ASSAM EARTHQUAKE AND OUR BUILDINGS
Md Ali Akbar MOLLICK*

Introduction An earthquake of magnitude 8.7 on the Richter Scale (8.1 on the Moment Magnitude Scale) occurred on 12th June 1897 at 5.11pm in the Assam Basin, some 250 kilometers (km) north from Dhaka, 130 km north-west from Sylhet and 70 km north from today's Bangladesh and Indian border. The epicenter was 26o (N) and 91o (E) and focal depth was 32 km. The shaking was felt over an area of 650,000 square-kilometer (sq-km) and destruction was taking place over an area of 390,000 sq-km. Many masonry buildings including temples, over the destructive area such as Dhubri, Goalpara, Guwahati, Kuch Bihar, Agortola, Kolkata and other cities inside India, either totally or partially damaged due to this earthquake. The earthquake created general panic in Dhaka and other areas of today's Bangladesh including Rangpur, Dinajpur, Sylhet, Rajshahi, Natore, Mymensing, Dhaka, Chittagong, Comilla, Noakhali, Jamalpur etc. A total number of 1542 persons were killed due to this earthquake out of which five were from Dhaka including two foreigners. Three buildings such as Shaheen Medical Hall, Temple Nazi's Shabagh Math and house occupied by Mrs Stansbury were totally damaged and the government houses for commissioner, collector, judge and civil surgeon were partially damaged in Dhaka, which were rebuilt. The total cost for the rehabilitation was 150,000 taka. The intensity of shaking in Dhaka was VIII+ on the Modified Mercili Intensity (MMI) Scale. The intensities in other major cities in Bangladesh were believed to vary from V to VIII+. There are two ways to determine the intensity of shaking on MMI Scale, one is by acceleration meter and the other is by physical examinations. Perhaps there were no installation of acceleration meter in Dhaka at that time and the intensity was determined by the extent of damages on masonry buildings. Only a few number of masonry buildings were in Dhaka or other area of Bangladesh in 1897, but now the number is some millions. So Bangladesh should prepare to withstand a similar shaking created by Assam Earthquake, especially a shaking equivalent to VIII+ on MMI Scale (equivalent to 140 to 260 cm/sec2). Because the trends of earthquakes are: (a) the regions experienced earthquake in the past are prone to it, (b) the magnitude of future earthquake may be uniform to the past ones, and (c) the earthquake occurrence, geological data and tectonic history all has close relationships.
*Notes: (1) Dr Md Ali Akbar Mollick, Earthquake Specialist (Structural Engineer) and Technical Adviser, JICA Expert Team, Email: drmollickmaa@yahoo.com and (2) This paper is presented at a seminar in Hotel Star Pacifc, East Dorga Gate, Sylhet on 25 September 2013, organized by Bangladesh Steel Re-rolling Mills (BSRM) Ltd.

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Since early eighties, apartment housings are being constructed due to the change of socioeconomic conditions and its popularity in Bangladesh. Also the birth of Bangladesh National Building Code (BNBC) in 1993 [1] made the structural designers for being more interested in seismic design. The author believes that the designers are practicing the seismic design within the scope of estimating lateral seismic minimum static forces by using some given formulas in BNBC, which is the same version of The Uniform Building Code (UBC) 1991 [2]. In addition to the consideration of static seismic forces, horizontal and vertical irregularities in buildings should be checked and replacement of solid bricks as in-filled materials in RC frames should also be in considerations to avoid poor performance in the event of earthquakes. Due to the lack of proper awareness, most of the buildings in Bangladesh are vulnerable to earthquakes. Bangladesh cannot afford to face the similar catastrophes of Haiti (Haiti Earthquake on 12 January 2010, M7.0) but should show a better performance like Chili (Chili Earthquake on 27 February 2010, M8.8). Horizontal irregularities and vertical irregularities are mainly responsible for the damage or collapse of a building in the event of moderate to major earthquakes. Especial attentions are to be given to the following areas of horizontal and vertical irregularities: HORIZONTAL IRREGULARITIES Irregular plans, incompatible span lengths, wrong orientation of columns, non-eccentric beam-column joints, non-uniformity in column sizes and uneven placement of lateral force resisting elements are to be addressed to eliminate horizontal irregularities as follows: (a) Irregular Plans Regular plans such as I, H, O are preferred and articulated plans such as T, L, C are not preferred as shown in Fig.1 for better seismic responses. To avoid articulated plans on a T or L shaped plot of land, subdivided plans should be preferred to make them in simpler forms. Buildings at street corners are more vulnerable to earthquakes due to difficulties in the achievement of regular plans than in those along streets whose plans can be outlined regular easily.

Fig.1: Regular (I O H), irregular (T L) and sub-divided (L) plans

(b) Incompatible Span Lengths Usually rectangular plans are preferred by architectures due to the rectangular shape of plot of lands. For example, the length of a particular plan in East-West direction is longer than its North-South direction. The former is provided with three and the latter is provided with two spans respectively as shown in Fig.2. The length of two spans in the North-South direction
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should be equal if possible to ensure a balanced structural building. At least the length of two end spans of three in East-West direction should also be equal and the middle one may be equal or longer/shorter with respect to the two end spans. The compatibility in span lengths should be maintained as far as practicable to avoid unbalanced planning of buildings. These ensure structural regularities in the planning.
(North)

L1 (West) L1
(East)

(South)

L2

L2/3

L2

Fig.2: Regular plan with compatible span length

(c) Wrong Column Orientation Most of the columns of buildings in Bangladesh are designed as rectangular in shape. These practices are not only for the demands of flexural and shear capacities of columns but also for convenience in the entry of motor vehicles and good looking. Let us consider the seismic force is to be acted in the lateral or weaker direction (North-South) of the building shown in Fig.2. If depth of any column section is h and width is b, then the moment of inertia is bh3/12 in lateral direction. If the column section is oriented in longitudinal direction, then the moment of inertia is hb3/12, which is much smaller than bh3/12 as shown in Fig.3. Therefore, the depth of column sections should be oriented in lateral direction of buildings as shown in Fig.2. But sometimes the structural calculations may require the orientation of some columns in longitudinal direction when they are subjected to higher bi-directional moments. Except these cases, all other columns are to be oriented in lateral direction of the buildings.

b h b

h

I= hb3/12

I = bh3/12

Fig. 03: Moment of inertia of column section with orientations 3

(d) Eccentric Beam-Column Joints Usually the beams in exterior frames are justified (flushed) with outer faces of columns for good looking. So the corner and intermediate columns of the exterior frames (Fig.4) are subject to bi-directional and unidirectional eccentricities respectively. Eccentricities in the beam-column joints are undesirable because many buildings collapsed due to eccentric beam-column joints during earthquakes. One of such research reports [3] founded that shear strength of eccentric beam-column joints is diminished by 40 percent due to seismic force resulting collapse of the beam-column joints. So good looking should not be a preferential factor while keeping the structural joints vulnerable to earthquakes. A designer should bear in mind that the joints of any structural system are most vital part of its fabrication.

Column

Beam

Column

Beam

Desirable positions of beams

Undesirable positions of beams (beam-column joints subject to torsion) Column

Fig.4: Beam-column joints with and without eccentricities shown by broken and solid lines respectively

(e) Non-uniformity in Column Sizes The sizes of corner and exterior columns usually are designed to be smaller than the interior columns. The sizes are subjugated by the gravitational forces. The lateral seismic minimum static forces in addition to gravitational forces results larger dimensions of beam and column sections somewhat proportionately. So the sizes of corner or exterior columns remain smaller in comparison to the interior columns. If the dimensions of the corner and exterior columns are the same as the interior ones (Fig.5), if needed with lower ratio of reinforcing bars, the performance of the frame will be better in the event of shakings, due to uniform compressive area of columns. The corner or exterior columns are also to be provided the same size of interior columns due to proper rebar detailing.

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Corner Column

Exterior Column

Desirable Uniform Size Exterior Column Interior Column

Undesirable Smaller Size

Fig.5: Beam-column joints with and without eccentricities shown by broken and solid lines respectively

(f) Uneven Placement of Lateral Force Resisting Elements Torsional effects on buildings can be minimized by placing vital elements in strategic locations in even form in the aim of reducing distances between center of mass (CM), where seismic floor forces are applied, and the center of rigidity (CR). A number of examples for both undesirable positioning of major lateral-force-resisting elements, consisting of structural walls and frames, and desirable locations are given in Fig.6.

Undesirable

Desirable

CR CM

CR

CM

CR

CM

CR

CM

CR CM

CR CM

CM CM = CR CR

Fig.6: Examples of undesired and desired placement of lateral force resisting elements 5

Response in Plan: In relation to the response in plan, two important concepts such as center of mass and center of rigidity must be defined. During shaking, acceleration-induced inertia forces be generated at each floor level, where the mass of an entire story may be assumed to be concentrated. Hence the location of a force at a particular level be determined by the center of the accelerated mass at that level. In regular buildings, the positions of the centers of floor masses differ very little from level to level. However, irregular mass distribution over the height of a building may result in variation in centers of masses, which should be avoided as far as possible. The center of rigidity of a level depends on the infilled RC frames of that level. If infills are made in the frames that make the plan uneven as shown in Fig.7(a), the center of rigidity will be shifted from the center of mass. This will cause the building subject to eccentricities (torsion) in lateral or in longitudinal or in both axes. Therefore, designer should achieve minimum distance between the center of rigidity and center of shear (mass) force by a deliberate assignment of stiffness to lateral force-resisting components, such as bracing and/or walls (Fig.7(b)).
CV: Center of Storey Shear; CR: Center of Rigidity CV: Center of Storey Shear; CR: Center of Rigidity

ex CR CV CR CV

Guard Room

ENTRANCE

ENTRANCE

(a) Typical Example of Ground Floor of a House Building in Bangladesh

(b) Ground Floor of a House Building with Seismic Resitance Elements in Addition to Columns

Fig.7 Ground floor with columns and with RC elements in addition to columns

Check for Eccentricity [4]: A building with irregular plan is subjected to greater vulnerability of earthquake than a building with regular plan. The eccentricity factor should be checked for building to avoid poor performance in the event of a major earthquake. As shown in Fig.8, (gx, gy) is the center of gravity of the total mass above the storey are being considered. The center of gravity can be obtained from the following equations:
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ex

ey

(l x , l y )

(gx , gy )

y

x
Fig.8 Building with eccentricity

gx =  (N x) / W gy =  (N y) / W

Total =  K y
 K x3

 K y1

 K y2

 K y3

 K y4

Y

X
 K x2

 K x1 Total =  K x

(1a) (1b)

where, N is the axial force on column, shear wall, and W =  N in the storey being considered. The center of gravity i.e. the center of rotation (lx, ly) under the action of torsional moment can be obtained from the following equations: lx =  (Ky x) /  Ky ly =  (Kx y) /  Kx (2a) (2b)

where, Kx and Ky are the transitional stiffness in x and y axes. The eccentric distances in the axes of x and y are ex and ey, as shown in the Fig.8 and can be obtained from the following equations: ex =  lx  gx  ey =  ly  gy  (3a) (3b)

The eccentricity factors Rex and Rey are to be checked by the following expressions: Rex = ey / rex  0.15 Rey = ex / rey  0.15 where, rex and rey are the elastic radii defined by the following equations: rex = (Kr /  Kx )1/2
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(4a) (4b)

(5a)

rey = (Kr /  Ky )1/2 where, KR is the rotational stiffness, defined by the following equations: Kr = Ix + Ix =  (Kx Y2)+  (Ky X2)

(5b)

(6)

where, Ix and Ix are the components of rotational stiffness in the axes of x and y. The X and Y are defined by the Fig.8. If one or more stories do not satisfy the requirement of Eq.4, the building must be checked for the horizontal load carrying capacity.

VERTICAL IRREGULARITIES Weak-beam strong-column design philosophy, easy frame systems, soft-storey, flat plate, short-column and masonry brick works are to be addressed in the vertical irregularities as follows: (a) Seismic Frame Design Philosophy It is very undesirable that the summation of flexural strengths of all beams is higher than the summation of flexural strengths of all columns in a particular plane of a joint. If the joints of a frame are designed in this manner, they are subjected to very high shear forces during seismic activity, result in excessive loss in strength and stiffness of the frame, and even collapse. The seismic energy is considered in the floor whose joints are designed in this manner, collapsing the building at least above that floor. To minimize collapse that may be caused by a significant seismic event, buildings should be designed by weak-beam strongcolumn philosophy. In weak-beam strong–column frame, as shown in Fig.9(b), plastic hinges are developed in all the beam-ends. Even with a weak-beam strong-column design frame, which seeks to dissipate seismic energy primarily in well-confined beam plastic hinges, a column plastic hinge must still form at the base of the column. The beam end and column base act as ductile hinge forming spring type mechanism. Therefore, the seismic energy is dissipated in all the beam ends and base of the columns, distributing the energy all throughout the building instead of concentrating the energy in a particular storey, which is very likely to occur in strong-beam weak-column frame as shown in Fig.9(a). For this reason, a designer must have to design the building following weak-beam strongcolumn philosophy so that only the mechanism shown in Fig.9(b) can develop to save the building from possible collapse. Special care should be taken to determine the region of plastic hinges and to make adequate confinement of the plastic hinges by transverse reinforcement. According to the ACI Committee 318 [5], weak-beam strong column design may be obtained if flexural strengths of the column satisfy:

 Mc  1.20  Mg

(7)

8

 Mc = sum of moments, at the center of the joint, corresponding to the design flexural strength of the columns framing into that joint. Column flexural strength shall be calculated for the factored axial force, consistent with the direction of the lateral forces considered, resulting in the lowest flexural strength.  Mg = sum of moments, at the center of the joint, corresponding to the design flexural strengths of the girders framing into that joint.



(a)Strong Beam-Weak Column Frame Building

(b)Weak Beam-Strong Column Frame Building

Fig.9 Seismic energy dissipation in strong-beam weak-column and weak-beam strong-column frame

Also ACI Committee 318 suggests that the column reinforcement ratio based on the gross section ρg must meet the requirement: 01 ≤ ρg ≤ 0.06. Unless adequate, closely spaced, well-detailed transverse reinforcement is placed in the potential plastic hinge region, spalling of concrete followed by instability of the compression reinforcement will follow. Therefore, the following guideline should be strictly followed in the design of transverse reinforcement. Transverse reinforcement shall be provided over a length of lo from each joint face and on both sides of any section where flexural yielding is likely to occur in connection with inelastic lateral displacements of the frame. The length lo shall not be less than (a) the depth of the member at the joint face or at the section where flexural yielding is likely to occur, (b) onesixth of the clear span of the member, and (c) 457-mm (18-in).
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Minimum transverse reinforcement is specified in terms of the volumetric ratio of transverse reinforcement to the volume of the core confined by the reinforcement ρs, for spiral or circular hoop reinforcement as

ρs = 0.12 f'c / fyh but not less than

(8)

ρs = 0.45 (Ag / Ach - 1) f'c / fyh

(9)

The total cross-sectional area of transverse rectangular hoop reinforcement Ash along the length of the longitudinal reinforcement shall be Ash = 0.30 (s hc f'c / fyh) (Ag / Ach - 1) but not less than Ash = 0.09 s hc f'c / fyh (11) (10)

where, f'c : compressive strength of concrete; fyh : Specified yield strength of transverse reinforcement; Ag : gross area of section; Ach : cross sectional area of column core, measured out-to-out of the transverse reinforcement; s : spacing of transverse reinforcement and hc : cross sectional dimension of the column core, measured center-to-center of the confining reinforcement.

(b) Easy Frame Systems A number of undesirable and desirable configurations are illustrated in Fig.10. An abrupt change in elevation, such as shown in Fig.10(a), called a setback, may result in the concentration of structural actions at and near the level of discontinuity. Such actions are difficult to predict in the event of seismic force. So the separation into two simple and regular structural systems, with adequate separation between them, is preferred as shown in Fig.10(b). Irregularities within the framing system, such as a drastic interference with the natural flow of gravity loads and that of lateral-force-induced column loads at the center of the frame in Fig.10(c), must be avoided. Staggered floor arrangements, such as in Fig.10(e), may invalidate the rigid interconnection of all vertical lateral-force-resisting units. Horizontal inertia forces in the event of seismic force, may impose severe demands, particularly on the short interior columns. If two buildings appear to be identical (Fig.10(g), there is no
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assurance that their response to ground shaking will be in phase. Hence, any connection (bridging) between the two may be desired to prevent horizontal force transfer between the two structures (Fig.10(h)).

Undesireable

Desireable

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Fig.10 Examples of undesired and desired structural frame systems

(c) Soft-Storey Since mid-1980s, RC frame buildings with car parking in ground floor have been developed as a local construction practice in Bangladesh. Amongst some other factors, keeping the car parking floor with columns only has become a prominent factor for making the buildings vulnerable to earthquakes. The main factors of vertical irregularities including this factor are discussed as follows: Response in Elevation: When subjected to lateral force only, a building will act as a vertical cantilever. The resulting total horizontal force and the overturning moment will be transmitted at the level of the foundations. It should be noted that the ground story is subjected to highest storey shear force and bending moment developed due to earthquake-induced horizontal forces as can be seen from Fig.11. The storey shear at the ground storey is the summation of all horizontal forces acting on the edge of every floor and roof above the ground storey. Also the bending moments developed at the base of columns are the highest. The storey

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1 2
F6 R

F5

5F

F4

4F

F3

3F

F2

2F

1 >> 2 2

F1

1F

1
A Typical 6-Storey House Building Storey Shear Bending Moment Deflection Mode

Fig. 11 A typical 6-storey house building subject to seismic force

shear force and overturning moment at any storey level must be resisted by shear and axial forces. The resistance capacity shall not be adequate if the ground and/or other stories, which are used for car-parking, are not supplied with elements like reinforced concrete shear walls and/or bracing with exterior columns and some other transverse bays as can be seen from Fig.7(b). The elements should be designed taking the following points in account such as: (i) the car-parking floor must have provision for free access of vehicles; (ii) the center of rigidity must not shift substantially from the center of storey shear to minimize torsional effect; (iii) the summation of shear strengths and flexural capacities of all columns and other elements must be greater than the storey shear force and bending moment induced by horizontal seismic force, and (iv) the lateral stiffness of the car-parking storey and its neighboring upper storey should not differ substantially. The said elements will enable the building to sustain large deformations, and a capacity to absorb energy by hysteretic behavior. For this reason, ductility is the single most important property sought by the designer of buildings located in regions of significant seismicity. If the shear walls and/or bracing are not supplied in the car-parking story (a common tendency usually observed in Bangladesh), it may become a soft-storey or weak-story. A lesson has repeatedly been learned from many earthquakes that the soft-storey and/or weak-storey is one of the most common causes of building failure. To avoid such a failure, the following checks are essential: Check for Storey Drift [4]: The storey drift at every floor level I under the action of lateral seismic shear force Qi are calculated by the elastic analysis. Then the story deformation angle Ri is to be checked by the following expression: Ri = i / hi  1/200 rad (12)

where hi: height of i-th story. The value of Ri can be released up to 1/120 rad in case of nonstructural members.
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Check for Rigidity Factor [4]: Check for rigidity factor Rsi is to be done by the following expressions: Rsi = rsi / rs  0.60 (13) rsi = 1 / Ri rs   rsi / n i 1 n

(14) (15)

n: number of stories. (d) Flat Plate in Residential Buildings Construction of residential buildings by RC flat plates became popular in Bangladesh from late 90s. Many real estate developers in the country initiated this practice and attracted the apartment buyers because of: (1) good looking due to absence of beams, (2) rooms inside an apartment could be rearranged as per requirements and (3) building construction time period could be shorter in comparison to the conventional beam-slab system. It seems that the structural designers of such flat plate RC buildings are not concerned to the seismic response behavior of such flat plate buildings. The flat plates make the buildings more vulnerable during earthquakes because: the weight of every storey becomes higher resulting action of higher seismic lateral force and the frame type is not compatible to the weak beamcolumn strong-column philosophy, which is recommended in seismically active zone (Eq.7).

70% of span Length from center-to-center of columns

Fig.12 RC flat plate-column joint subject to seismic force

As shown in Fig.12, a flat plate becomes an imaginary beam having width approximately equal to 70% of span length from center-to-center of two neighboring columns during earthquakes. Since the thickness of flat plate usually used as 180- to 200-mm (7- to 8inches) with sufficient rebar, it possesses higher flexural strength than the column. This is a clear violation of Eq.(7) and opposite to the weak-beam strong-column philosophy.
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Especially low-rise buildings (6-storied in Bangladesh is typical example) are more vulnerable because while the properties of columns are designed by considering number of building stories, the supplied flat plates are thin as much as 200-mm irrespective to the number of building stories. (e) Short-Column Providing walls with columns in the balcony, providing openings for windows and ventilations connected to columns make columns short. These short columns when subject to earthquakes, are attacked by larger seismic horizontal force that the tall columns as shown in Fig. 13 (a). Due to the larger horizontal force, short column suffer damage as shown in Fig. 13(b, left). To prevent such damage, slits are provided as shown in Fig. 13 (b, right).





Beam

Beam

Short-Column
Short Columns: Attracts larger horizontal force

Short-Column Wall Wall

Wall

Wall

Tall Columns: Attracts smaller horizontal force

Slits

(a) Short columns subject to larger seismic force than tall columns

(b) Short column damage(left)and prevention of damage by providing slits (right)

Fig. 13: Tall and short columns subject to seismic forces

(f) No to Solid Bricks Most of the buildings with RC frame, are supplied with masonry bricks as infill materials. The masonry infill is composed of solid bricks, hollow bricks and hollow blocks supported only by sand-cement mortar. When the frames are subject to seismic force, the behavior of lateral deformation becomes more complex as a result of the frame attempting to deform in a flexural mode while the panel attempts to deform in a shear mode. The result may be splitting, crushing, shear sliding and separation of infill, with shedding masonry into streets below or into stairwells with great hazard to human life. The vital part of the masonry works in infill RC frames is given below:

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Solid Bricks: Where earthquake is a concern, the use of solid bricks as masonry infill in RC frames should be avoided as far as possible. The use of solid bricks makes the weight of each storey heavier and so the total weight of the building becomes much higher in comparison to the building, which uses lighter hollow bricks/blocks. Since the seismic force in horizontal direction is directly proportional to the weight of the building, it will be much higher if solid bricks are used. Therefore, the story shear and bending moment at ground base will be much higher and that will subject the building more vulnerable to earthquake. Moreover, solid bricks can be supported only by sand-cement mortar but not by rebar in the horizontal and/or vertical direction. Hollow Bricks/Blocks [6]: Lighter hollow bricks/blocks masonry is encouraged to use as infill materials in RC frames (Fig.14). They are consists of masonry units, most commonly with two vertical flues or cells to allow vertical reinforcement and grout to be placed. They are typically constructed to a nominal 200x400 (8 in x 16 in) module size in elevation, with
Vertical Rebars Vertical Rebars

Horizontal Rebars

(a)Hollow Bricks/Blocks with Vertical and Horizontal Rebars

(b)Hollow Bricks/Blocks with Vertical Rebars

Fig. 14 Strengthening and securing hollow bricks/blocks by rebar

nominal widths of 100, 150, 200, 250 or 300 mm (4, 6, 8, 10, 12 in). Actual dimensions are typically 10 mm (3/8 in) less than the nominal size to allow the placement of mortar beds. The grouted cavity is typically 50 to 100 mm (2 to 4 in) wide. Concrete masonry units vary in compression strength from 12 MPa (1750 psi) for some lightweight (pumice aggregate) nominal widths of 100, 150, 200, 250 or 300 mm (4, 6, 8, 10, 12 in). Actual dimensions are typically 10 mm (3/8 in) less than the nominal size to allow the placement of mortar beds. blocks to over 30 MPa (4350 psi) for blocks made with strong aggregates. A minimum strength of about 12.5 MPa (1800 psi) is typically required by design codes. The modulus of elasticity for concrete masonry is 1000f'm and for brick masonry 750 f'm (f'm is the compressive strength of masonry units). Grout [6]: The grout used for reinforced masonry construction can be characterized as a small-aggregate sized, high-slump concrete. High water/cement ratios are necessary to enable the grout to flow freely under vibration to all parts of the masonry flues or grout gaps.
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A minimum compressive strength of 17.5 MPa (2500 psi) is typically required for grout, but strength up to 30 MPa (4350 psi) can be obtained. Combinations [6]: Hollow bricks/blocks are inherently brittle materials. Although their tensile strength cannot be relied on as a primary source of resistance, they are eminently suited to carry compression stresses. The primary objective of the detailing of composite structures consisting of masonry and steel with grouting is to combine these materials in such a way as to produce ductile members, which are capable of meeting the inelastic deformation demands imposed be severe earthquakes. Hollow bricks/blocks can be supplied with rebar in the horizontal and vertical direction securing with the columns and beams of the frame (Fig. 14). The author believes that in the context of Bangladesh, the infilled materials should be secured at least in the vertical direction for upgrading flexural strength and so that they cannot fall over and create situation hazardous to human lives. The amount of rebar may be determined by designing the masonry wall for out-of-plane bending and in-plane bending as described below: Design of Rebar in Hollow Bricks/Blocks for Out-of-Plane Bending [6]: The out-of-plane design is mainly involved in the design of rebar to provide a required ultimate moment capacity Mu with a given axial load level of Pu (Fig.15). The required ideal design actions are thus Mi ≥ Mu / φ Pi = Pu where φ is the flexural strength reduction factor as follows: 0.85 ≥ φ = 0.85 - 2 (Pu / f'm Ag) ≥ 0.65 (18) (16) (17)

It should be noted that the axial load has not been factored up, since in the typical seismic design situation Pu is widely or largely due to gravity loads, and Mu is wholly or largely due to seismic forces. It is convenient to consider the ideal flexural strength Mi to consist of two components viz. a moment Mp sustained by axial load and a moment Ms sustained by rebar. Hence Mi = Mp + Ms where, a1 = Pu (t/2 - a1/2)
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(19)

(20)

Pu Mi
Hollow Block

Rebar

Mortar

Hollow Block

a1+a2

Hollow Block

t

t
PLAN

Rebars

t/2

Pu+As f y

Forces for Analysis a1 a2 Pu+As f y

Subdivision of Compression Forces into Components for Design
Cm = Pu+As fy

Fig.15 Design of masonry wall section subject to out-of-plane bending

The moment to be sustained by rebar with As is Ms = Mi - Mp Assuming that (a1+a2) is small compared to t/2, a2 ≈ a1(Ms / Mp) Hence Ms = As fy (t/2 - a1 - a2/2) where fy is specified yield strength of non-prestressed rebar. And thus As = Ms / fy (t/2 - a1 - a2/2) (24) (23) (22) (21)

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Design of Rebar in Hollow Bricks/Blocks for In-Plane Bending [6]: Referring Fig.16: Mp = Pi (lw/2 - a1/2)
Pi Mi

(25)

lw
ELEVATION

lw/2 a1 a2

Ast f y

Pi

Ast f y
RESULTANT FORCES

Fig.16 Design of masonry wall section subject to in-plane bending

where a1 = Pi / α f'm t α :stress block parameter :12 Ec / Ic2 :Ic is story height The moment to be sustained by rebars with As is Ms = Mi - Mp and from Eq. (19) a2 ≈ a1 (Ms / Mp). Therefore, Ms = As fy (lw/2 - a1 - a2/2) and thus
18

(26)

(27)

(28)

As = Ms / fy (lw/2 - a1 - a2/2)

(29)

Conclusions The following conclusions may be made: (1) Bangladesh has got a natural destructive energy located 32-km below earth surface in the Indian area but only some 250-km away from its capital city of Dhaka. This energy may come up with a major shaking all over Bangladesh having a serious impact on the structures as well as all kinds of lifelines. Preparedness and awareness may minimize the impacts, especially preparedness in construction of seismic resistant buildings should be adequate enough to resist a major shaking. (2) The main objective of this paper is to convey the basic conception of planning and designing buildings with undesirable horizontal and vertical irregularities and for replacement of solid bricks into lighter hollow blocks to the structural designers. There may have some phenomena which are not encountered in any code by the author but given in this paper from professional experiences. (3) In the context of Bangladesh, the measures that have been suggested in the paper to minimize the vulnerability of buildings to earthquake may be compared with insuring the building through applying them with structural elements.

References [1] [2] [3] Bangladesh National Building Code 1993, by Housing and Building Research Institute and Bangladesh Standard and Testing Institution. The Uniform Building Code (UBC) 1991 by The International Conference of Building Officials (ICBO). Masaya HIROSAWA, Tomoaki AKIYAMA, Tatsuya KONDO and Jiandong ZHOU, "Damage to Beam-to-Column Joint Panels of R/C Buildins Caused by the 1995 Hyogo-ken Nanbu Earthquake and the Analysis", 12th World Conference on Earthquake Engineering, 2000, Auckland, New Zealand. Mollick, MAA, “Seismic Design Provisions of Buildings in Japan”, Journal of the Civil Engineering Division, The Institution of Engineers, Bangladesh, Vol. CE 23, No.1, 1995, pages 7-58. ACI Committee 318, “Building Code Requirements for Reinforced Concrete (ACI 318-89)”, ACI Manual of Conference Practice 1995, part , pages 18/18R-1 to 18R-45. Pauly T and Priestley MNJ, "Seismic Design of Reinforced Concrete and Masonry Buildings", Wiley 1992.

[4]

[5] [6]

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