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Tuesday, September 24, 2013

Safe Design of Multistorey Buildings


STEPS FOR SAFE DESIGN AND CONSTRUCTION OF MULTISTOREY REINFORCED CONCRETE BUILDINGS - PART 1


 1.      Introduction:

A large number of reinforced concrete multistoreyed frame buildings were heavily damaged and many of them collapsed completely in Bhuj earthquake of 2001 in the towns of Kachchh District (viz., Bhuj, Bhachao, Anjar, Gandhidham and Rapar) and other district towns including Surat and Ahmedabad. In Ahmedabad alone situated at more than 250 kilometers away from the Epicentre of the earthquake, 69 buildings collapsed killing about 700 persons. Earlier, in the earthquake at Kobe (Japan 1995) large number of multistoreyed RC frame buildings of pre 1981 code based design were severely damaged due to various deficiencies. Such behaviour is normally unexpected of RC frame buildings in MSK Intensity VIII and VII areas as happened in Kachchh earthquake of January 26, 2001. The aim of this paper is to bring out the main contributing factors which lead to poor performance during the earthquake and to make recommendations which should be taken into account in designing the multistoreyed reinforced concrete buildings so as to achieve their adequate safe behaviour under future earthquakes. The Indian Standard Code IS:1893 was suitably updated in 2002 so as to address the various design issues brought out in the earthquake behaviour of the RC Buildings. The paper highlights the main provisions of this code.

2.      Causes of the Collapse of RC Frame Buildings and Recommendations

2.1  Ignorance of the Architects and Structural Engineers about the Contents of the relevant earthquake resistant Building Codes :

Recommendation:-

The following BIS Standards will be mainly required for the design of RCC Buildings. Architect’s and Structural engineer’s design office should have the current copies of these standards available in their offices and all their staff should fully familiarize with the contents of these codes:-

1.      IS: 456 -2000 “Code of Practice for Plain and Reinforced Concrete”

2.      IS: 875 Part 1 “Unit weights of materials”.
3.      IS: 875-1987Design loads ( other than earthquake ) for buildings and structures, Part2 Imposed Loads
4.      IS: 875-1987Design loads ( other than earthquake ) for buildings and structures ,Part 3 Wind Loads
5.      IS: 1904-1987 “Code of Practice for Structural Safety of Buildings: Foundation”
6.      IS: 1498-1970 Classification and identification of soils for general engineering purposes (First Revision)
7.      IS: 2131-1981 Method of Standard Penetration Test for soils (First Revision)
8.      IS: 1905-1987, Code of Practice for Structural Safety of Buildings: Masonry
9.      IS:1893(Part-I)-2002 "Criteria for Earthquake Resistant Design of Structures (Fifth Revision)”.

10.  IS:13920-1993, "Ductile Detailing of Reinforced Concrete Structures subjected to Seismic Forces - Code of Practice"
11.  IS: 4326-1993, "Earthquake Resistant Design and Construction of Buildings - Code of Practice (Second Revision)"
12.  IS-NBC-2005: National Building Code of India.


Note: The design offices should keep in touch with BIS-CE division to keep track of any amendments issued or further revisions.

2.2 Softness of Base Soil:

The soft soil on which most buildings in Ahmedabad were founded would have affected the response of the buildings in three ways:

(i)     Amplification of the ground motion at the base of the building;

(ii)   Absence of foundation raft or piles;
(iii)Relative displacement between the individual column foundations vertically and laterally, in the absence of either the foundation struts as per IS: 4326 or the plinth beams;
(iv)  Resonance or, semi-resonance of the whole building with the long period ground waves;
(v)    In the absence of the beam at plinth or, ground level, the length of ground storey columns gets increased, which increases the flexibility of the ground storey and if the columns become ‘long’ the buckling moments due to P- effect will increase bonding to cause collapse of the columns.
(vi)  If the soil is sandy and water table is high, it may liquify. See IS:1893-2002 Cl 6.3.5.2 and Table 1 for minimum N (corrected values) for safety and carryout soil liquefaction analysis by standard procedures available in the literature. The adverse effects of liquefaction may be seen in Figs. 1, 2 & 3.
 Fig. 1                                                                  FIg. 2                                                                    Fig.3

Fig.1 - The Building Sank evenly about 1 m due to soil liquefaction. The displaced soil caused a bulge in the road.
Fig.2 - This inclined building sank unevenly and leans against a neighbouring building
Fig.3 - The solid building tilted as a rigid body and the raft foundation rises above the ground

Recommendation:-

Soil exploration at the buildings site must be carried out at sufficient points and to sufficient depth so as to give the following data:

(i)     Soil classification in various layers and the properties like grain size distribution, fields density, angle of internal fritting and cohesion a plastic and liquid limits and coefficient of consolidation of cohesive sites.

(ii)   Position of water table just before and just after monsoon.
(iii)SPT values and CPT values.
(iv) The output results should include liquefaction potential, safe bearing capacity and the type of foundation to be adopted, viz. (i) individual column footing of given width (ii) combined row footing or (iii) raft foundation or (iv) Pile foundations.

(v)   Chemical analysis of soil to find if it has any harmful elements to the concrete, if so, precautions to be taken in making the foundations.
Chemical analysis of water to be used in making the Concrete mixtures

2.3 Soft-first Storey:

Open ground storey (stilt floor) used in most severely damaged or, collapsed R.C. buildings, introduced ‘severe irregularity of sudden change of stiffness’ between the ground storey and upper storeys since they had infilled brick walls which increase the lateral stiffness of the frame by a factor of three to four times. Such a building is called a building with ‘soft’ ground storey, in which the dynamic ductility demand during the probable earthquake gets concentrated in the soft storey and the upper storeys tend to remain elastic. Hence whereas the ‘soft’ storey is severely strained causing its total collapse, much smaller damages occurs in the upper storeys, if at all.

Behaviour of soft first storey buildings (buildings on stilts or with open plinth) during earthquakes may be seen in Figs. 4, 5 & 6.
Fig.4                                                Fig.5                                                 Fig.6
Fig.4 - Sway mechanisms with soft storey ground floors (Izmit, Turkey 1999
Fig.5 - Soft first storey collapsed, upper part of the building fall onto the ground, (kachchh, 2001)
Fig.6 -Soft Storey (Open Plinth), Vertical Split between two blocks (Bhuj)

Recommendation:-

In view of the functional requirements of parking space under the buildings, more and more tall buildings are being constructed with stilts. To safeguard the soft first storey from damage and collapse, clause 7.10 of IS: 1893-2002 (Part 1) provides two alternative design approaches
(i)     The dynamic analysis of the building is to be carried out which should include the strength and stiffness effects of infills as well as the inelastic deformations under the design earthquake force disregarding the Reduction Factor R.
(ii)   The building is analysed as a bare frame neglecting the effect of infills and, the dynamic forces so determined in columns and beams of the soft (stilt) storey are to be designed for 2.5 times the 
 storey shears and moments: OR the shear walls are introduced in the stilt storey in both directions of the building which should be designed for 1.5 times the calculated storey shear forces.

Some remedial measures to counter the bad performance are shown in Fig. 7.

Some times a soft storey is created some where at mid-height of the multi-storey building, for using the space as restaurant or gathering purposes, see fig.8. Such soft storey in building also collapsed in Kutch and Kobe earthquakes. For such a case also, the storey columns should be designed for the higher forces OR a few shear walls introduced to make up for the reduced stiffness of the storey

2.4 Bad Structural System:

The structural system adopted using floating columns, for reasons of higher FSI is very undesirable in earthquake zones of moderate to high intensity as in Zone III, IV & V since it will induce large vertical earthquake forces even under horizontal earthquake ground motions due to overturning effects.

Recommendation:-

The structural engineer should provide for the load path in the building from roof to the foundation. For example, a building with floating columns requires transfer of the floating column loads to horizontal cantilever beams through shear forces. The load path, therefore, is not vertical but changes from vertical to horizontal members before reaching the foundation. Sometimes similar situations arise within the frames where, for any reason, either the beam is missing or a column is missing. These are structural discontinuities and should better be avoided as far as possible. Other irregularities such as those defined in Table 4 & 5 of IS: 1893-2002 (Part 1) become the cause for large torsional moments and stress concentration in the buildings which should better be avoided by the architect and structural engineer in the initial planning of the building configuration. Otherwise, they should be carefully considered in structural analysis and properly detailed in the structural design.


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