Seismic effects

Most people are familiar with the concept of a shock wave emanating from an epicentre below ground being the source of a seismic event. For the purpose of considering the forces developed against structures, this shock wave can be considered as travelling at ground level, which, in effect, laterally displaces the ground and objects attached to the ground. These objects experience a momentary shift, or series of shifts, with intermediate replacements, in the direction of the wave propagation. It follows from this that anything which has an appreciable part of its mass above the ground surface, such as tall trees, columns or walls, will be at risk from its own dead weight. This is because inertia will try to retain the bulk of mass in its former position, whilst the base of the object is forced to displace due to the ground-level wave. Slender, flexible objects, such as tall reeds, saplings, etc. are at low risk, even if quite tall, as they are able to bend to accommodate the shock wave. Medium to tall trees do not fare so well, whereas rigid structures, such as most buildings, all suffer a degree of damage.

Because the shock wave passes across the structure in a continuous succession of increments, each element in a wall, say, will suffer compressive, then tensional, forces. A low-height, rubble-cored wall may appear to survive fairly well from the experience, and be seen to lose some outer masonry blocks. In fact, the whole core will have been rumbled by the passage of the wave, and much core mortar will have been detached or de-bonded with the core stones.

This basic mechanism was recognised by some ancient peoples who specialised in the construction of interlocking masonry able to absorb the alternating forces developed by the shock wave, and built without loose core material to be disturbed by it. The high seismic resistance of the Inca ruins and those at Rhodes, etc. show how very effective this type of masonry has proved to be.

Tall structures built of conventionally coursed masonry, and which lie parallel to the wave front, will sway due to inertia and are likely to topple. As the inertia of the above ground mass resists the inertia, a secondary shear zone is initiated, which due to primary stress distribution will occur above ground level. With a homogeneous material, the initial compression will spread at 45 degrees upwards, and a horizontal shear will be initiated since no further material exists to accept the compression. In simple terms, a column 1400 mm in diameter is likely to shear at about 1200 —1600 mm above its base, and such shear will tend towards a joint where the resistance is least. Some concomitant stone damage will also occur, and this will be at a sloping shear plane on the leeward edges, but will be minor.

In active seismic areas, which experience only low-level tremors, such as south-eastern France, some ruins display timber beams built into masonry walls at regular vertical intervals, which are seismic attenuation devices. These are often mistaken for former lintels abandoned due to alterations. Further south in Europe, the Balkans, etc., these timbers occur in vernacular architecture, as full perimeter ties, rather like bond-beams in concrete masonry. The timber tie beams provide a tensional capacity, which the masonry lacks, and some minor flexural capacity. These features, coupled with the softer, fibrous nature of the timber, help to redistribute some of the ground wave stresses, and to absorb some of the shock by local bending and by dampening.

This same phenomenon helps to explain the Roman fascination for bands of brickwork within their masonry buildings.

Some minor tensional capability is available from the brick layers via the mortar bonding, and the frequently repeated layers, as for example in the Baths of Constantine in Arles shown in Figure 2.11, provide

Figure 2.11 Baths of Constantine, Arles.

a cumulative capability for tension between the layers. The different dynamic response of fired brick, as opposed to masonry, has a dampening effect also. As can be seen, the brick courses extend the full width of the walls.

In the low seismic areas of southern France, there are many examples in vernacular architecture oftaper-ing walls or buttresses, with splayed quoins, which reduce the effects of minor tremors. These have often been added to with quite massive sloping buttresses, which taper towards their tops, and provide the same effect (Figure 2.12).

A similar add-on seismic buttress can be seen here provided to the seventeenth century church at Manosque, France (Figure 2.13). Notice the neat, but massive, buttress of extra large quoins which has been added to stabilise the fine limestone ashlar work of the façade, which shows signs of having been shaken and cracked by an earth tremor. The position of the crack and loose joints, just back from the corner, indicates that the shock wave passed from right to left. This is borne out by signs of light spalling to the ashlar faces of the two town gateways, which lie on the same axis of the wall shown. This is typical of light tremor damage and it shows how the masonry damage will accumulate on the lee side of the pressure wave, as the masonry rebounds, thereby realising tensional forces in the masonry. An examination of building complexes damaged by seismic activity will illustrate these principles.

Some experiments have been made of reinforcing re-erected ruined columns with steel rods through their centres. This has a potential for further worse damage to occur, than if solely pegged together in the classical manner. In the event of a seismic occurrence, grouting of the rods is likely to initiate a secondary shear plane starting from the centre of the rod group,

Figure 2.12 Tapered corner designed to reduce the effects of minor earth tremors.

and should therefore be avoided. As the magnitude of a future seismic event is unpredictable, a satisfactory model for analysis may not be possible, and it may be better policy to avoid this type of refinement. The probability of a lightning strike to the top of a reinforced column is a further factor to consider.

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