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INDEX 1)Abstract 2)Introduction 3)Causes of earthquakes 4)Basic terminology: a)Hypocentre, b)Epicentre c)Focal depth 5)Earthquake size a)Magnitude b)Intensity 6) Earthquake hazard a)Primary effects b)Secondary effects 7)Earthquake loads on buildings 8)How Buildings Respond to Earthquakes 9)Common Modes of Failure a)Structural failure A)sliding shear B) Diagonal cracks C)Effect of overturning )Nonstructural failure c)Site Failures d)Foundation Failures 10)Designing Masonry Buildings for Earthquakes a) Masonry Materials b) Construction Systems A) Walls B)Door and window openings C)Floors and Roofs 11)Conclusions 12) References Abstract The paper summarizes and analyses the basic requirements associated with initial conceptual design of reinforced concrete and masonry buildings that are included in Eurocode 8 (EC8) into an easily understandable synopsis accompanied with practical examples and pictures.
The synopsis is intended to be used by architects and other types of engineers; its purpose is to increase the knowledge about the rules of earthquake resistant design, especially about those that should be respected during initial conceptual design of the building. In this way we can approach the problem of constructing an earthquake resistant structure from another end, by trying to avoid the “bad” structure layouts already during the initial design phase. Introduction
The experiences from the past strong earthquakes prove that the initial conceptual design of a building is extremely important for the behaviour of the building during an earthquake. It was shown repeatedly that no static analysis can assure a good dissipation of energy and favourable distribution of damage in irregular buildings, such as, for example, structures with large asymmetry or distinctively soft storeys. The responsibility for a “good” initial conceptual design lies with the architect, as well as with the structural engineer providing numerical proof of the structure’s safety.
The guidelines for a “good” conceptual design are included in building codes, however, the codes are much more suited to the needs of structural engineers as to the needs of architects. This can be seen also in recent EC8, where many requirements related to initial design include formulae with parameters that could be obtained only by preliminary static analysis. On the other hand, same requirements are formulated only as recommendations and their fulfilment depends on experience and judgment of the designer.
From this point of view the cooperation between architect and structural engineer would be therefore necessary also during the initial phase of the design of the building. In practice it is difficult to perform static analysis if, for example, the floor plan is still under discussion, so this cooperation is not working properly in many cases (especially for less complex buildings). It is evident that architects should be familiar with the basic rules of earthquake resistant design, so that they can be incorporated in their building solution already from the first sketch.
The present paper summarizes and analyses the requirements associated with initial conceptual design that is included in EC8 into an easily understandable synopsis accompanied with practical examples. Causes of earthquakes Earthquakes are caused by a sudden rupture of crustal blocks. Their size (? magnitude) depends on the dimensions of the rupture plane and the degree of displacement. Rupture is caused by the constant movement of large tectonic plates. Earthquakes may occur on the contact plane between two plates – e. . the San Andreas Fault in California – or on the numerous secondary faults in plate boundary zones or – less frequently – within a plate. Though more seldom, such intraplate earthquakes are, in terms of their loss potential, by no means insignificant, as their hypocentres may be very near or directly beneath densely populated areas. Depending on the type of relative movement, a distinction is made between thrust faults, normal faults (both with vertical displacement) and horizontal faults (with horizontal displacement).
Depending on the depth of the hypocentre, the rupture plane may reach the earth’s surface and result in measurable displacements. This is often not the case with thrust faults (which are then called blind thrusts) so that they often go unnoticed, making them the most dangerous kind of all. Basic terminology: Hypocentre: The hypocentre (also known as the focus) is an idealised point which designates the place in the depths of the earth where the earthquake rupture has its origin. Epicentre: The epicentre is the point derived from a geometric projection of the hypocentre onto the earth’s surface.
It usually coincides with the zone in which the worst damage is recorded. Focal depth: The depth of the hypocentre measured in kilometres. It may be anything between a few kilometres and over 600 km. The focal depth of damaging earthquakes is not expected to exceed 150 km and in the vast majority of cases it is 5 to 50 km. Earthquake size There are basically two different ways of defining the size of an earthquake – in terms of its magnitude and in terms of its intensity. Magnitude: Magnitude is a measure of the size of an earthquake based on instrument readings.
According to the original definition after the Californian seismologist Charles Richter (Richter Scale, 1932), magnitude is a function of the maximum amplitude of ground motion and the distance between the hypocentre and the site of the recording. As there are other definitions of magnitude which have been introduced since Richter’s, the magnitudes quoted for earthquakes often differ. The magnitude providing the best basis for global comparison is moment magnitude (Mw). One of its advantages is that it measures all the energy radiated in the form of seismic waves.
One of its drawbacks for practical applications is the saturation of ground motion at a magnitude of about 7. Consequently, there is no longer any direct relation to the effect of ground motion above that level of magnitude. This is best represented by local magnitude (ML), which is essentially identical to Richter’s original magnitude. The magnitude scale is a continuous logarithmic scale. Appreciable damage is only to be expected above a magnitude of 5 in the immediate epicentral area.
To sum up, magnitude is always a fixed value which is linked to the vibration energy radiating from the hypocentre and is not directly related to the extent of damage since this is also determined by the distance between the hypocentre and potentially affected objects. Intensity: Contrary to magnitude, macroseismic intensity is a purely empirical measure of an earthquake’s local size, which is linked by definition to the damage observed. It is established by experts following a statistical analysis of the damage in the places affected.
The scale most widely used in the world today is the 12-level Mercalli scale, which has been repeatedly modified in order to objectify the assignment of intensity grades. The scale that has achieved this objective best is the European Macroseismic Scale of 1998 Earthquake hazard Primary effects The primary effects of earthquakes are the ground motions caused by the incident seismic waves and, if the rupture plane reaches the earth’s surface, the relative displacement. Secondary effects The extent of damage caused during earthquakes is also influenced to a great extent by secondary effects.
These effects include in particular the following: – Local amplification of ground motion, especially on young, uncompacted sediments – Liquefaction of fine water-saturated sands – Landslides in geological formations prone to landslides (clay, marl) – Tsunamis (erroneously called seaquakes), waves of water caused by earthquakes in the sea that can reach run-up heights of up to 20 m on the shore – Fires caused by short circuits, leaks in gas pipes, and sometimes open fires Earthquake-resistant construction Earthquake loads on buildings
The acceleration of buildings by seismic waves depends both on the acceleration of the soil and on the dynamic behaviour of the building itself. The forces which are induced by the acceleration of the building’s mass and which resist the motion are called earthquake loads; they are a proportional function of the building’s mass and the degree of acceleration. Buildings on soft and uncompacted soil are generally much more highly exposed than buildings on firm or rocky ground. The resonant frequency of stiff buildings is hardly ever excited.
Their earthquake load is purely the result of ground acceleration. The resonant frequency of soft or moderately stiff buildings can be excited by ground acceleration. The resulting acceleration of the building’s mass can be many times as great as the ground acceleration. The resulting forces represent an elevated risk for the buildings involved. Very soft buildings whose resonant frequency is not excited may resist ground motion by means of distortion. The earthquake loads on these buildings are the lowest.
All types of buildings, from stiff to very soft, can, in principle, be built to resist earthquakes insofar as they are designed to cope with the earthquake loads and the specific structural demands are observed. The choice of loadbearing structure will mainly depend on the manufacturing costs and the purpose of the building. The design of buildings for sensitive machine parks should be stiff rather than soft because of the danger of business interruption due to the machines being damaged or decalibrated. Seismic waves generate both horizontal and vertical accelerations of the soil.
Vertical acceleration is usually less than two-thirds as strong as horizontal acceleration. And as the majority of buildings are constructed in such a way that they can easily transfer their own dead load and the maximum live load in a vertical direction and their loadbearing structures are calculated and designed with sufficient safety factors to cope with these loads, damage caused by earthquake loads usually occurs because buildings are not designed in a way that adequately allows for the transfer of horizontal loads. This danger therefore relates in particular to buildings that have a large mass at the top and weak horizontal stiffening.
Potentially catastrophic losses could be avoided in the majority of cases by adequate horizontal stiffening. How Buildings Respond to Earthquakes An earthquake is the vibration of the earth’s surface that follows a sudden release of energy in the crust.. During an earthquake, the ground surface moves in all directions. The most damaging effects on buildings are caused by lateral movements which disturb the stability of the structure, causing it to topple or to collapse sideways. Since buildings are normally constructed to resist gravity, many traditional systems of construction are not inherently resistant to horizontal forces.
Thus design for earthquakes consists largely of solving the problem of bracing a building against sideways movement. The actions demonstrate combinations of the vertical gravity effects with the lateral effects of earthquakes. In most buildings this system consists in some combination of horizontal distribution elements, such as roof and floor diphragms and vertical bracing elements such as shear walls and rigid frames. An earthquake shakes the whole building. A major design consideration must therefore be that of tying the building together to prevent it from being shaken apart.
This means that the various separate elements must be positively secured to one another. The detailing of construction connections is a major part of the structural design for earthequake resistance. Common Modes of Failure Earthquakes subject the structure to a series of vibrations which cause additional bending and shear stresses in structural walls. Four modes of failure which are given below: 1)Structural failure: A)sliding shear: It reults in a building sliding off its foundation or on one of the horizontal mortar joints. It is caused by low vertical load and poor mortar.
If the building is adequately anchored to the foundation, the next concern is for adequate resistance of the foundation itself, in the form of some combination of horizontal sliding friction and lateral earth pressure. Sliding shear failure can also occur within the building structure, a classic case being the dislocation of a lightly attached roof. B) Diagonal cracks: Failure namely in masonry walls when the tensile stresses, developed in the wall under a combination of vertical and horizontal loads, exceed the tensile strength of the masonry material.
C)Effect of overturning: This may result in the building tipping over. The critical nature of the overturning effect has much to do with the form of the building’s vertical profile. Buildings that are relatively squat in form are unlikely to fail in this manner, while those with tall, slender forms. 2)Nonstructural failure While sructural elements of a building should be the prime concern for earthquake resistance, everything in the building construction should resist forces generated by earthquakes.
Nonstructural walls, suspended ceilings, window frames and fixtures should be secure against movement during the shaking actions. Failure here may not lead to building collapse, but it still constitutes danger for occupants and requires costly replacements or repair. Interior partitions, curtain walls, wall finishes, windows and similar building elements are often subjected during earthquakes to shear stresses, for which they do not have sufficient resestive strength. The most common damage resulting from this is breakage of window panes and cracks in internal plaster and external rendering.
A possible remedy for the former is to isolate the window frames from the surrounding walls by the introduction of flexible joints; the latter can be avoided by reinforcing the plaster or to precrack it by introducing control joints (groves). 3)Site Failures Five common site failures that may occur during an eathquake. If significant in dimension site failures can cause damage to fences, retaining walls, pavements, drains and other buried piped services. 4)Foundation Failures Site failures described above can cause damage to the building foundations.
If the supporting ground moves, the foundations will move. It is essential that the foundation system move in unison during an earthquake. When supports consist largely of isolated column footings, it is advisable to add ties of the type illustrated in Figure 6 in order to achieve this and to enable the lateral loads to be shared among all the independent footings. Designing Masonry Buildings for Earthquakes Masonry has been popular through the ages for its fire resistance, its thermal capacity and its durability.
However the combination of weight, stiffness and weakness against tensile forces makes traditional masonry buildings highly vulnerable to earthquakes. This is not only the case in developing countries but it is also the case in the most developed regions of the world. Vibrations caused by earthquakes generate additional loading. Shear stresses develop which cause damage to structural elements. Since masonry, which can be stressed relatively high in compression, is weak in resisting bending and shear, collapse is often the result.
Consequently, masonry has, for a long time, been considered unsuitable in earthquake resisting constructions. However, in the last few decades, considerable research on the behaviour of masonry walls and buildings subjected to seismic actions has been carried out in many countries. Use of strong mortars, high strength masonry, added reinforcement, improved detailing and the introduction of good anchorage between masonry walls and floors and roofs have enhanced the resistance of masonry to seismic stress. These improvements enable the masonry building to act as a box-type structure.
Vertical gravity loads are transferred from the floors and roof, which act as horizontal tension elements, to the bearing walls, which support the floors and act as vertical compression members. During earthquakes, however, floors and roofs act as horizontal diaphragms that transfer the seismic forces, developed at floor levels, into the walls. In addition to this, floors and roofs connect the structural walls together and distribute the horizontal seismic forces among the structural walls in proportion to their lateral stiffness. Tie (ring) beams are provided at floor levels to assist the floors in connecting the structural walls.
Masonry Materials Masonry Units Fired bricks, concrete blocks (hollow or solid) and natural stone are used for the construction of masonry walls. In all cases the quality of masonry units should comply with the local national requirements with regard to materials and manufacture, dimensions and tolerances, mechanical strength, water absorption, frost resistance, soluble salts content, fire resistance, etc. Stone units should be square dressed with parallel faces. Random rubble is not adequate in earthquake zones. Hollow concrete blocks should meet the following requirements: The holes of hollow units should not exceed half the volume of the unit. • The minimum thickness of shells is 15 mm. • The vertical webs should extend over the entire horizontal length and width of the unit. Mortar The minimum allowable mix is 1 part of cement to 4 parts of sand, or 1 part cement to 1 part of hydrated lime and 5 parts of sand. 14 Concrete Infill The concrete mix used to fill holes of hollow concrete blocks where steel bars are placed should be not weaker than 1:2:4. The maximum aggregate size for blocks with 50 mm voids is 10mm; for blocks with 100 mm voids 20 mm.
Reinforcing Steel Plain or deformed bars may be used for structures, reinforced masonry and confined masonry. Especially shaped prefabricated ladder-type or truss-type reinforcement is to be sued in mortar bed-joints. The vertical distance between reinforcements should not exceed 600 mm. The reinforcing bars should be anchored adequately into the tie-columns or intersecting walls. Minimum thickness of mortar cover above reinforcing bars should be 15 mm. Construction Systems Unreinforced Masonry This form of construction is not considered earthquake resistant and its use should be disallowed.
Reinforced Masonry Two systems of reinforced masonry are in common use: 1. Reinforced hollow units masonry. This is achieved by placing bed joint reinforcement of the type illustrated in Figure 10 at 600 mm centres, and verticals bars as shown in Figure. The holes containing the vertical bars are filled with concrete as the construction of the wall progresses. 2. Reinforced cavity masonry. This system consists of two leaves of masonry units, separated by a cavity into which the vertical and horizontal reinforcement is placed and the cavity is filled with either concrete infill or mortar.
The leaves are usually 100 mm thick and the cavity 60-100 mm. Confined Masonry This is a construction system where masonry structural walls are surrounded on all four sides with reinforced concrete In order to ensure structural integrity, vertical confining elements should be located at all corners and recesses of the building, and at all joints and wall intersections. In addition, they should be placed at both sides of any wall opening whose area exceeds 2. 5 m2. Walls: General Principles • Walls are to be uniformly distributed along each principal axis of the plan. The minimum thickness of structural walls should be 240 mm. The total crosssectional area of structural walls along each of the two axes should not be less than 3% of the gross floor area. • Adequate foundations and good anchorage between walls and floors are essential. • Distances between structural walls of reinforced masonry should not be more than 6m; distances in confined masonry should not be more than 8m. • Partitions should be reinforced with 6 mm o bars placed at the bed joints with vertical spacing of 600 mm in order to prevent their out-of-plane instability.
Partitions should be anchored to structural walls or tie columns with steel anchors. Door and window openings The sizes and positions of wall openings have strong effect on the in-plane resistance of masonry shear walls. When subjected to seismic loads, stress concentration takes place in the opening zones, causing cracking and deterioration of masonry. In order to improve the behaviour of masonry buildings when subjected to earthquakes, the following requirements should be met: • Openings should, where possible, located in those walls which are subjected to smaller intensity of vertical gravity loads. An opening should be located not closer than 600 mm to the inside corner of its wall. • On each storey, openings should be located in the same position along the vertical line. • In order to provide uniform distribution of resistance and stiffness in two orthogonal directions, openings should be located symmetrically in the plan of the building. • The tops of openings in the storey should be at the same horizontal level. Lintels Lintels should have a minimum of 250 mm bearing length at both ends to prevent local collapse due to crushing of supports during an earthquake.
The width of a lintel should not be less than150 mm. If the distance between top of lintel and underside of beam above is less than 60 cm, the two should be united. In the case of openings larger in area than 2. 5 m2, the lintel should be anchored to the tie columns. Double-Leaf Walls • The traditional stone masonry construction with two outer layers of uncoursed irregularly sized rubble stones with an inner infill consisting of smaller pieces of stone bound together with lime mortar is not recommended in earthquake zones. • Generally speaking, single-leaf walls should be preferred to double-leaf walls.
Double-leaf cavity walls, where the cavity is filled with concrete, should be preferred to normal cavity walls, since they ensure monolithic behaviour of the wall under seismic conditions. Floors and Roofs During earthquakes, floors and roofs should act as rigid horizontal diaphragms, which distribute the seismic forces among structural walls in proportion to their stiffness. One of the main reasons for the poor behaviour of existing masonry buildings is a lack of proper horizontal diaphragm action of floor and roof structures and or lack of proper connections between them and the structural walls which carry them.
Use of timber floors and roofs in high-risk seismic zones is only recommended where the requisite carpentry skills exist and if specially designed details to ensure the integrity of these elements and their anchorage to the supporting walls. Jack arches in lime mortar spanning between steel joists are adequate, provided the spans do not exceed 900 mm and steel cross bracing welded to corners of the outer joists above on the upper surface of the floor or roof is provided. Use of deformed bars for this is not allowed because they produce brittle welded joints.
In the case of reinforced concrete floors and roofs, two-way slabs are to be used in preference to one-way slabs. FOR LOAD BEARING MASONRY STRUCTURE, remember a few simple points that minimize the risk of damage in the event of an earthquake. i. Check with your structural engineer that he has followed the Indian Standard IS 4326. This standard covers earthquake resistant features. ii. Keep the building form as simple as possible. Avoid too many projections, twists and turns in the building. iii. Ensure that there is vertical corner einforcement at the junction of walls. iv. Verify that there is a reinforced lintel band, a roof band and a plinth band. v. Make sure that all the windows and doors have the same lintel level. vi. Do not have more than 50% openings in any single wall and provide continuous reinforcement around openings larger than 600 mm X 600 mm in size. vii. Ensure that the openings for doors and windows are not at the corners of the walls but are placed towards the center. viii. The walls uniformly distributed in the lan of the building in both horizontal directions. Conclusions: The article briefly summarizes EC8 requirements that are important to an architect and should be considered already during initial conceptual building design. The final version of the synopsis is still in progress; in the paper only the most important requirements are presented. It is important to stress that the buildings with extremely unfavourable/unregular floor plan cannot be transformed into a safe design simply with the help of good static calculation.
Any safety verified in such a way is only imaginary and can be easily disproved by a first stronger earthquake impact. The authors hope that architects – constructors will find the synopsis helpful when designing/planning earthquake resistant reinforced concrete or masonry structures. References [1] EUROPEAN STANDARD prEN 1998-1, Revised Final PT Draft (preStage 49), Draft May 2002 prEN 1998-1:200X, Doc CEN/TC250/SC8/N317. Eurocode 8: Design of structures for earthquake resistance Part 1: General rules, seismic actions and rules for buildings, CEN, European Committee for Standardization. 2] Paulay, T. , Priestley M. J. N. , Seismic Design of Reinforced Concrete and Masonry Buildings. Birkhauser-Verlag, USA, 1992. [3] Nilson, A. H. , Winter, G. , Design of concrete structures. McGraw-Hill, Inc. , USA,1991. [4] Fischinger, M. , Cerovsek, T. & Turk, Z. , EASY: a hypermedia learning tool. Electronic journal of information technologies in construction, vol. 3, pp 1-10,1998. [5] Dorris, -V. K. , Seeking structural solutions. Civil engineering, v. 66, no. 11, pp 46-49, November, 1996.