Necesidad de una solución libre de costo y de código abierto

Allasab Gudihal¹ and T.H. Sadashiva Murthy²

1Post Graduate Student, ²Associate Professor

Department of Civil Engineering, The National Institute of Engineering, Mysore, India

ABSTRACT

Many buildings in the present scenario have irregular configurations both in plan and elevation. In case of devastating earthquakes, it is necessary to identify the performance of the structures to withstand against disaster for both new and existing one. In recent years, it has become a common practice in growing cities located in mountainous regions to undertake the construction of buildings of considerable height and large base areas on slope terrains. Hill buildings are different from those in plains; they are very irregular and unsymmetrical in horizontal and vertical planes, and torsionally coupled. Hence, they are susceptible to severe damage when affected by earthquake ground motion. It was found that the step back set back buildings is found to be more suitable on sloping ground, as most of hill buildings fall under vertical geometric irregularity.

Criteria and limits specified for vertical irregularities as defined by different codes of practice and types of different geometric configurations used are discussed. This paper summarizes the research works carried out regarding effect of vertical irregularity on buildings when subjected to seismic loads.

Keywords— Seismic performance, Sloping ground, Vertical irregularities, Dynamic characteristics.

INTRODUCTION

Irregular buildings constitute a large portion of the modern urban infrastructure. The group of people involved in constructing the building facilities, including owner, architect, structural engineer, contractor and local authorities, contribute to the overall planning, selection of structural system, and to its configuration. This may lead to building structures with irregular distributions in their mass, stiffness and strength along the height of building.

When such buildings are located in a high seismic zone, the structural engineer’s role becomes more challenging.

Therefore, the structural engineer needs to have a thorough understanding of the seismic response of irregular structures. In recent past, several studies have been carried out to evaluate the response of irregular buildings. During earthquake, torsionally coupled &

irregular buildings in hilly areas suffer more damage.

Dynamic characteristics of hill buildings are somewhat different than the buildings on flat ground. Torsional effect of such buildings is demanded for having the difference stiffness and mass along horizontal and vertical plane during earthquake ground motion. Short column of RC frame building suffer damage because of attracting more forces during earthquake. And also architectural buildings which are irregular in plan and elevation suffer more. This paper is an attempt to summarize the work that has been already done pertaining to the seismic response of vertically irregular building frames.

Design Code Perspective on Vertical Geometric Irregularity

In the earlier versions of IS 1893 (BIS, 1962, 1966, 1970, 1975, 1984), there was no mention of vertical irregularity in building frames. However, in the recent version of IS 1893 (Part 1)-2002 (BIS, 2002), irregular configuration of buildings has been defined explicitly. They are: stiffness irregularity (soft story), mass irregularity, vertical geometric irregularity (set-back), in-plane discontinuity in lateral-force-resisting vertical elements, and discontinuity in capacity (weak story). National Earthquake Hazards Reduction Program (NEHRP) code (BSSC, 2003) has classifications of vertical irregularities similar to those described in IS 1893 (Part 1)-2002 (BIS, 2002) as shown in table 1. As per this code, a structure is defined to be irregular if the ratio of one of the quantities (such as mass, stiffness or strength) between adjacent stories exceeds a minimum prescribed value. These values (such as 70-80%

for soft story, 80% for weak story, and 150% for set-back structures) and the criteria that define the irregularities have been assigned by judgment. Further, various building codes suggest dynamic analysis (which can be elastic time history analysis or elastic response spectrum analysis) to come up with design lateral force distribution for irregular structures rather than using equivalent lateral force (ELF) procedures.

The stepped building form is recognized by several design codes, such as IS 1893:2002 and ASCE 7:2005, as a

Proceedings of the National Conference on Advances in Civil Engineering and Infrastructure Development

typical form of vertical geometric irregularity that merits special design consideration. As per IS 1893:2002, such building forms are to be treated as vertically irregular as shown in Fig 1. As per ASCE 7:2005, when the horizontal dimension of the building in any story (Li) is more than 130% of that in an adjacent story (LiC1) this building will be considered as vertically irregular as shown in Fig 2. Evidently, the codes consider the ratio of geometric lateral dimension of one storey of a building to the other storey as a parameter to define vertical geometric irregularity.

Fig. 2: Showing irregularity limits as per ASCE 7: 2005.

This does not account for the offsets in the other floors.

Also, the definitions of vertical geometric irregularity in design codes do not account for gradual variation in irregularity. Moreover, they treat all kinds of geometrically irregular buildings as one category. Table 1 shows different code limits for vertical irregularity REVIEW OF RESEARCH WORKS REGARDING VERTICAL GEOMETRIC IRREGULARITIES B.G. Birajdar and S.S. Nalawade (2004) carried out seismic analyses on 24 RC buildings with three different configurations like, Step back building, Step back Set back building and Set back building as shown in Fig 3-5.

3 –D analysis including torsional effect has been carried out by using response spectrum method. The dynamic response properties i.e. fundamental time period, top storey displacement and, the base shear action induced in columns were studied with reference to the suitability of a building configuration on sloping ground. It is observed that Step back Set back buildings are found to be more suitable on sloping ground.

Fig. 1: Showing irregularity limits as per IS: 1893:2002.

Table 1: Irregularity limits prescribed by different codes

Type of irregularity ECB 2004 IS 1893:2002 ASCE 07:2005 UBC 1997

Mass Should not reduce

abruptly Mi < 2Ma Mi < 1.5Ma Mi < 1.5Ma

Stiffness Si < 0.7Si+1 or Si <

0.8(Si+1 +Si+2 + Si+3) Si < 0.7Si+1 or Si <

0.8(Si+1 +Si+2 + Si+3) Si < 0.7Si+1 or Si <

0.8(Si+1 +Si+2 + Si+3) Si < 0.7Si+1 or Si <

0.8(Si+1 +Si+2 + Si+3) Soft storey Si < 0.7Si+1 or Si <

0.8(Si+1 +Si+2 + Si+3)

Si < 0.7Si+1 or Si <

0.8(Si+1 +Si+2 + Si+3)

Si < 0.7Si+1 or Si <

0.8(Si+1 +Si+2 + Si+3) Weak storey Si < 0.8Si+1 Si < 0.6Si+1 or Si <

0.7(Si+1 +Si+2 + Si+3) Si < 0.8Si+1

Setback irregularity Rd < 0.3 Tw at any

level SBi <1.5SBa SBi <1.3SBa SBi <1.3SBa

A Review on Seismic Analysis of Vertical Geometric Irregularities of Buildings

Fig. 3: Step back building

Fig. 4: Step back Set back building

Fig. 5: Set back building

Pradip Sarkar et al. (2010) studied `Stepped building' frames, with vertical geometric irregularity, The authors have proposed a new method of quantifying irregularity in such building frames, accounting for dynamic characteristics (mass and stiffness). The proposed

`regularity index' provides a basis for assessing the degree of irregularities in a stepped building frame. They also proposed a modification factor for the empirical formula for estimating fundamental time period for stepped building frames. The proposed equation for fundamental time periods is expressed as a function of the regularity index. It had been validated for various types of stepped irregular frames. A measure of vertical irregularity, suitable for stepped buildings, called `regularity index', is proposed, accounting for the changes in mass and stiffness along the height of the building. This is simple in concept, and is shown to perform better than existing measures. An empirical formula (modification of the existing code formula for regular RC framed building) is proposed to calculate the fundamental time period of stepped building, as a function of regularity index. This has been validated by free vibration analysis, performed on 78 stepped frames. A case study of an existing stepped building located at New Delhi demonstrates that the proposed correction to the code specified empirical

formula results in an accurate estimate of the fundamental period, even for three dimensional building models.

C. V. R. Murty et al. (2012) studied the damages occurred during Sikkim earthquake in 2011. The earthquake near the Nepal-Sikkim border on 18 September 2011 (18:10 local time) (re) emphasized many lessons on technical, techno-legal and techno-financial systems related to development of built-environment in the region. From their study, it was observed that there were insufficient precautions in land slide potential assessment of hill slopes and the RC structures were not designed for earthquake resistance.

A.R. Vijaya Narayanan et al. (2012) observed that RC frame buildings in the Himalayan region are constructed without even designing them for gravity loads. Many features of these buildings render them vulnerable to lateral shaking, including column bases rest on ground at different heights and half brick masonry infill walls distributed irregularly in plan within each storey. This paper presented results of nonlinear analyses performed on typical buildings on steep hill slopes with two possible types of column base connectivity-one as column bases fixed and other one is roller support except lower two columns fixed to ground. It was found that RC buildings with large plan size vulnerable to strong seismic shaking.

The bending moment, shear force and axial force variations are studied and critical portions of hill buildings observed.

Tian Chunyu et al. (2012) conducted Shake Table test on an office building of 30 storey with very irregular plan and elevation having a height of 112.4m as shown in Fig 6. Shaking table test was carried out on the structure to investigate its seismic performance. A 1/20 scaled test model was designed and built, then tested on the shake table of China Academy of Building Research under small, moderate and large earthquake levels. Test results show that main seismic performance objectives of the structure can be achieved. However, the irregularities lead to some severe damage under strong earthquakes.

Fig. 6

Proceedings of the National Conference on Advances in Civil Engineering and Infrastructure Development Based on the research results some suggestions were

proposed to contribute favorable effect on the seismic capacity of the structure, the structural irregularities, including plan reduction, little top tower and uncontinous columns, cause some obvious damage under large earthquakes. The little top tower has obvious whipping effect and in the structure design some measures had be taken to ensure its safety. It was suggested to strengthen the internal corner joint at 7th, 15th and 23rd storey and the corner column-brace joint at 11th storey by adding steel shape in beam and columns around the joint, and adding reinforcement in the floor slab connected to these joints

Ravikumar C M et al. (2012) made an attempt to study two kinds of irregularities in the building models namely plan irregularity with geometric and diaphragm discontinuity and vertical irregularity with setback and sloping ground. These irregularities are created as per clause 7.1 of IS 1893 (part1)2002 code. In order to identify the most vulnerable building among the models considered, the various analytical approaches are performed to identify the seismic demands in both linear and nonlinear way. It was also examined that the effect of three different lateral load patterns on the performance of various irregular buildings in pushover analysis using ETABS 6.0. This study creates awareness about seismic vulnerability concept on practicing engineers. The work conclude that (i) The equivalent static method doesn’t consider the irregular effects in the building (ii) In pushover analysis the codal type of vertical distribution of lateral force was found more detrimental in low rise models (iii) The performances of all the models except sloping ground are lies in between life safety and collapse prevention. This shows the buildings resting on sloping ground are more vulnerable to earthquake than rest of the models and capacity of the buildings may be significant but the seismic demand varies with respect to the configurations.

Rayyan-Ul-Hasan Siddiqui and H.S.Vidyadhar (2013) studied the mechanism for failure of multistorey buildings on levelled and sloped ground using ETABS 9.7. They were concentrated on structural behavior of RC building with and without infill walls and shear walls and also behavior of building on levelled and sloped ground. The results were obtained in forms of storey displacements and base shear. The analysis is carried base on Indian standards using equivalent static, response spectrum and pushover analysis. The work concludes that the sloping ground buildings possesses relatively more maximum displacements and shear forces which may give rise to critical situations than the buildings on leveled ground.

SUMMARY

The presence of structural irregularity changes the seismic response and change in the seismic response depends

upon types of structural irregularities. It seems to be more works required on hill buildings and vertical irregularities.

The main focus of the earlier research works was to vary the mass, stiffness and geometric configurations of buildings. It is also suggested suitable configuration for hill buildings. Many works were carried out on shake table test on irregular buildings and comparative studies on methods of analysis such as Response spectrum analysis and Pushover analysis using softwares ETABS and SAP Review of Seismic Response of Vertical Irregular Building Frames”, ISET Journal of Earthquake Technology, Technical Note, Vol. 43, No. 4, December 2006, pp. 121-132

[3] Pradip Sarkar, A. Meher Prasad, Devdas Menon. “Vertical geometric irregularity in stepped building frames”, Eng Struct 2010;32:2175-2182.

[4] A.R. Vijaya Narayanan, Rupen Goswami and C.V.R.

Murty., “Performance of RC Buildings along Hill Slopes of Himalayas during 2011 Sikkim Earthquake”. 15^th WCEE LISBOA 2012.

[5] Tian Chunyu，Liu Junjin，ZhangHong，Cao Jinzhe.,

“Experimental study on seismic behavior of an Irregular high-rise building”, China Academy of Building Research, Beijing, 15^th WCEE LISBOA 2012.

[6] C. V. R. Murty, Arun Menon, Rupen Goswami, A. R.

Vijaya Narayanan, S. R. Gandhi, K. N. Satyanarayana, S.

T. G. Raghukanth., “Observations from Damages Sustained in India during 2011 (India-Nepal) Sikkim Earthquake”, 15^th WCEE LISBOA 2012.

[7] S.Varadharajan, V.K.Sehgal and Babita Saini,. “Review of different Structural irregularities in buildings”. Journal of Structural Engineering Vol.39, No.5, Dec 2012-Jan 2013 pp.538-563.

[8] Rayyan-Ul-Hasan Siddiqui and H. S. Vidyadhara.,

“Seismic Analysis of Earthquake Resistant Multi Bay Multi Storeyed 3D - RC Frame”, International Journal of Engineering Research & Technology (IJERT) Vol. 2 Issue 10, October – 2013, ISSN: 2278-0181

[9] Dr. Sanjaya Kumar Patro, Susanta Banerjee, Debasnana Jena, Sourav Kumar Das., “A Review on Seismic Analysis Of a Building on sloping ground”, IJERT Vol. 2 Issue 10, October – 2013

[10] IS 1893 (part 1) 2002. Indian standard criteria for earthquake resistant design of structures. New Delhi:

Bureau of Indian Standards; 2002.

[11] ASCE 7. Minimum design loads for buildings and other structures. American Society of Civil Engineers; 2005.

Proceedings of the National Conference on Advances in Civil Engineering and Infrastructure Development (ACEID-2014), Vasavi College of Engineering, Hyderabad, A.P. 6 - 7 February, 2014. pp.107-112.