Dubai Seismic Design Paper

8th Pacific Conference on Earthquake Engineering 5 -7 December 2007, Singapore

Seismic design of high-rise structures in Dubai, UAE D.R. Wood Beca Carter Hollings & Ferner (SEA) Ltd, Singapore.

D. Whittaker Beca Carter Hollings & Ferner Ltd, Wellington, New Zealand.

ABSTRACT: Dubai is enjoying an unprecedented high-rise building construction boom. The Dubai area is not highly seismic, but a large earthquake in southern Iran could cause strong shaking and have serious effects on structures in Dubai. Tall structures tend to be more vulnerable to large distant earthquake sources and therefore pose a potential high seismic risk. A similar situation exists in Singapore, due to its proximity to the active seismic area of Sumatra, and many high-rise buildings may not have been designed for seismic effects. Like Singapore, Dubai does not have its own seismic design code. Buildings are designed to British Standards, plus the seismic loading and detailing requirements of the US Uniform Building Code (UBC), generally for Zone 2A (low to moderate seismic zone comparable to Auckland, New Zealand). The mixed use of British and US design standards provides a challenge to designers to understand which parts of the codes should be applied concurrently. Recent experience by the authors designing several tall buildings in Dubai has shown that wind loading tends to dominate over seismic, and wind vibration control tends to govern the stiffness of lateral load systems. However seismic overstrength can produce critical loading on some building elements and piled foundations. Architectural planning requirements often lead to vertical load transfer structures within these buildings. UBC deals with seismic design of such elements which require a margin of protection against inelastic behaviour, by means of a simple overstrength coefficient applied to earthquake design actions. Design to UBC seismic requirements is commonly fully automated, using design modules in software packages. By contrast, the more rigorous capacity design approach common in New Zealand makes the direct use of design software more difficult.

1 INTRODUCTION 1.1 Design Illustration This paper outlines the aspects of seismic design incorporated in a recent 50 storey building designed by the authors for Dubai. The design requirements of international codes commonly used in this region are discussed. Design practice in Dubai appears to incorporate most but not all of the US Uniform Building Code (UBC 97) requirements. There are implications for possible seismic design of structures in other low to moderate earthquake zones at long range from potentially very high magnitude earthquakes, especially for tall structures.

Paper Number 053

2 DUBAI PRACTICE 2.1 Local practice UBC Zone 2A is generally applied in Dubai, as required by the regulatory authority, Dubai Municipality (DM). Discussion with local engineers highlighted the opinion that the seismic zoning for Dubai ought to be 1, rather than 2A. Use of Zone 1 would reduce base shear to 53% of the Zone 2A requirement. We note that the development of a seismic code specific to Dubai is currently stalled. In the meantime, the requirements of Zone 2A seem to be implemented in the main, but with some key omissions. 2.2 Detailing of gravity framing The choice of seismic zone has implications for design detailing of gravity frames for example, which have typically been detailed in accordance with British practice. UBC and ACI 318 require detailing similar to moment resisting seismic frames for buildings in UBC Zone 2 and above. The level of detailing is dependent on the size of bending moments induced in the gravity members when subjected to the building displacements under seismic action, and can exceed the requirements for the lateral force resisting frames in some instances when applied to Zone 2 structures. We suggest that this need never exceed the detailing associated with Intermediate MRFs when incorporated in a building in seismic zone 2, but ought to exceed British code standard detailing in the beam column joints and end regions of frame members. 2.3 Amplification of seismic forces at discontinuities UBC requires that a stipulated amplification factor (dependent on the building type chosen but generally set at 2.8) be applied to the seismic forces to account for structural overstrength in certain circumstances and locations. This is to be applied generally to elements supporting discontinuous parts of the lateral load resisting system. These may include design of diaphragms for offsets in shear walls and design of transfer beams for transfer of column axial loads. The amplification factor is to ensure structural robustness and avoidance of catastrophic collapse rather than being directly related to the capacity design considerations of strain hardening and yield overstrength of reinforcement. It appears that Dubai practice is generally not to include this factor in designs. We did incorporate amplification for key elements such as transfer beams and transfer diaphragms. 2.4 Detailing of reinforcing steel It has been suggested that reinforcing steel ought to be specified and detailed in accordance with ASTM standards for consistency with UBC. However, the common practice in Dubai is to design for UBC seismic forces and to detail in accordance with UBC/ACI 318 for loads and bar layout in lateral load resisting elements and in accordance with British standards in all other respects of detailing. We consider this appropriate given the low ductility demands typical of the tall structures being built in the region. Our reinforcement details applied to the example building were largely to BS 8666:2005 with modification of the standard hook tail lengths.

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3 OUTLINE OF A RECENT BUILDING DESIGN 3.1 General building description The building consists of three basement carparking levels under a seven storey podium structure with 43 storeys of office and hotel tower above. Services transfer occurs at mid-height of the tower. The gravity load system is a combination of reinforced concrete core walls and perimeter reinforced concrete frames, generally with precast hollowcore floors. The podium structure at the front of the site accommodates entrance lobbies and ballroom / function spaces, requiring large spans to create column-free areas. These spaces and the open lobby entrance area are achieved by heavy steel trusses spanning up to 37 m. At the rear of the site, a multi-level carpark supports a landscaped deck structure which spans 50m from the tower edge. The deck and carpark are seismically separated from the tower by a movement joint achieved by corbel supports for the deck trusses at the tower edge. 3.2 Piled foundations Piles are of the bored concrete type and are 1200 mm and 1500 mm diameter, extending to depths of approximately 45 m below the lowest basement floor. 3.3 Precast floor detailing Hollowcore slabs are supported on corbels at the perimeter beam and core wall, with a structural topping for composite and diaphragm action. Two of the four voids in each unit are concrete filled at the ends, allowing additional dowel reinforcement to improve the vertical shear transfer and providing positive structural connection of the precast units to the main structure, in line with New Zealand recommendations. Where corbels cannot be formed due to height constraints, such as at some doorway openings in the core walls, steel ledge angles are installed to provide the floor unit seating.

Figure 1 – Example structure

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Figure 2 – Hollowcore end support detail

3.4 Columns The tower perimeter columns have been sized for minimum dimensions within strength and elastic shortening constraints. They carry substantial axial loads and are constructed of high grade concrete (C75 or f’c = 60 MPa) with up to 4% steel reinforcement. The tower perimeter changes from a curved shape in the upper floors at two opposite corners to the fully rectangular shape in the lower floors via transfer beams at the tower mid-height, together with some column overlaps. 3.5 Core walls The core walls transfer from a curved layout to a rectangular arrangement at the tower mid-height by way of a vertical overlap and wall thickenings. 3.6 Seismic and wind framing Seismic and wind loads on the building are resisted by a combination of the core walls and the perimeter framing. The seismic design philosophy is described in more detail later in this paper. Diaphragm action between the lateral load resisting elements is provided by structural topping concrete at each level. Specific transfer diaphragms at ground level, top of podium and tower midheight are of thicker in-situ concrete.

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3.7 Mid-height transfer floor Deep transfer beams are required at building mid-height to support setback columns at two opposite corners, due to a change in floor plate shape. The transfer is achieved by 4.2 m deep x 1.5 m thick reinforced concrete beams spanning between the core and the perimeter columns. 3.8 Upper level transfer structure The three reduced floor plate levels at the top of the tower require a further inward transfer of the building perimeter. Radial post-tensioned beams at the level 47 floor achieve this transfer. 3.9 Design basis and codes of practice The codes of practice used for the structural and seismic design are listed in Table 1. Table 1 – Codes of practice

Subject Loading

Reference Dubai Municipality Regulations on Building Conditions & Specifications Uniform Building Code (UBC) – 1997 ASCE 7 – 98 – Minimum Design Loads for Buildings and Other Structures BS 6399 – Design Loads

Seismic loading

Uniform Building Code (UBC) – 1997

Wind loading

Dubai Municipality Regulations on Building Conditions & Specifications Site and building specific wind tunnel study (for design) ASCE 7 – 95 – Minimum Design Loads for Buildings and Other Structures (for concept design and benchmarking) BS 6399 – Design Loads (for benchmarking)

Reinforced concrete

Steelwork

Uniform Building Code (UBC) – 1997 ACI 318-2005, Building Code Requirements for Structural Concrete Uniform Building Code (UBC) – 1997 (Seismic Design) BS5950-2000, Part 1 (Non-seismic Design)

It is normal in Dubai to design tall buildings for UBC 97 Seismic Loads, incorporating the detailing provisions of that code for the lateral load resisting elements depending on the level of ductility implied by the design. ACI 318 was chosen for all reinforced concrete design on the example project, so that loading and detailing were completely consistent with the UBC seismic loading.

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4 SEISMIC DESIGN PHILOSOPHY FOR TALL BUILDINGS 4.1 Structural classification Although Dubai does not currently have a seismic code of its own, buildings in Dubai are typically designed in accordance with the Uniform Building Code (UBC 97), using Seismic Zone 2A (low to moderate in terms of US seismic zones) in accordance with DM regulations. A dynamic lateral force procedure is required by UBC for buildings of the type considered here. A building response factor, R=4.5, was conservatively adopted for this building, since the building type strictly falls between the bearing wall and building frame categories. The R factor is defined by UBC as representing the “inherent overstrength and global ductility capacity” of the building lateral force-resisting system. The selection of R factor is not critical for long period structures in terms of the design seismic loading applied, because the response spectrum is scaled up to meet the minimum base shear level required by UBC 97.

WIND

SEISMIC 250 m ROOF

SLS accelns 10 yr Melbourne criterion a < 20milli-g SLS drifts (50 year wind) Drift < h/500 (BS 8110:Pt 2)

235 m TOC 210 m TRANSFER

Tower 1.6W Wind study

Parts load to UBC Fp = 0.35W p (R p = 4.0)

Transfer beams Em = Ω 0 E = 2.8 E

Check shear margin at critical sections MRS scaled up to min V b 125 m TRANSFER

Deep transfer beams Em = Ω 0 E = 2.8 E

50 m PODIUM

Shear transfer from core Nom elastic diaphragm shear

0 m GL

Shear transfer to basement walls

-12 m FDN

Equiv static E

Figure 3 – Governing lateral design criteria

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For the example building, the calculated dynamic base shear was approximately 0.9% g. The modal response spectrum was scaled up to the minimum base shear level of 1.7% g. While wind loads were found to govern the design of the tower portion of the building, seismic load combinations governed much of the pile and pile cap design under the core walls. Seismic loading generally governed the podium bracing elements by a small margin. 4.2 Foundations UBC requires that the piled foundations be designed to resist the Equivalent Static seismic forces defined in the code. The axial load effects in the piles due to overturning from the Equivalent Static forces were found to be very close to the effects obtained by applying the “overstrength” factor to the Modal Response Spectrum forces used in the design of the superstructure. It does not necessarily follow that this would be the case for all building sizes, and we regard the use of the “overstrength” forces as a necessary check on the UBC required actions to ensure some margin of strength over the superstructure. The transfer of base shear between building and ground was conservatively considered to be via flexure of the piles. Piles were designed to resist the amplified modal response spectrum shear, providing at least a 2.8 factor of safety against shear failure. The top section of the piles was detailed in accordance with the confinement requirements of ACI 318. Although UBC does not require the full potential plastic hinge zone confinement for piles of structures in Zone 2, we adopted it as good practice. The pile-soil interaction capacity was designed to allowable working load with normal factors of safety as defined by the codes, confirmed by in-situ load testing. The alternate basic load combinations of UBC 97 Clause 1612.3.2 were used, allowing for a 30% increase in allowable stresses for W or E combinations and a simultaneous reduction factor of 1/1.4 on the seismic effects. 4.3 Core walls As a bearing wall/building frame structure, the core walls were designed to carry all of the building lateral design loading, in strict accordance with the definitions in UBC 97. However, it is common to apportion the lateral load effects between walls and frames according to the stiffness-based allocation of the building model and design directly for these. There is no requirement in UBC for a margin against shear failure of walls when compared with flexural failure. However, for this design it was confirmed that a strength margin did exist at the tower/podium interface to allow the limited ductile response required of the core walls. 4.4 Frames The perimeter frames play an important part in the torsional rigidity of the building and carry around 15% of the lateral load in the tower according to the results of three dimensional computer modelling. They were designed as Intermediate Moment Resisting Frames (IMRF) in accordance with the specific loading and detailing requirements of UBC 97. For Zone 2 structures, the additional requirements for Intermediate Moment Resisting frames are limited to: •

Increased seismic shear contribution at member ends by a factor of two, in combination with gravity effects.

•

Increased confinement at the member ends.

4.5 Transfer elements It is appropriate to provide a margin of overstrength to elements within the structural framing system in which yielding should not be concentrated. The amplification factor, Ω0 = 2.8, has been applied to seismic loads acting on transfer structures (including transfer beams at the mid-height and upper levels 7

of the tower). Similarly, transfer diaphragms required at locations of shear reversal or specific transfer in the core walls were designed for UBC loads factored up by the ratio of nominally elastic (R=1.25) to ductile (R=4.5) shears at the key locations (ie. essentially elastic loads) to suppress potential brittle failure mechanisms in these important transfer elements. Because the default minimum shears are greater than the R=4.5 shears by a factor of about 1.9, the effective load factor (margin) for diaphragm shears is approximately 1.9 (= 4.5/1.25/1.9). The major transfers of shear occur at: •

Mid-height, where the core wall layout changes from three piers above to four piers below.

•

Top of podium, where the additional podium walls attract shear from the tower core.

•

Ground level, where basement perimeter walls provide the stiffer structural system and the reaction for the flexural cantilever of the core wall.

5 CAPACITY DESIGN 5.1 Comparison with NZ code approach The New Zealand Reinforced Concrete design standard, NZS 3101, requires that limited ductile wall structures be designed to withstand the shear derived from flexural overstrength of the walls, at the critical region. Detailing principles such as confinement of boundary zone regions and limitations on lapping of longitudinal bars at critical regions are also applied. Coupling beams in limited ductile wall systems are designed to ensure that flexural overstrength can be attained. UBC does not make provision for consideration of flexural overstrength in wall systems. We consider that it is appropriate to review the margin of shear strength available at the critical regions of walls, given the effectively limited ductile demand on the structures considered. 6 WIND EFFECTS 6.1 Wind studies required The tall structures considered are typically classed as wind sensitive and fall outside the bounds of the normal loading codes. A wind study was commissioned for the example building, incorporating derivation of windspeeds, combination of dynamic effects from wind tunnel testing, review of comfort level criteria and the effects of local wind environments. The resultant wind loading was found to be lower than those calculated from ASCE 98 and BS 6399. 6.2 Design criteria •

Wind induced drift at the unfactored characteristic 50 year wind load < h/500 limit defined by BS 8110:Part 2.

•

5 and 10 year wind induced accelerations at the upper floors are less than currently accepted limits. Generally Melbourne’s (1988) 20 milli-g peak acceleration at 10 year wind was the governing criterion.

7 AUTOMATED DESIGN 7.1 Software used The ETABS design module was used to design typical elements such as shear walls, beams and columns to ACI 318. Atypical elements such as many of the podium beams and transfer elements and members adjacent to discontinuities were designed and detailed manually.

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It was noted that axial shortening can be overestimated by ETABS because the load is applied instantaneously rather than incrementally, leading to unconservatively low axial loads in perimeter columns for example. In addition, the transfer walls and primary beams linked to the core tended to “hang up” parts of the structure off the core in the ETABS model. This was rectified by designing for column loads from the larger of ETABS output or the results of traditional tributary load take-off. 7.2 User beware In-house studies have indicated that there are several limitations to the automated design module in ETABS, requiring manual review and / or amendment. •

For beams, patterned live loading and layering of beam reinforcement are not considered.

•

Moment redistribution needs to be applied manually.

•

For columns, confinement is not considered fully.

•

Care (and manual review) is required in the definition of shear wall boundary elements.

•

Coupling beams and deep transfer beams are treated as ordinary beams, which is often inappropriate.

We would emphasize the need to conduct manual review (at least) of critical elements (eg. any transfer elements) and yielding regions of members. 8 SINGAPORE SEISMIC HAZARD 8.1 Current estimates of peak ground acceleration Megawati et al and Balendra et al refer to seismic design forces potentially well in excess of minimum code lateral forces currently applied to buildings in Singapore. Significant commercial buildings in Singapore are currently designed to resist lateral loading of the greater of the British code wind loadings (based on wind speed of between 30 and 35 m/s) and 1.5% of self weight. Wind loading is expected to govern the tower portion of very tall structures, however for intermediate height buildings of around 20 storeys, there is a strong suggestion that seismic forces may well govern the lateral design. With amplification of response spectra for the deep and flexible soils in parts of Singapore, it is apparent that building base shear could approach 6-7% g. Such levels of load may be possible from a Sumatran subduction earthquake of Mw = 8.0. Megawati et al refer to the very large events of 1833 and 1861 (estimated Mw = 9.0 and 8.5 respectively) together with 7.7 and 7.9 magnitude earthquakes in 1935 and 2000, as evidence that such earthquakes are a very real risk in the region. Effects in Kuala Lumpur may be significantly worse due to the shorter range of 500 km. 8.2 Comparison with UBC For the sample building on its type SB (rock) soil site, UBC sets a minimum base shear of 1.7% g, to which the modal response spectrum is scaled. A displacement ductility of about 2.4 is implied. According to the attenuation relationships derived by Megawati et al for Sumatran subduction earthquakes, a similar 50 storey building on a soft soil site in Singapore would most likely be adequately proportioned to resist seismic forces without ductility demand under the Mw = 8.0 earthquake. At Mw = 9.0, displacement ductility of around 2 might be required. A 25 storey building on a soft soil site in the Singapore CBD, however, might need to be detailed to a ductility of 2-2.5 at current strength levels, to resist a Mw = 8.0 earthquake at 700 km range and 15 km focal depth. A Mw = 9.0 earthquake in a similar location might require that the building be designed for lateral forces of 2.5-3 times existing practice, even if the building was detailed for a high level of ductility.

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Figure 4 – Comparison of response spectra ! "# $

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8.3 Performance of tall buildings It is of concern that reinforced concrete buildings designed to low levels of lateral load, and detailed in accordance with British code practice, may not possess sufficient ductility to survive a very large Sumatran earthquake. Global displacement ductility demands of up to 4 (at rock sites) and potentially much higher (allowing for site amplification) could be made on many structures which have been designed and detailed generally in accordance with the (non-seismic) British standard. Enquiries were received regarding assessment of the seismic safety of several tall buildings, after a strongly felt tremor arising from a 6.4 magnitude Sumatran earthquake in March this year. There is growing awareness of the potential effects of large earthquakes in the region. 8.4 Suggested supplementation of British codes To guard against the effects of a very large Sumatran earthquake, UBC building types and seismic loading regimes could be applied on top of the regular British loading code approach. For many very tall shear wall buildings, it may only be found necessary to detail potential yielding regions for a moderate level of inelastic response and to design for a margin of safety against shear failures. For medium to tall buildings, especially where considerable frame action is implied, the detailing of element end zones might be more critical but could still potentially follow the simplified methods outlined by UBC for structures in Zone 2, but with primary lateral load resisting elements designed for greater strength than has been the case and with greater attention to the type and layout of structural

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system chosen. Gravity frames should also be detailed to survive higher drifts than typically considered. Manual review of critical regions such as transfer beams, foundations and potential hinge zones would be implied by a revised approach. With improved detailing of potentially critical regions of structures, together with some nominal increases in lateral loading, new buildings in Singapore can be much better equipped to survive very large distant earthquakes. More detailed study of the seismic hazard in Singapore together with a relatively small amount of change to the loading and design codes (especially where applied to soft soil areas) would appear to be warranted. 9 REFERENCES American Concrete Institute. ACI 318-05. Building Code Requirements for Structural Concrete. Balendra, T. Lam, N.T.K. Wilson, J.L. & Kong, K.H. 2001. Analysis of long-distance earthquake tremors and base shear demand for buildings in Singapore. Engineering Structures Vol 24, No 1, Jan 2002. British Standards Institution. BS 8110:Part 1:1997. Structural use of concrete, Part 1. Code of practice for design and construction. International Conference of Building Officials, California, USA. Uniform Building Code (UBC 97) Paulay T. and Priestley M.J.N. 1992. Seismic Design of Reinforced Concrete and Masonry Structures. Megawati, K. Pan, T-C & Koketsu, K. 2004. Response spectral attenuation relationships for Sumatransubduction earthquakes and the seismic hazard implications for Singapore and Kuala Lumpur. Soil Dynamics and Earthquake Engineering 25 (2005) 11-25. Standards New Zealand. NZS 3101:Part 1:1995. Concrete Structures Standard – The Design of Concrete Structures.

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