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Tuesday, September 24, 2013

STEPS FOR SAFE DESIGN AND CONSTRUCTION OF MULTISTOREY REINFORCED CONCRETE BUILDINGS - PART 2


2.5 Heavy Water Tanks on the Roof:

Heavy water tanks add large lateral inertia forces on the building frames due to the so called ‘whipping’ effect under seismic vibrations, but remain unaccounted for in the design. See the fall of such water tank in
Fig.10
Fig.10 - 5 storey R.C., collapse of open plinth, water tank at top dislocated (Bhuj)

Recommendation:-

All projected systems above the roof top behave like secondary elements subjected to roof level horizontal earthquake motions which act as base motions to such projecting systems. To account for such heavy earthquake forces, IS:1893-2002 (Part 1) provides in clause 7.12 that their support system should be designed for five times the design horizontal seismic co-efficient Ah specified in clause 6.4.2. Similarly any horizontal projections as the balconies or the cantilevers supporting floating columns, the cantilevers need to be designed for five times the design vertical co-efficient as specified in clause 6.4.5 of IS: 1893-2002 (Part 1)                                                                                                   

2.6 Lack of Earthquake Resistant Design:

Many buildings in Gujarat were not designed for the earthquake forces specified in IS:1893, which was in existence from 1962, revised in 1970, 1976 & 1984. The applicable seismic zoning in Gujarat had remained the same as adopted in 1970 version. It is the same even in 2002 version of IS:1893 (Part I).
Fig.11:- Lateral Strength of Building Frame
All the upper floors weak in long directions (Izmit, Turkey 1999)
Recommendation:-

It does not need emphasizing that all buildings including the multistoried buildings should be designed in accordance with IS: 1893 (Part 1) and IS: 4326 – 1993. The salient features of the design will be presented in Para 3.0 in this guide.

2.7  Improper Dimensioning of Beams & Columns: 
The structural dimensioning of beams and columns was inadequate in terms of provisions in IS: 13920-1993 and also for proper installation of reinforcements in Beam-Column joints as per IS: 456 and IS: 13920.
Fig.12:- Plan of Reinforcement in Beams & Columns

Recommendation:

The relative dimensions of beams & columns become very important in tall buildings from
the point of view of provision of longitudinal & transverse reinforcement in the members as well as the reinforcement passing through and anchored in the beam-column joints, permitting enough space for proper concreting and without involving any local kinking of the reinforcing bars. The practice of using small dimension columns like 200 or 230 mm and beams of equal width is totally unacceptable from the reinforcement detailing view point. Infact for permitting the beam bars passing through the columns, without any local bending then straightening (introducing kinks), the proper scheme would be to use wider columns than the beams. Minimum dimensions of beams and columns, also limiting aspect ratios of the two members, are specified in IS: 13920 which need to be adhered to.


2.8 Improper Detailing of Reinforcement:

In detailing the stirrups in the columns, no conformity appeared to satisfy lateral shear requirements in the concrete of the joint as required under IS 4326- 1976 and IS: 13920-1993. The shape and spacing of stirrups seen in collapsed and severely damaged columns with buckled reinforcement was indicative of non-conformity even with the basic R.C. Code IS: 456-1978.

Recommendation:

In respect of proper detailing of reinforcement in beams, columns, beam-column joints as well as shear walls, all the provisions in IS:13920 have to be carefully understood and adopted in design. The philosophy of over-design of beams in shear to force flexural hinge formation before shear failure, confining of highly compressed concrete in columns and the use of properly shaped shear stirrups with 135 degree hooks are some low-cost but extremely important provisions. For overall safety of the frame, design based on the concept of strong-column, weak-beam system should be adopted as far as practical. It may be mentioned that the full ductility details as specified in IS: 13920 permit the use of the High Reduction Factor R=5 which would make the design economical. But if such ductility details are not adopted, the Reduction Factor is permitted as only 3.0, which means that the design force will become 1.67 times the case when full ductile detailing is adopted which may indeed turnout to be more expensive and at the same time brittle and relatively unsafe (see fig.13).
Fig.13:- Detailing of reinforcement (Overlapping Hoops & Crosstie)



2.9 Short Column Detailing
In some situations the column is surrounded by walls on both sides such as upto the window sills and then in the spandrel portion above the windows but it remains exposed in the height of the windows. Such a column behaves as a short column under lateal earthquake loading where the shear stresses become much higher than normal length columns and fail in shear. (See fig. 14)

Recommendation:
To safe guard against this brittle shear failure in such columns the special confining stirrups should be provided throughout the height of the column at short spacing as required near the ends of the columns.
 
          Fig 1 4 - Damage to buildings due to short column effect on columns

3.      Some Important Codal Design Provisions:

In the last few years the author has had the opportunity of reviewing many reinforced concrete building designs prepared by well-established consulting companies as well as individual consultants and felt the need of preparing brief guidelines so that no important Codal provisions are missed out and the various design details for achieving better construction in the field and better ductile performance in the event of a great earthquake are ensured. Thus a safe and ductile building could be achieved.

3.1 Building Configuration

For achieving basic structural safety of buildings under postulated earthquake forces the first important requirement is that the building should be designed with symmetrical configuration both horizontally and vertically. In any case the seismic force resisting elements must be planned symmetrically about the centre of the mass of the building. IS:1893 (Part 1-2002) presents in detail in cl.7.1 the various types of irregularities which should be avoided as far as possible or corrected by planning the structural resisting elements. The present day requirements of large column free spaces inside can be met by designing strong frames on the periphery of the building so as to resist most of the horizontal design seismic forces and relieving the internal columns relatively from the earthquake forces. For this purpose shear walls may be provided in the building perimeter to increase the stiffness in both principal axes of the building as compared with the internal columns which could be designed basically for vertical loads.

3.2 Calculation of Loads

The loads will include the following:

(i)                 Dead Loads: These will include the weight of all components at each level, viz., roof including water tanks, Barsatis, Parapets, roof finishes, slabs, beams, elevator machine room etc. and including all plasters and surface cladding etc., and each floor level including fixed masonry or other partitions, infill walls, columns, slabs and beams, weight of stairs, cantilever balconies, parapets and plastering or cladding wherever used. The unit weights may be taken from IS:875 (Part 1) or ascertained from the manufacturer.

(ii)               Imposed Floor Loads: IS 875 (Part 2) deals with the imposed loads on roofs, floors, stairs, balconies, etc., for various occupancies. There is a provision for reduction in the imposed loads for certain situations, e.g. for large span beams and number of storeys above the columns of a storey. The earthquake code IS: 1893 (Part 1)-2002 permits general reduction in roof and floor imposed load when considering the load combination with the earthquake loading. But the two types of reductions, that is, in IS: 875 (Part 2) and IS: 1893 (Part 1) are not to be taken together.

3.3 The Earthquake Load:

For working out the earthquake loading on a building frame, the dead load and imposed load and weights are to be lumped at each column top on the basis of contributory areas. The imposed load is to be reduced as specified in IS: 1893 (Part1)-2002 for seismic load determination. Let us call them Wi at ith floor and Wn at the nth level at the roof level for a n-storey building. Hence the total load at the base of the building just above the foundation will be

n
W        = Σ i=1 W i + Wo

where Wo is the weight of elements in the ground storey.

3.4 Earthquake Resistant Design

Now the following steps may be taken:

(a) Estimate fundamental time period Ta using empirical expressions given in the Code IS: 1893-2002.

Ta = 0.075 H0.75, IS: 1893 Cl.7.6.1 for bare frame along each axis

Tax = 0.09h/d along x -axis IS: 1893 Cl.7.6.2 for frame with substantial infills Ta z = 0.09h/b, along z-axis, IS: 1893 Cl.7.6.2 for frame with substantial infills

where h is the height of the building and d and b are the base dimensions of the building along x and z axis respectively.

(b)   Calculate the design horizontal Seismic coefficient Ah

Now compute the fundamental time periods T/x and T/z for the bare frame along the two axes by dynamic analysis. These are generally found to be higher than Tax and Taz respectively.

The design horizontal coefficient Ah is given by

Ah = (Z/2). (I/R). (Sa/g)


Take Z for the applicable seismic zone
(IS: 1893
Cl.6.4.2),
Take I for the use importance of the building
(IS: 1893
Table 2),
Take R for the lateral load resisting system adopted
(IS: 1893
Table 7),

and take Sa/g for the computed time period values T/x, Tax , T/z and Taz with 5% damping coefficient using the response spectra curves IS: 1893 Fig 2 for the soil type observed. Thus four values of Ah will be determined as follows:-

In x-direction A/hx for T/x & Ahax for Tax

In z-direction A/hz for T/z & Ahaz for Taz

(c)    Calculate the total horizontal shear (the base shear) The design value of base shear VB

VB = Ah W

as per 1893 Cl.7.5.3.

For design of the building and portions thereof, the base shear corresponding to higher of Ahax and A/hx, similarly between Ahaz and A/hz will be taken as minimum design lateral force.

(d)   Seismic Moments and Forces in Frame Elements:

Calculate the seismic moments and axial forces in the columns, shears and moments in the beams by using the seismic weights on the floors/(column beam joints) through an appropriate computer software (having facility for using floors as rigid diaphragm and torsional effects as per IS: 1893:2002).

It may be performed by Response Spectrum or Time History analysis. The important point is that according to IS: 1893 Cl.7.8.2., the base shear computed in either of the dynamic method, say V/B shall not be less than VB calculated under Cl.7.5.3 using Ahax and Ahaz. If so, then all shears, moments, axial forces etc worked out under dynamic analysis will be increased proportionately, that is, in the ratio of VB/V/B.

(e)    Soft Ground Storey

It must be designed according to Cl.7.10 of IS: 1893-2002.


4. Method of Design

Structural design of various members has to be done by Limit State Method, as per IS 456-2000 for which the following load combinations should be used to work out the maximum member forces:-

Using


DL
for
DEAD LOAD
LL
for
LIVE LOAD
EQX
for
SEISMIC LOAD (X) DIRECTION
EQZ
for
SEISMIC LOAD (Z) DIRECTION


The load combinations for analysis and design will be taken as follows:

1.
(DL+LL)*1.5
8.
(DL-EQX)*1.5
2.
(DL+LL+EQX)*1.2
9.
(DL-EQZ)*1.5
3.
(DL+LL+EQZ)*1.2
10.
0.9DL+EQX*1.5
4.
(DL+LL-EQX)*1.2
11.
0.9DL+EQZ*1.5
5.
(DL+LL-EQZ)*1.2
12.
0.9DL-EQX *1.5
6.
(DL+EQX)*1.5
13.
0.9DL-EQZ*1.5
7.
(DL+EQZ)*1.5



The members (beams, columns, shear walls etc.) and their joints will be designed for the worst combination of loads, shears and moments.


MATERIALS:

a)      Cement: Ordinary portland cement conforming to IS 269 - 1976 shall be used along with fly ash after carrying out the design mix from approved consultant.

b)      Reinforcement: Cold twisted high yield strength deformed bars grade Fe 415 conforming to IS: 1786-1985, or preferably TMT bars of standard manufacturer e.g. TATA Steel, SAIL or equivalent shall be used.

The following grades of concrete mix may be adopted or as required for safe design:

(a) For RCC columns in lowest few storeys
:
M35
(b) For RCC columns in the middle few storeys
:
M30
(c) For RCC columns in the top few storeys
:
M25
(d)
For beams, slabs, staircase etc.
:
M20
(e)
For raft foundation
:
M 20 or 25
(f)
Max. Water cement Ratio
:
0.45
(g) Minimum cement content
:
300 kg/m3 of concrete.
(h) Admixtures of approved brand may be used as per mix design


CLEAR COVER TO ALL REINFORCEMENT:

For mild Exposure and fire rating of 1 hr. following clear covers may be adopted

(a) For foundation R.C.C.:

i)      Footings             :  60 mm.
ii)  Raft
:
60 mm.
(b) For columns
:
40 mm
(c) For Beams
:  25 mm or main bar dia. whichever is more.
(d) For Slab
:
20 mm.


4.1  Ductile Detailing

After designing the frame column-beam, shear walls and foundation by limit state theory as per IS: 456:2000, all details of longitudinal steel, overlaps, shear capacities, confining reinforcement requirements, stirrups and ties etc. shall be worked out using the provisions of IS: 13920-1993.

The drawings should clearly show all the adopted details.

5.  Concluding Remarks

In a nut-shell, the seismic safety of a multi-storeyed reinforced concrete building will depend upon the initial architectural and structural configuration of the total building, the quality of the Structural analysis, design and reinforcement detailing of the building frame to achieve stability of elements and their ductile performance under severe seismic lading. Proper quality of construction and stability of the infill walls and partitions are additional safety requirements of the structure as a whole. Any weakness left in the structure, whether in design or in construction will be fully revealed during the postulated maximum considered earthquake for the seismic zone in the earthquake code IS: 1893.

Acknowledgement:

The figures have been taken from various sources to suit the text message and are anonymously acknowledged.




Inspite of that, the structural designers ignored the seismic forces in design. It may also be stated that most buildings are designed against lateral load in the transverse direction. Hence they collapse in the longitudinal directions.

Proper arrangement of columns is shown in Fig. 11 which would give adequate seismic resistance along both axes of the building.

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