21-11-2012-SUMMARY OF E-CONFERENCE ON TALL BUILDING

*************************************************************

1.SUMMORY OF 20-11-12:

Posted by Er.T.RangaRajan, Raconteur E-Conferenc.

2. Under No title topic Er. Sureshkumar_Kumaresan has commented that Tall

building definition shall also consider slenderness ratio not just the height. There is no

definite answer to this and diverse definitions/limits all over the place. In my opinion,

we need not worry too much on whether the building is tall or low. Our concern shall be

whether any element or the entire structure is wind sensitive or not. For instance, in

low buildings such as stadiums/airports, lateral wind force is not an issue, but

uplift/down force on the long-span roof is an issue and should be carefully studied.

With regard to which load dominates, as madam Alpa rightly said this depends on many

parameters. However,typically when number of floors above about 35 and slenderness

ratio above 5-6, wind may govern the design. Rule of thumb, as the buildings go

higher and slenderness increases, the building becomes more flexible and

will interact with wind more than earthquake as there is lot more energy

with wind at low frequencies than earthquake does.

3.Welcome to the Econference on Design and Construction of Tall Building:

Dr.N.S has informed the availability of Stud rails or Stud reinforcement and it does

not have any connection with tall buildings, but has connection with flat

plates.

They are to be used only when the punching shear strength of the slab is less than the

applied punching shear at the column-slab joint. There are other methods of increasing

the punching shear strength, though stud reinforcement has been found to be more

efficient.

Also cited a reference article on the use of studs in Corbels:

Birkle,G., Ghali, A. and Schafer, K., Double-Headed Studs Improve Corbel

Reinforcement, Concrete International, Vol. 24, No.9, September 1, 2002, pp.

4. A new topic How to optimize tall buildings for wind is posted by

Dr.Sureshkumar_Kumaresan

In the above topic he stressed that plan form geometry can play a huge role in the

optimization procedure before one would look into the structural aspects.

Most of the time the geometry is fixed by the architect and then the building goes

into a wind tunnel study mode. But then we have the only option of fixing the issues

only structurally which is quite a task for the structural engineer. So structural fix means

more concrete (for stiffening and increasing mass, not always sometime reorienting the

frames would do the job) and less space. This way of fixing is not a sustainable way as

well.

He also expressed his views as when we come with tall/slender buildings, geometry

should be given considerable importance .

He needs more comments and questions in this regard from the participating Sefians.

5. Again Er. SureshKumar has put a new topic as WIND TUNNEL TESTING:

As informed in my previous summary he is an expert in Wind engineering and has a

good and vast experience in that field. He informs that in my career as a wind engineer,

I have used many approaches to calculate wind loads on any wind sensitive structures

not just limited to tall buildings. The approaches that I have used are codes/standards,

wind tunnel testing, analytical (stochastic) and full-scale measurements. I have to admit

that out of all these approaches, wind tunnel testing approach becomes the most

sought out approach in the industry considering their accuracy and practicality.

He outlined the rule of thumb when to conduct the wind tunnel test for a project as one

should be looking into this once the structure is above 35-40 floors and slenderness is

above 5-6. The reasons for doing is multifold. The key reason is to obtain accurate

loading on the structure and accelerations at the top of the building to evaluate human

comfort. Without a wind tunnel test, there is no way of find realistic loading on the

building unless the building is boxy and located without any immediate surrounding

buildings. Mostly, the wind tunnel test will provide lower loads than an international

code would provide. So here there will be huge saving for the owner. However, this is

not the reason to do a tunnel test.

He welcomes a deep discussions and comments on this.

6. Dr.Swaminathan Krishnan has given a brief comments on various codes that are

applicable for Tall buildings and explained vividly on the US code and Altenate Design

Guideline (ADG).

The most recent US codes fall into three categories:

1. ASCE 7 (Standard).

2. International Building Code (IBC).

3. Material-specific codes such as ACI-318 for concrete and AISC Manual of Steel

Construction for steel.

Under the heading Background to the development of alternate design

guidelines for tall buildings he explained that because the prescriptive codes

have strict height limits. In particular, ASCE 7 requires a "dual-system" to be employed

for all buildings greater than 160 feet (BTW, the IBC classifies any building with an

occupied floor located more than 75 feet above the lowest level of fire department

vehicle access as a high-rise building; nominally, I think, this would work out to >10

stories). Dual systems can be configured by combining a braced-frame or a shear wall

core and a perimeter moment frame; the moment frame should be configured to resist

25% of the prescribed seismic forces. To get around this problem, engineers decided

to draft "alternate design guidelines (ADG)"with contributions and input from several

prominent academics. The first document was for San Francisco city followed closely

by a similar document for Los Angeles by the Los Angeles Tall Building Structural

Design Council (LATBSDC). Subsequently, the CTBUH drafted its own guideline (along

the lines of the LATBSDC document) which is the document you reference.

Key differences between ADG (alternate design guidelines for Los Angeles)

and IBC (prescriptive code):

(a) The ADG eliminates height limits on all structural systems.

(b) For the code-level check in the ADG, the prescriptive code provisions were

adopted, but with certain key exclusions.

(i) The code provides a simple formula to determine the time period for

various structural systems as a function of the height (termed Method A

period, Ta).

(ii) The ADG eliminates two other minimum design base shear requirements

that were first introduced in the 1997 UBC to account for near-source

effects, replacing it with a hard lower bound of 2.5% instead.

(iii) ADG eliminates the drift limit imposed by the prescriptive code. The result

of these three "relaxing" exceptions is that the ADG makes it possible to

conceive buildings that are more flexible than what the prescriptive code

would allow.

My thoughts about ADG:

Under this he listed that:

(a) Eliminating the hard lower bound on the fundamental period (1.3Ta)in the base

shear computation:

(b) Redundancy.

(c) Nonlinear analysis software.

(d) Ground motion selection.

(e) The last point has to do with the "subjectivity" of the design process that is

afforded within the ADG.

http://www.sefindia.org/forum/viewtopic.php?t=13132

Interested Sefians can browse the link for more details as explained by him.

He has placed the questions to Sefians to ponder and reply as

(a) Flexible vs stiff (see my original post)?

(b) ADG vs Prescriptive?

6. In response to the above comments by Prof.Dr.S.K, engineer Rangarajan

has the following questions:

a. Dual systems can be configured by combining a braced-frame or a shear wall

core and a perimeter moment frame; the moment frame should be configured

to resist 25% of the prescribed seismic forces.

Can you explain how to model in any software like STAAD etc so that the frame

takes at least 25% of base shear?

b. FRAME 3D program is only on line use or available separately on price?

Also asked why there is no comment on UBC as adopted in US?

7. Accessing construction materials to higher floors:

Fast Track Construction of Stories/structural floors: Current practice for

general & tall structures-Cast in situ

For the above Er.Suraj has listed 16 notes like

1 Cast in situ construction applies to various structures, where required

working & material handling area is available on sites.

2 Cast in situ also, applies to structures that require complex coordination for

allied services & various non regular plan shapes.

3 Cast in situ is also, helpful for better site controls on line management

level.

4 Site fabricated formwork can be conveniently, used for preparatory works.

5 Thick & massive members can be handled conveniently in all cases .

6 Detected errors or mistakes can be reworked if so required.

7 Construction vertical members & horizontal members do not cause major

issues with exception of site time consumption, stretching schedule bars.

Loss Prevention on Buildings/Fire Suppression:

29 notes are given for the above by Er.Suraj.

Under the main topic Urbanisation of Metropolis Surrounding Areas-

General Discussion–High Rise Er.Suraj has given more notable points for

the

Electrical Standby Supply & Distribution: A few out of 14 points are:

1. Cannot depend exclusively, on state board supply & distribution of electrical

energy to serve occupancies.

2. Without continuous electricity supplies, vertical movement in buildings stops,

which can cause adverse issues.

3. Some persons can trap in elevators at all levels, jeopardizing lives, which

situation necessitates requirement of standby emergency electrical supplies.

4. Though all services may not require standby supplies, yet necessary services

such as water distribution, lifts, electric LEDs, access card system etc. should be

provided operational all times.

5. Such services loads should be calculated for procurement of electrical

generators of required capacity, without producing sound & operable at low

running cost.

For Combination of cast in situ & cast in factory he has given 21 notes

out of first few of them are listed below:

1 High rise or tall buildings are in line to current practice prosecuted on fast

track basis, so that major work is carried out off site in factory & limited volume

of work affected on site.

2 Construction method adopted for such combination is cast in situ

application for columns & beams, while factory production of precast hollow core

slab/panels to be installed on site born over cast in situ columns & beams

framework.

3 .Columns as usual are constructed on site by conventional methods of cast in

situ procedures.

4 .Supporting beams are located on particular coordinates on plan that respond

with wall locations.

*****************************************************************

8. The new topic of to-day is use of structural steel ans other alternative

materials:

Er. Jignesh V Chokshi has asked the query

Can anyone highlight the statistics of Steel buildings and concrete buildings?

Since R.C.C preferred materials for almost all Tall building in India.

He stated that in order to deal with loads and other services requirement, use of

alternative materials or methods is quite extensive.

He has few questions/observations for following two points:

A. Use of Steel in Building industry.

B. Alternative materials.

For item A he has listed :

1. Steel has much capacity compared to concrete. I understand that steel

construction in general is perceived to be expensive. But this is not always

true. For large loads, stel is always a preferred choice of engineer. Is Steel

construction very expensive compared to RCC even for all high rise buildings?

2. Steel construction of high rise building require sections having large sectional

area by large flange and web thicknesses. Does our steel producers make such

sections?

3. Reduced plan dimensions of sections will give lot of room for floor area and

may also be suitable for aesthetic purposes dur to slim sections.

4. Steel construction will require adequate measures for fire safety. Do we have

adequate resources in terms of material and contractors who can do it?

5. Steel construction is faster compared to RCC. The sections may be brought

directly from shop and very less field work will be required. Do builders give due

consideation to time value in their commercial analysis of project?

6. Bolted connections can make building erection much faster. We are yet to

adopt this philosophy. Experts can highlight pros and cons of bolted construction

use as of today in India.

For item (B) he has raised the questions as

1. Why such hollow precast planks not popular in India? Can the same be used

with RCC construction also?

2. Light weight partitions are not much seen in India. Is it that the consumer is

scared to adopt new material or is it not suitable to our environment?

In response to the postings by Er. Jignesh V Chokshi, Er.Suraj has listed 31

items which can be read under the link:

http://www.sefindia.org/forum/viewtopic.php?t=13175

Er.R L DINESH has informed about the choice of RCC & Steel depends on factors like

construction cost, less weight of construction, stiffness, flexibility of plan, behavior in fire,

construction time and usable area. A general guide lines collected from tall structures

references is enclosed.

+ sign indicate suited. ++ sign indicate more suited

CRITERIA RCCNormal

RCC HighStrength STEELCOMPOSITE

Construction Cost + ++ 0 ++Less Weight ofconstruction 0 + ++ +

Stiffness ++ ++ 0 +Flexibility of plan 0 0 ++ +Behavior in Fire ++ ++ _ +Construction time + + ++ ++Usable area _ + ++ +

9. Pointed discussions on tall buildings - Use of Flat Slab:

In response to this earlier posting Er. shekhar at shekharpana... on 20-

11-12 has quoted that The three basic framing systems to resist lateral loads

in high-rise buildings are:

(1) Frames,

(2)shear walls coupled or acting individually and

(3) frames interacting with shear walls.

As an economical form of construction a shear wall structure incorporating a flat

plate system is almost ideal. The flat plate system is very efficient in

resisting gravity loading while the shear wall provides the resistance to

lateral load.

In response to the above to-day 21-11-12 Er. Kapildingare has sought more

information and get expert opinion on relative extent of damage with respect to

these three forms when such structure is exposed to severe earthquake than

designed earthquake.

To answer to the above question Dr.N.S has offered the followings:

The failures that occurred are mostly in flat slab systems without shear walls. He

quoted the places of failure of FLAY SLAB construction as:

Several failures of flat slab structures have been reported in the literature, which

include New York Coliseum on May 9, 1955 (waffle slab), 2000 Commonwealth

Avenue: January 5, 1971, Five story Harbour Cay Condominium collapse at

Cocoa Beach, Florida, March 27, 1981(11 workers killed and 23 injured), The

Tropicana Casino parking garage in Atlantic City, New Jersey: October 30,2003,

Four story warehouse at Ontario, Canada: January 4, 1978, five story Sampoong

Department store, Seoul, Korea: June 29th 1995 (The collapse is the largest

peacetime disaster in South Korean history - 502 people died, 6 missing, and 937

sustained injuries), Piper’s Row Car Park, Wolverhampton, UK, 1997, Geneva,

Switzerland 1976, Bluche, Switzerland 1981, Cagliari, Italy 2004, and parking

garage flat slab at Gretzenbach, Switzerland, 2004. In addition several flat plate

systems failed during earthquakes. Many slab column connections in flat-plate

structures were damaged and failed after the 1985 Mexico City earthquake, the

1989 Loma Prieta earthquake, and the 1994 Northridge earthquake.

He suggested that Prof. Murty, the moderator of this conference to conduct

some expt. investigations at IITM to provide guidelines on this aspect, and also

to provide a overview of the behaviour of such dual systems in case of larger

earthquakes- how the walls and flat plates interact, failure mechanisms, how

punching shear can be prevented, etc.

He also wanted to do research on Transfer Girder Behavior and he

stated as

It is also necessary to do experiments for the so called 'Floating columns' that

terminate at a transfer girder. Design Engineers do not know about the

behaviour at the beam-column junction where the columns are terminating,

under EQ loads. Guidelines are necessary to detail such beam-column junctions,

so that failure is prevented. As the transfer girders are usually provided at the

first floor level to accommodate column free parking or shopping areas, such a

joint assumes at most importance, as the failure at the point will trigger

catastrophic failure of the whole building.

A study of the combination of flat plate-shear wall-transfer girder system is also

desirable.

Er.Anandkamath has asked Why are not using steel structures with precast

wall for tall buildings?

Under this post Er. Mjnasar has requested the clarification on Fire safety &

evacuations issue and structural engineers role in it

Following are the some points:

Standards of Fire rating in Hrs for Various structural elements

1. Role of Concrete covers / concrete encasement in Fire safety

2. Chemicals to improve fire ratings

3. Evacuation issues

4. Progressive collapse issues

5. Blast Resistant construction for certain elements

9. Geotechnical Issues in Tall Buildings - Note from Geotechnical consultant

Jaydeep Wa:

P.K.Mallick has raised the question on 20-11-12 that How do we define

soft rock and hard rock from soil investigation point of view ?

To clarify the above Er.Suraj has posted the following information:

Rocks/Compressive strength/BS 5930

1. Extremely strong/Rocks ring on hammer blows/>200MN/m^22. Very srong/Core chipped only by heavy hammer blows/100-200MN/m^23. Strong/Broken by heavy hammer blows/ 50-100MN/m^24. Moderately strong/Broken by hammer blows while hand held/12.5-50MN/m^25. Moderately weak/Thin slabs or edges broken by heavy hand pressure/5-

12.5MN/m^26. Weak/Gravel size lumps broken by heavy hand pressure/1.25-5MN/m^27. Very weak/Gravel size lumps crushed between fingers & thumb/<1>8. RQD 70/75 may be considered hard range

Granular

1. # of blows 50/Very dense2. # of blows 30-50/Dense

Cohesive soil

1. Undrained shear strength>300 KN/m^2/Hard

10. Books and References :

1. SP240: Performance-Based Design of Concrete Building for Wind Loads .

2. Proceedings of the Nationa Workshop on HIGH RISE BUILDINGS(NWHRB) -HELDBETWEEN aPRIL 25TH AND 26TH @ Hyderabad.

3. Multi-purpose High-rise Towers and Tall Buildings.H.R. Viswanath (Editor), Jurek Tolloczko (Editor), J.N. Clarke (Editor)

The above 3 are added by by Er.T.RangaRajan.

1. Construction Technology for Tall Buildings By Michael Chew Yit Lin is posted by Er.P.K.Mallick.

11. Tall Building design:

A new topic is posted by Er. Sriprakash_shastry. In his posting he explained thatIndia is still a developing country we do not find clients who would like to invest thiskind of money and even the one of the richest tycoon Mr. Mukesh Ambani has nothaving a tall building for his office.

In this context he has modeled a fictitious model which is a Super Tall Skyscraper witha bundled tube design. Lateral force resisting elements are a combination of outriggerbraced trusses and core walls. The structure would be very similar in design to theSears Tower in Chicago. I have tried to work on it only during my free time as my workkeeps me busy during the regular hours.

The model has been created using STAADPRO software and he need thecomments/views from Sefians.

To respond to this Er.Suraj has listed 11 reasons for not building Tall Buidlings in India.

12. In response to the Welcome address in which Madam Alpa listed the topics

Er. R L DINESH posted that Elastic compression of columns shall be taken intoconsideration in the analysis & design. Storey Height of tall buildings at lower level willbe kept more than at the upper levels to account for this deformation.

13. New topic: Axial Shortening and Damping

Er. vikas.pai has opened up the above new topic and stated that there are twomore important issues that need some discussion in design and construction of tallbuildings:

1. Axial Shortening of compression members and how to deal with it during the design,detailing and construction2. Damping systems that are useful for wind damping, seismic damping and which oneare available to use in India

Any thoughts from the current designers of tall buildings as well as contractors wouldbe welcome.

14. Time period-Tall Building Design.(New topic)

Er. Sukanta.adhikari has asked is there any limitation on time period of structure fortall structures?

15. Tall structure structural arrangement?

On the above topic Dr.N.S has replied as:

Location of shear walls should be at the extreme boundaries, so that we will get a goodleaver arm for resisting the lateral loads, and hence the forces induced will be less.Many engineers adopt core walls, as it is easy to locate them around the stairs and lifts.Also such walls will have box configuration instead of the simple rectangular walls;hence provide much stability- However, much care should be exercised in designing thelink beams around openings in the shear walls- Read the book by Smith & Coull, TallBuildings Structures-Analysis and Design, Wiley, 1991, which still remains as anexcellent book on this topic.

Are you talking about pre-tensioned slabs in Tall buildings. Most of these slabs areexecuted by proprietary firms and you need to consult them. However visit thefollowing sites, which may be useful:

http://www.vsl.net/Portals/0/vsl_techreports/PT_Slabs.pdfhttp://www.post-tensioning.org/pti_journal.phphttp://www.pci.org/publications/journal/index.cfm

Er. BMR again asked the question :

How effective would be the precast RCC building systems and an guidelines are therefor the connection design of elements mentioning the tolerance limits for theconnections as the connections would be defining the performance of the building?

16. Er.Rabinder Shekher has a very good suggestion saying that the outcome of the e-conference if put in CD form can be distributed to those interested on payment basis.

For this Er.T.RangaRajan has informed that the day-to-day Summary is being postedin PDF format and interested Sefians can download them.

Er.Rabinder Shekher has informed that In his state there is no building more than10 storeyed and probably Building construction permission authorities have clear

orders to accord the sanction to a building having more than specified storeyes or aparticular height.

Now should we consider adoption of tall buildings in our state which falls in seismiczone IV and V?

17. Seismic Design of Cast-in-Place Concrete Diaphragms, Chords:

The post appeared yesterday has been responded by Dr.N.S as:

NIST-NEHRP has published a number of reports useful for practicing engineers whichcan be downloaded at

http://www.nehrp.gov/library/index.htm

http://www.nehrp.gov/library/guidance_steel.htm

I have already given the link to download the previous reports 1-6 in my earlierpostings at SEFI-Er Rangarajan once again has given link to Report 4 on Cast-in-PlaceConcrete Diaphragms, Chords, and Collectors- the latest report is:

NIST Report on Seismic Design of ReinforcedConcrete Mat Foundations-A Guide for Practicing Engineers,

http://www.nehrp.gov/pdf/nistgcr12-917-22.pdf

Other reports available at this site are:

Selecting and Scaling Earthquake Ground Motions for Performing Response-HistoryAnalyses (NIST GCR 11-917-15)

Research Plan for the Study of Seismic Behavior and Design of Deep, Slender WideFlange Structural Steel Beam-Column Members (NIST GCR 11-917-13)

NIST GCR 12-917-21 “Soil-Structure Interaction ofBuilding Structures” (NIST 2012).

Links to other FEMA, PEER, MCEER reports are also available.

In short it is a very useful website for design engineers, researchers and students.

NIST is a Govt. Body and hence we can download it free of cost.

Hope further lively discussions will continue.

T.RangaRajan.

E-Confer. Raconteur