|
View previous topic :: View next topic |
Author |
Message |
sikandarsiddiqi ...
Joined: 26 Jan 2003 Posts: 294
|
Posted: Wed Sep 15, 2004 10:38 am Post subject: soft storey > DESIGN APPROACH REPORT WITHOUT PREJUDICE wi |
|
|
Radio Link International E-mail
Dateline Dhaka - Bangladesh/Asia
IMPORTANT
DESIGN APPROACH REPORT WITHOUT PREJUDICE with full explanation CGS FPS
Your Building Size : 60 x 40m = 196.8 x 131.2 feet
Storey : G + 4 stories
Dear Engr.Surendra, > Chennai , India
I would like to appreciate you for taking a bold step in designing an earthquake resistant building (ERB) of such a mega dimension from seismic point of view but approach is some what incorrect as it is not in conformity with certain basic & fundamental requirements related to seismic design of structure and fire safety engineering which are :
Findings
Ø flat plate slab and weight reduction
Ø inferno –fire safety
A building is a machine to live in and it must be safe in all respects.
The size of a building is very important in designing an earthquake resistant building and also it is equally important for fire safety reason i.e. how quickly a fire can be extinguished in case of a reinforced concrete building-longer time to extinguish may lead to total collapse of the building as reinforcement will become weak due to intense heat.
The defects are :
First : LENGTH
My report : An increase in the length of a building may have an adverse effect on its seismic resistivity. The reasons are given below :
1. When the length of a building is close to the wave length of incoming seismic wave the horizontal torsional moment due to inertia may reach a very large value resulting in collapse .
2. Theorem – in determining seismic forces, it is usually assumed that the structure vibrates as a system in which all the points of a plan at the same level and at the same moment in time are in the same phase of displacement,velocity and acceleration, their amplitude being the same. Such an assumption in designing may be close to actual conditions of structure’s behavior when its dimensions in plan are small and considerably less than the wave length of the seismic wave.
3. In any building temperature and shrinkage stresses and also stresses due to the non-uniform settlement of the base of the structure are to be taken into account .These increase with an increase in length of the building.
4. An increase in the length of a building increases the stresses in a floor working as a horizontal distribution diaphragm in transverse direction
5. As the propagation of seismic waves is not instantaneous but has a final velocity depending on the density of the soil and characteristics of the structural elements,the various parts of the base of the building along its length vibrate asynchronously with different accelerations causing additional longitudinal compressive & tensile stresses and horizontal displacements.; this means the effect of these stresses will be less if the dimensions of the building are smaller.
That is to say as per 5 above, less advanced is the information available related to the nature of an earthquake for design of an earthquake resistance building the more is the need for RESTRICTION on over all dimensional SIZE OF THE BUILDING. The very nature of an earthquake is unknown because the exact property of a locked position is unknown; therefore, factor of safety is essential so that it works as a back up in case of an emergency, conservative attitude may lead to a CATASTROPHIC FAILURE. Seismic forces determined from acceleration diagram of actual earthquake are often more than the design value.
6. It is to be noted that the base of a structure when its length is considerable, is not homogeneous. Some parts of the base may be more compact and others may be less compact.
An other problem is layering under the foundation is not ideally horizontal and because of this reason the velocity of propagation and direction of seismic waves at different parts of the building may be different causing torsion even in a symmetric plan .
Other problems are roof & beam are also not ideally horizontal.
Second : FLAT PLATE SLAB
My report :
In a seismic zone a flat plate slab is not at all desirable as it increases the weight of the building and in this case columns resist the seismic force causing an increase in dimension of each column as well as reinforcement alternatively a shear wall structure can be designed which too will increase the weight of the building and columns will carry only the gravity load. So both the cases are not desirable as the base shear depends on the over all weight of the structure, instead frame structure is to be designed by following weak beam- strong column procedure in combination with shear wall .The weight of the structure can be reduced further by using hollow block and ceiling block in combination with 60 – grade deformed bar which will reduce the weight of reinforcement by about 30%. The weight reduction may vary from 45 to 55% and cost saving may vary from 15 to 20% ; so these are the appropriate materials for construction in a seismic zone which is the desire of a designer as well as the building owner.
Third : INFERNO – THERMAL _ FIRE SAFETY
My report:
The size of a building is equally important for fire safety reason i.e. how quickly a fire can be extinguished in case of a reinforced concrete building - longer time to extinguish may lead to total collapse of the building as reinforcement will become weak due to intense heat.
Recommendations CGS FPS
Your Building Size : 60 x 40m = 196.8 x 131.2 feet
Storey : G + 4 stories
1. Change to 4(four) units each of 30 x 20m = 98.4 x 65.6 feet separated by earthquake separation joint in both directions.Separation joints between adjacent sections should pass through the entire height of the building.Separation joints are not made in foundation.The width of separation joints depends on the height of a building. For taller buildings the width of joints should be increased by 2cm for each 5m of height ( 3 cm for building upto 5m high ).In addition , the width of joints should not be less than twice the sum of maximum horizontal displacement of the structural elements.Separation joints should ensure that there is no contact between elements under displacements due to seismic forces.
2. No. of span : Odd in both short and long direction to maintain symmetry.
3. Frame structure : Seismic Resistant Frame ( SRF ) is to be designed by following ‘weak beam strong column technology’ which seeks to absorb seismic force by spring type action.
4. Seismic zone : Chennai is in zone II but as a factor of safety assume it to be in zone III for reasons explained in 5 above (Findings) for ERB design. Moreover Chennai is in a very severe stormy zone; it may experience typhoon upto 275 kmh as it is on the coast line of the Bay of Bengal at the samemoment of an earthquake – such a possibility can not be ignored.
( Watch ‘SHOCKING’ in AXN TV Channel for this sort of back up safety )
6. Use DITHA method - this is probably the most accurate level of analysis taking into account time domain.
Design status :The grading and status of design should be like that of modern skyscrapers in San Francisco considered to be the most sophisticated earthquake proof building in the world vibrated sidewise like corn stalks in a strong breeze during anearthquake on Oct.17,’89 which measured 7.1 on the Richter Scale. The epicenter was only 97 km (65 miles) away on the San Andreas fault line which runs through Crystal Springs Reservoir along the western coast of California,USA.
7. Fire Safety : 4(four) units for fire safety reason as a modern office complex contains lot of costly materials like furniture,decoration,computer,air conditioner etc and also to prevent structural failure due to intense heat and above all offical files,documents etc. If space is available each building unit is to be separated by 2.5 m for easy movement of fire fighting officials.
With regards and best wishes.
With regards and best wishes.
Engr.Sikandar Hayat Siddiqi Project & Seismic Design Management Engineer & Builder/Developer Selection Consultant & Co-ordinator,Earthquake Anti-disaster Defence Management System Initiative
Copy to
Prof. Dr.Engr.Sudhir K Jain, IITK, India (skjain@iitk.ac.in)
Engr.Sanjay jha, Government of Delhi (hqadm@hub.nic.in)
Mr.Stacey Martin, Director, ASC (martin@asc-india.org)
Engr.Anup Karanth (anup.karanth@undp.org)
Engr.Alpa,India ( alpa_sheth@vakilmehtasheth.com )
Engr.Rudra Nevatia,India( rudra_nevatia@yahoo.com)
Engr.Sangeeta,India (sdec@bol.net.in)
-----------Original Message--------
To:sikandarsiddiqi@yahoo.comSubject:soft storeyFrom:surendra@lntecc.com Date:Thu Aug 19 19:58:14 2004
surendra@lntecc.com wrote: Dear friends,
We have analysed a 60x40m (approx) building with G+4 stories. The building has four L-Shaped RC shear walls at four corners, central RC core for lifts and other intermediate columns in between.
Please note that all columns and shear walls are continued from foundation to roof level.
As per the analysis (shear walls and columns with flat slabs modelled in ETABS) shear walls are taking almost 95% of base shear and remaining by columns.
The ground floor of the building is like car parking/open storey, with columns and shear walls. But in all other floors (office building) outer walls are blockwork/glazing and Internal infill walls are not decided.
My question is whether i have to design ground floor columns, RC walls and foundation elements for soft storey condition or not as per 1893-2002 ?
Please clarify
Thanking you,
SURENDRA BABU .J Assistant Engineering Manager, EDRC, L&T - ECC, Manapakkam, Mount poonamalle high road, Chennai - 600 089
PS : Dr. Jain - Requested for your kind comments please
---------------------------------
Posted via Email |
|
Back to top |
|
|
|
|
You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum You can attach files in this forum You can download files in this forum
|
|
|
|