www.www.bussiapp.com

[swamikrishnan]

Dear SEFIans,

我想欢迎结构的所有成员ral Engineers Forum of India to this much-anticipated e-conference on Tall Building Design and Construction. The big headline from the 2010 M8.8 Maule earthquake was that of the 3000+ tall buildings (>10 stories) in Chile, 80 buildings were damaged and only one collapsed. Given the size of the event, there is a broad consensus in the global engineering community that this outcome is more than satisfactory as far as tall building performance is concerned. Much of the credit has been attributed to the building code revisions undertaken after the 1985 M7.8 Valparaiso earthquake that caused extensive damage. With few exceptions, the seismic provisions of the American Concrete Institute’s ACI-318 building code for structural concrete were adopted for the design of new reinforced concrete structures in Chile (as in India, the material of choice for tall buildings in Chile has been reinforced concrete). Damage was mostly limited to concrete crushing and spalling, and reinforcing bar buckling and fracture at the ends of thin shear walls. The shear wall boundary element detailing provisions in ACI-318 were omitted from the revised Chilean code and these thin walls lacked the required extent of confinement reinforcement. Engineers have concluded that thicker shear walls incorporating boundary elements would have prevented most of the observed damage. On the face of it, this seems to be a ratification of the ACI-318 seismic provisions and suggests that such a code could be adopted for tall building design the world over. While this may be the best course of action, this in itself may not be sufficient to produce the Chilean outcome. The ratio of wall area to floor area in Chilean tall buildings is far greater (2%-4%) than that in US tall buildings (1%-1.5%) despite the fact that they are designed nominally to the same provisions. The Chilean tall buildings are thus much stiffer than their US counterparts. The stiff Chilean tall buildings have been tested to a certain extent in this earthquake, whereas the flexible US tall buildings are yet to be tested. According to data collected by Prof. Moroni of the University of Chile, Chilean tall buildings have been getting progressively more flexible over the last 5 decades. During the same period, the damage rate in these buildings has been increasing with increasing flexibility (ref.: Prof. Jack Moehle’s EERI/PEER presentation). These observations raise important philosophical questions in the Indian context. Should Indian tall buildings go the stiff way of Chile or the flexible way of the US? How would one achieve this through the building code? I would like to see this question of “flexible vs stiff” vociferously debated in this conference. I would also like to understand and learn about the state of ductile detailing in Indian tall buildings. Do engineers and contractors recognize the importance of ductility detailing for earthquake resistance? How do we codify, implement, and regulate this? How do we educate all the stakeholders on the critical need for seismic detailing and get their “buy-in”? I hope to find answers to all these questions in this conference.

The Chilean example has also raised other important questions for India. The peak ground velocity in Concepcion was quite strong at 67cm/s; however, the peak ground displacement was only 21 cm. The duration of significant ground motion was quite long at 88 s, but not as much as the 1960 Chile and the 1964 Alaska earthquakes. Had the displacements and durations been greater, the outcome may have been quite different. Do we know what ground shaking would result in Delhi from a great earthquake on the Himalayan Frontal Thrust? Do we have models that can predict ground motion from such an event? I hope to find out answers to these questions in this conference. I hope similar seismological questions can be addressed for other large metropolitan cities in the northern belt and the western front including, but not limited to, Mumbai, Ahmedabad, Kolkata, Surat, Pune, Jaipur, Vadodara, and Allahabad.

I am also deeply concerned about the buildings on stilts that seem to be ubiquitous (at least in the south where I have traveled extensively in the last decade). We engineers have to come up with creative solutions to address the architectural drivers of such glaring seismic vulnerabilities without compromising structural (seismic) integrity. I hope to see creative solutions for such problems outlined in this conference. Tall first and second stories, termination and/or offsetting of gravity/lateral force resisting elements such as columns and walls, also fall in this category. I hope to see case histories of buildings with such idiosyncrasies and the novel approaches that SEFI engineers have undertaken to tackle such architecturally driven situations.

最后,但肯定不是最少,环境cientists warn us that global warming is going to result in more intense and violent storms and hurricanes in the coming decades. Our coastal cities are going to be most affected. The latest storms in the eastern US (New Jersey) and in eastern India (Chennai) are harbingers of storms to come. Are our tall buildings prepared to face these “Frankenstorms”? How do we deal with the moving target of wind hazard and our ever-increasing density of tall buildings in our mega-cities? During the lifetime of our building how will the wind loading patterns change with the rapidly changing landscape. How do we anticipate these changes at the design stage and build in contingencies into our designs, without being overly conservative and jacking up the costs? Seems like a pretty stiff challenge to me. I hope to hear your considered and deep thoughts on these “gray-area” questions.

In closing, I am looking forward to two weeks of exciting, rejuvenating, informative and enlightening debates and discussions on tall building design and construction in India. I sincerely hope that the SEFI community takes full advantage of this wonderful opportunity to share and elevate the state of this art, made possible by the remarkable vision and efforts of my co-moderators, Er. Alpha Sheth and Prof. C. V. R. Murty.

Swaminathan Krishnan, Co-Moderator
California Institute of Technology
http://krishnan.caltech.edu

Posted via Email

[sureshkumar_kumaresan]

Prof. Swaminathan,

With regard to wind storms and loading, your thoughts are very important and should be addressed.

(1) As you know our wind code IS875-Part 3 is 25 years old though many drafts are on the works. With regard to speed, we need to have good measurements at various levels (probably we need computer simulations of cyclones are also required to get pool of data) and then if any potential change in speed is required bring this to the code level for practioners to use this. In US, ASCE is updated almost every five years. In this way unceratinty in wind speeds can be eliminated. For those buildings already existing, we are relying on the factor of safety used for design.

(2) One another factor which will be of help for the existing buildings is the ever changing skyline of a city. Cities are growing as a result more and more buildings are getting built and as a result the wind loading on existing buildings shall get reduced due to shielding.

(3) While doing any specific studies such as wind tunnel studies, one could simulate different surrounding scenario to address the effect of change in surroundings.

Suresh

[nrk]

Dear Dr. Suresh Kumar,

从27.4.6第3期7 - 10的条款,它是明确的all buildings should be designed for case 1 and case 3 of Figure 27-4.8 of ASCE 7-10. In clause C27.4.6 of ASCE 7-10, it is stated: "In buildings with some structural systems, more severe loading can occur when the resultant wind load acts diagonally to the building. To account for this effect and the fact that many buildings exhibit maximum response in the across-wind direction (the standard currently has no analytical procedure for this case), a structure should be capable of resisting 75 percent of the design wind load applied simultaneously along each principal axis as required by Case 3 in Fig. 27.4-8�. In the book, 'Wind and Earthquake Resistant Buildings' by Dr.Bungale S. Taranath, the author explains, "In buildings with unusual structural systems, such as the one used for the City Corp. Tower in New York, more severe loading can occur when the resultant wind load acts diagonally to the building. To account for this effect and the fact that many buildings exhibit maximum response in the across-wind direction, a structure should be capable of resisting 75% of the design wind load applied simultaneously along each principal axis, as required by case 3 in Fig. 6.9 of ASCE 7-02". From the above, it can be inferred that the mandatory Case 3 of ASCE 7-10 is a result of the Citicorp Center issue. In fact, the same conclusion can be drawn by going through the Citicorp Center issue discussed in 'Beyond Failure: Forensic Case Studies for Civil Engineers' by Prof.Norbert J. Delatte, where the author provided pages of his class notes on the Citicorp issue, discussed by the late Er.Le Messurier, at MIT.

Although, a structural system like that of the Citicorp Center is not common, I believe that a load combination similar to that of the case 3 of Figure 27-4.8 of ASCE 7-10 should be made mandatory in codes like Eurocode 1, to address the issues of quartering winds and across-wind response.

I was advised to refer to Chapter 6 of the AIJ guidelines and CNR-DT 207/2008 (Guide for the assessment of wind actions and effects on structures) for better guidelines on dealing with across-wind response.

I request your views on the issue of quartering winds and across-wind response and throw some light on the relevant clauses of IS 875 part 3 that deal with these issues.

[nrk]

Dear Prof.Swaminathan Krishnan:

In my experience, misinterpretation of codes and lack of understanding of the behaviour of structures is more dangerous in the field of structural design rather than 1% or 2% errors in the calculation of forces and deformations. A couple of examples on the misinterpretation of codes and lack of understanding of the behaviour of structures that I came across in my structural design experience are discussed hereunder.

Empirical expression for the calculation of approximate time period of a moment-resisting frame building without brick infill walls
Clause 7.6.1 of IS 1893 part 1:2002 gives the following empirical expression for the calculation of approximate fundamental natural period of vibration (T_a), in seconds, of a moment-resisting frame building without brick infill walls:

T_a= 0.075h^0.75for RC frame building

The code explicitly states that the empirical formula is applicable only to moment-resisting frame buildings without brick infill walls. However, the clause is misinterpreted by some structural engineers. They interpret that the empirical formula is applicable to all moment-resisting frame buildings whose mathematical/analysis model does not include brick infill walls. In my opinion, use of the above empirical expression should be done away with, for the following reason, besides others:

The expression doesn't include the plan dimensions. The formula gives the same time period for all the buildings with the same height irrespective of the plan dimensions. For example, a building of height h and plan dimensions dxd and a building of height h and plan dimensions 100dx100d will have the same calculated time period, which is not true. Similarly, a building of height h and plan dimensions dxd and a building of height h and plan dimensions dx100d will have the same calculated time period, which is not true either.

Third mode of vibration of a moment-resisting frame building (bare frame)
It is quite common in design practice to model moment-resisting frame buildings as bare frames. Eurocode 8 recommends avoiding torsion in the first two modes of vibration. Some academics in India insist on avoiding torsion in the first three modes of vibration. I believe that avoiding torsion in the first three modes of vibration of an MRF is not possible. A simple study using SAP2000/ETABS on various bi-symmetrical (in both the principal orthogonal horizontal directions) moment-resisting frames reveals that the first two modes are always translational and with the same magnitude of time period of vibration, while the third mode is always torsional. Hence, insisting on avoiding the torsional mode in the first three modes of vibration seems meaningless.

Structural design paradigm in India
The 2001 Bhuj earthquake has changed the structural design paradigm in India. Although IS 1893:1984 listed the area of Bhuj in seismic zone IV, the building designs did not comply with the earthquake code. Most buildings in India were designed for gravity loads alone before the release of the updated earthquake code IS 1893 part 1:2002. Many practicing engineers in India understand well, the behaviour of structures for gravity loads, but not earthquake loads, as the design of structures for earthquakes in India is relatively a new phenomenon.

Earthquake design Vs Gravity design
My belief that what is true for gravity loads may not be true for earthquake loads led me to do a simple study on the assessment of a 28-storey RC building designed according to IS 1893 part 1:2002. The same building is assessed when the building is occupied after construction of 9-storeys and 18-storeys. The assessment of the 9-storey and 18-storey buildings is done to reflect the real situations where construction of some buildings was stopped mid-way due to the global economic recession. From the gravity loads (dead and live loads) point of view, the 9-storey and 18-storey buildings are safe. My belief was that the 9-storey and 18-storey buildings do not meet the performance criteria of the earthquake code as the structural members are designed based on the 28-storey building. The purpose of my study was to prove that a building designed according to the provisions of the earthquake code when stopped mid-way and occupied will not meet the performance requirements of the earthquake code and hence has to be retrofitted to meet the performance requirements of the earthquake code. The results of my study confirmed my belief.

[nrk]