertically and horizontally must be considered early in the design phase, and sound structural engineering principles applied to avoid or mitigate the effects of these.
AS 1170.4 specifies that all parts of a structure shall be interconnected, in both the horizontal and vertical directions. Connections between structural elements are typically the weakest link in the chain and should be detailed to fail in a ductile manner to avoid rapid degradation of strength under earthquake actions as shown in Figure 3.
The connections must be capable of transmitting the calculated horizontal earthquake forces in order to provide load paths from all parts of the structure to transfer earthquake forces to the footings and foundation. In turn, the foundations must be robust enough to accommodate the overload due to large events without catastrophic loss of strength. Future excavations that change design assumptions must also be considered.
In Australia, stair and lift cores are typically constructed with concrete walls because of the fire rating and construction techniques which have developed over many years. As a result, most buildings in Australia will be either a concrete shear wall system or a combination of concrete shear walls and moment-resisting frames or moment-resisting frame only. The designer has to choose whether the shear walls are ductile or limited ductile elements. Once the structural system is chosen, the structural ductility factor [ and structural performance factor S can be determined in accordance with Table 6.5 (A) of AS 1170.4 or Table C3 of AS 3600. Ductile shear walls are often chosen where earthquake forces are high, as the seismic reduction factor ([S.sub.p]/[mu], will lead to smaller members, particularly foundations, and the detailing is not too onerous. The decision as to which design approach to take is left to the designer.
Because of the ratio of Structural Performance Factor, [S.sub.p], to the Structural Ductility Factor, [mu], the earthquake design actions will be increased by about 73% if the designer chooses an Ordinary Moment-Resisting Frame (OMRF) over an Intermediate Moment-Resisting Frame (IMRF). OMRFs only require detailing in accordance with the body of AS 3600.
One problem with Moment-Resisting Frames (MRF) is their lack of lateral stiffness and consequently, the large displacements (or drift) that can occur under earthquake actions. Other parts of the structure are often incompatible to resist such drifts. This can result in significant damage to adjoining structural elements and non-structural parts and components. In addition, the importance of any plastic hinges forming in the beams and not the columns in an extreme event must be considered.
Band beam floor systems are typically significantly stiffer than the columns, making the concept of strong columns and weak beams difficult to achieve with this structural system. However, the strong column/ weak beam requirement is only for IMRFs detailed in accordance with Appendix C of AS 3600. Also, for band beam systems designed in accordance with Appendix C, the lateral earthquake forces could be supported entirely by shear walls, avoiding the need for reliance on a moment-resisting frame, and columns could then be considered as simple struts carrying vertical loads. Note that the reinforcement detailing in columns would still need to allow for the expected drift of the structural system.
Also, where excess strength is provided above that theoretically required by the design through rationalising the design, less ductility is required for the element e.g. due to the provision of additional reinforcement for tying, or extra thickness or depth of section for fire requirements or deflections. Where structures are capable of carrying greater earthquake loads elastically, and detailing can typically be in accordance with the body of AS 3600, the reduced detailing required may also assist with buildability.
9. Responsibility for the design
It is recommended in the SRIA Guide that if a number of designers are working on the design and detailing of a concrete structure for seismic actions, the overall responsibility for the structural aspects of the project should be taken by one structural engineer called the Principal Designer.
The Principal Designer and the design team should preferably carry out all the structural design of the building. Where part of the design is assigned or subcontracted to others, the Principal Designer needs to understand and fully coordinate those designs and take overall responsibility for them. Examples of design by others are the design of precast concrete elements and post-tensioned floors.
The failure of the CTV building in Christchurch where 115 people lost their lives in this extreme event is attributed to the designer of the building. Not only was the designer not experienced in earthquake design and did not fully understand what was required, the senior engineer did not adequately supervise the inexperienced designer (Canterbury Earthquakes Royal Commission 2012).
10. Detailing and drafting of concrete elements
Conceptualisation, structural analysis and design are the first part of the overall design process of a structure and detailing and drafting the second part. Detailing and drafting consists of satisfactory plans, elevations, sections and details and an understanding of how each part of the structure will perform under seismic loads.
Detailing of the reinforcement is a vital part of the seismic design process for reinforced concrete. There must be sufficient transverse steel to prevent shear or crushing failures and anchorage of reinforcement into areas of confined concrete to avoid buckling of compression steel, once the cover to the concrete has been lost due to cyclic movements as shown in Figure 4. The main steel bars must not lose their anchorage into the concrete during the repeated reversing loading cycles in a major earthquake. The anchorage lengths connecting various parts of the structure together must be sufficient and allow for local failures.
Once designed, the art of reinforcement detailing is to ensure that the required reinforcement to meet the expected earthquake demands is provided in the correct locations. If the reinforcement is correctly placed and fixed in position and the concrete correctly placed around the reinforcement then the structure will comply with the intent of the design and should perform satisfactorily during its design life, including the rarer seismic events.
Detailing involves practical and detailed consideration of how and where the reinforcement should be placed. Experienced designers who understand the overall design and the seismic requirements of the building should be responsible for the clear detailing and specification of the reinforcement requirements on the drawings. Detailing must not be carried out by graduates, inexperienced engineers or drafters without senior supervision.
With the correctly detailed structural drawings, the reinforcement processer is able to process the reinforcement using the reinforcement schedules produced by the scheduler from the structural drawings, prefabricate any components and deliver the reinforcement to site. This will allow the steel fixers to place the reinforcement as designed and the builder/contractor to place the concrete around the reinforcement.
The detailing of reinforcement often occurs in the later phase of the documentation process, after the design is substantially completed, and the final drafting of the structure has commenced. Where possible, the structural design, including the drafting and detailing of the reinforcement should be completed prior to construction commencing.
In the design and detailing process, enough time must be left for adequate checking and coordination. Checking should occur prior to issue of the drawings for construction and processing of reinforcement.
The detailing requirements of AS 3600 generally follow those of ACI 318M-14 (American Concrete Institute 2014) with one notable exception for confinement reinforcement. Clause C4.4 in Appendix C of the current AS 3600 for an IMRF refers designers to Clauses 10.7.3 and 10.7.4 in the body of the Standard for confinement and restraint of longitudinal reinforcement to avoid the types of failures shown in Figure 4. In the 2001 version of AS 3600, the closed ties that extended over the distance D or [L.sub.o]/6 were required to be spaced at maximum centres, [s.sub.c] of [0.25d.sub.o], [8d.sub.b], [24d.sub.f] or 300 mm with the first tie located a maximum of 50 mm from the support face, or [0.5s.sub.c]. This requirement appears to have been lost in the current 2009 edition of AS 3600 and requires amendment. The SRIA Guide reflects the former confinement provisions and is supported by other Standards such as ACI 318M-14. This error is being addressed in the current AS 3600 revision.
With the trend to prefabrication of reinforcement off-site, attention needs to be given by designers as to how the components can be prefabricated and joined by drop in splice bars, known as loose bar detailing (CIA 2014). The positioning of the splice outside plastic hinge locations is critical for IMRFs designed to AS 3600 Appendix C.
Diaphragms in seismic design are the concrete floor and roof slabs that tie the structure together and transmit the seismic loads to the lateral force-resisting elements of the structure. They are a critical element in the design of any building for seismic actions and must be considered early in the design.
AS 1170.4 makes passing reference to the deflection of diaphragms in Clause 5.2.5. AS 3600 in Clause 6.9.4, states that insitu concrete can be assumed to act as horizontal diaphragms. Unfortunately, there is no guidance in either Standard on the loads, the design of the diaphragm or the transfer of actions from diaphragms into the vertical elements.
Diaphragms have a number of roles in a building including carrying gravity loads and imposed vertical loads; to provide lateral support to vertical load bearing elements and to transfer the lateral earthquake actions applied at each floor level into the lateral force-resisting system. They also have a number of other functions such as redistribution of loads around openings, redistribution of forces due to torsion, and for resisting inclined or offset columns.
One method for the design of diaphragms is to consider them as a horizontal deep beams, where the flanges take the tension and compression forces as shown in Figure 5. Designers can also use a strut and tie approach for diaphragm design. Diaphragms can also be rigid or elastic, regular or irregular, and have large penetrations, all of which can complicate their design and require trimming reinforcement arrangements.
Evaluating all the situations for the detailing of floor diaphragms requires experience and engineering judgement. For example, a building long and narrow in plan may be more flexible than thought, and the deformation of the diaphragm may not be able to be accommodated by the walls at either end, or by intermediate supports, resulting in separation of the walls from the diaphragm or failure of intermediate supports, and hence potential failure of the structural system below the design load.
Typically, edge beams form the edges of a diaphram. They need to be continuously reinforced with the longitudinal bars fully lapped for tension and compression, restrained for compression and adequately anchored to the concrete walls and columns.
Designers need to study how the forces from the diaphragm are transferred into and out of the vertical elements, particularly shear walls. A good understanding of how these forces are transferred is necessary to ensure adequate detailing.
Volume changes due to creep and shrinkage of the concrete, temperature changes and post-tensioning also need to be considered with diaphragms. Where floors are temporarily uncoupled from shear walls such as cores and lift shafts to allow for initial shrinkage, axial shortening, and post-tensioning effects, then correct detailing is required to ensure they will act as diaphragms in the final condition and are properly connected to the vertical supporting and lateral force-resisting elements.
Diaphragms will have a number of components depending on the design model adopted. Tension and compression members of the diaphragm are known as chords, and collector elements collect the shear forces and transmit them into the columns and walls. The earthquake forces must be transferred into the vertical supporting elements from the diaphragm, and these can be significant forces. The reinforcement used to transfer these forces is known as drag bars.
Failures of diaphragms in the recent high magnitude New Zealand Canterbury earthquakes were observed and a realisation that a more rigorous approach is required for the design of diaphragms and their connection to lateral force-resisting elements as shown in Figure 6. Designers need to consider these elements much more critically than they may have in the past (Gardner, Bull, and Carr 2008; NEHRP 2010).
Australia is an area of low to moderate seismicity, of low probability but high consequence in comparison to areas such as Japan and New Zealand. This is reflected in the provisions of both the design and detailing of reinforced concrete structures in Australia in accordance with the BCA and referenced Standards.
The reality is that earthquakes are a regular occurance in Australia and it is only a matter of time before a major capital city is struck with a Newcastle magnitude or greater earthquake. To satisfy a minimum sesimic level, building structures in Australia are required to at least be designed and detailed in accordance with the main body of AS 3600, using the specific Clauses in each section of the Standard to provide the minimum reinforcement detailing requirements, which are not that onerous. The minimum requirements will provide the levels of ductility and continuity of reinforcement, to allow the structure to meet the anticipated earthquake cyclic loading satisfactorily in a life safety event.
With some additional design and detailing to Appendix C of AS 3600, the building can meet higher levels of earthquake resistance more economically, while still providing adequate life safety.
It is important to provide a minimum level of ductility in both beams and columns framing into a joint and to ensure adequate confinement of column reinforcement, regardless of the type of structural system employed.
With a limited additional quantity of appropriately detailed fitments and continuity reinforcement at negligible cost, plastic hinges can be induced to form at predetermined locations. Yielding will be ductile (gradual), and even if the design earthquake load is exceeded, the formation of a plastic hinge will act as a 'fuse' preventing failure of the more critical column elements.
A fully elastic response by the structure is not economical and a non-elastic response is allowed by the BCA and referenced Standards. To reduce the risk of a catastrophic collapse and probable loss of life under a greater than design event, a ductile failure must still be ensured.
This minimum required level of ductility can be readily achieved by careful detailing and key design decisions such as reducing the axial stresses in the columns to ideally below the balance point. Close attention to walls and their axial stress levels plus boundary conditions is as critical as the primary columns.
Precast and tilt-up concrete construction requires additional care in detailing to ensure connection detailing is satisfactory and floors are adequately supported and will function as diaphragms in order to correctly transfer horizontal forces.
These simple performance improvements can be achieved using the SRIA Guide's effective design and detailing principles across key areas of weakness observed in past earthquake events. Adopting these principles will improve collapse robustness, reduce damage and provide confinement for resistance under cyclic loading. This redundancy is primarily achieved through practical and inexpensive detailing.
Real structural performance in earthquakes has demonstrated that this critical practice will deliver improved seismic structural system response for regions of low to moderate seismicity. To reduce risk, prevent future disasters, secure life and business continuity this publication has been made freely available on the SRIA website and is an essential resource for all building design professionals.
All Australian engineers should have a seismic design and detailing overarching goal of trying to improve the robustness of the built form to manage this real risk. To further assist the client/building owner, designers and the builder/contractor in this important process, specific seismic design checklists of questions have been developed to ensure that key issues are considered in the process for conceptualising, designing and detailing reinforced concrete for structural performance under earthquake actions.
Received 28 April 2017
Accepted 17 November 2017
Scott Munter and Eric Lume
Steel Reinforcement institute of Australia, Sydney, Australia
The SRIA would like to acknowledge the work of the contributing authors for the Guide to Seismic Design and Detailing of Reinforced Concrete Buildings in Australia who are:
* John Woodside, J Woodside Consulting
* Peter McBean, Wallbridge and Gilbert
No potential conflict of interest was reported by the authors.
Notes on contributors
Scott Munter is a structural engineer and executive director for the Steel Reinforcement Institute of Australia (SRIA). He represents the SRIA on various Standards Australia and industry committees. His research interests centre on the design and detailing of reinforced concrete structures where he develops design guides and education streams to assist practicing engineers and undergraduates. Over the past years, this research has focused on seismic design and has led to his co-authoring and editing of a new publication Guide to Seismic Design and Detailing of Reinforced Concrete Buildings in Australia.
Eric Lume is a civil and structural engineer with a wealth of industry experience including working for a consultancy in Christchurch, New Zealand following the devastating earthquake in 2011. He is currently the national engineer at the SRIA and his role includes ensuring that steel reinforcement is designed and detailed correctly in structures to ensure life safety in extreme events. Other interests include the design of (reinforced) masonry structures and concrete technology, both of which he lectured in at the University of Wollongong for many years.
Australian Building Codes Board (ABCB) NCC. 2016. Building Code of Australia, July.
American Concrete Institute, ACI 318M-14. 2014. Building Code Requirements for Structural Concrete. American Concrete Institute.
ATC/SEAOC (Applied Technology Council Jointly with the Structural Engineers Association of California). 1999. Briefing Paper 4 - Seismic Response of Concrete and Masonry Buildings Part B: The Role of Diaphragms.
Byfield, M., M. Wjesundara, C. Morrison, and E. Stoddart. 2014. "A Review of Progressive Collapse Research and Regulations." Proceeding of the Institution of Civil Engineers, Structures and Buildings 167 (8).
Canterbury Earthquakes Royal Commission, Part 3, Volume 6. 2012. Canterbury Television Building (CTV). Accessed http://canterbury.royalcommission.govt.nz.
Concrete Institute of Australia (CIA). 2014. Recommended Practice, Reinforcing Detailing Handbook for Reinforced and Prestressed Concrete, updated and republished 2014.
Eurocode 0. 2002. Basis of Structural Design, BS EN 1990.
Gardner, D. R., D. K. Bull, and A. J. Carr. 2008. Internal Forces in Concrete Floor Diaphragms of Multi-storey Structures During Seismic Shaking. 14th World Conference on Earthquake Engineering, Beijing.
The Institution of Engineers (IEAust). 1990. Newcastle Earthquake Study, 1990 and Conference on the Newcastle Earthquake, 1990.
Melchers, R. E. 2011. Investigation of the Failure of the Newcastle Workers Club, Institution of Engineers Australia. Australian Journal of Structural Engineering 11 (3).
National Earthquake Hazards Reduction Program (NEHRP). 2010. Seismic Design Technical Brief Number 3, Seismic Design of Cast in Place Concrete Diaphragms, Chords and Collectors, a Guide for Practicing Engineers.
New Zealand Concrete Society & New Zealand National Society for Earthquake Engineering. 1991. Guidelines for the Use of Structural Precast Concrete in Buildings.
Paulay, T., and M. J. N. Priestley. 1992. Seismic Design of Reinforced Concrete and Masonry Buildings. John Wiley & Sons.
Standards Australia AS 1170.0. 2002. Supplement 1, Structural Design Actions--General Principles--Commentary.
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The Institution of Structural Engineers. 2010. Practical Guide to Structural Robustness and Disproportionate Collapse in Buildings, October.
Wibowo, A., J. L. Wilson, N. T. K. L. Lam, and E. F. Gad. 2014. "Drift Performance of Lightly Reinforced Concrete Columns." Engineering Structures 59: 522-535.
Wilson, J. L., A. J. Robinson, and T. Balendra. 2008. "Performance of Precast Concrete Load Bearing Panel Structures in Regions of Low to Moderate Seismicity." Engineering Structures, July 30 : 1831-1841.
Woodside, J. W. 2012. "Discussion Paper on the Investigation of the Failure of the Newcastle Workers Club, Institution of Engineers." Australian Journal of Structural Engineering 12 (2).
Caption: Figure 1. Drop panel bottom layer reinforcement 3D view adjacent to service penetration (Photograph courtesy Peter McBean, Wallbridge and Gilbert). Note: top layer reinforcement not shown for clarity
Caption: Figure 2. Peak ground acceleration versus average return period (Paulay and Priestley 1992).
Caption: Figure 3. Failure of transfer beam, Copthorne Hotel, Christchurch (Photograph courtesy Peter McBean, Wallbridge and Gilbert).
Caption: Figure 4. Failure of column at the Hotel Grand Chancellor, Christchurch due to poor confinement (Photograph courtesy Peter McBean).
Caption: Figure 5. Floor as diaphragm (after ATC/SEAOC).
Caption: Figure 6. the shear walls remain amongst the ruins of the CTV Building, Christchurch, New Zealand (Photograph courtesy Peter McBean).
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|Author:||Munter, Scott; Lume, Eric|
|Publication:||Australian Journal of Structural Engineering|
|Date:||Oct 1, 2017|
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