SAFETY AGAINST EARTHQUAKES

Evening Star, Issue 22142, 24 September 1935, Page 2

SAFETY AGAINST EARTHQUAKES

REVISIDW OF OLD METHODS MODERN STANDARDS OF CONSTRUCTION CONSULTING ENGINEER’S VIEWS In comparatively recent' years the loss of life and the destruction caused by disastrous earthquakes in various parts of the world have indicated that the old methods of building construction, which took account 'only of the vertical forces due to floor and wall loads, would have to be revised to meet the stress conditions set up .by lateral earthquake forces, writes Mr R. A. .Campbell, in a booklet prepared by the Business ■ Development Service, advertising and publicity agents. Mr Campbell was the consulting engineer for the construction of the _ new State Fire Office building in Christchurch. In countries such as Japan, Italy, California, Chile, India, Burma, and New Zealand, which are subject to major seismic disturbances, there has been much thought given by scientists, architects, and engineers to, the nature of the stresses set up in. structures by earthquake forces and to the formulation of regulations governing the design and construction of such structures. 7

It is, of course, impossible to make full provision in design for resistance to a force which may bo of unlimited magnitude, but it has been proved, beyond doubt that. structures such as buildings can he constructed so that they will withstand heavy earthquake shock without endangering .the lives of the occupants, or suffering anything more than minor damage themselves. It has also been proved that the extra cost of making provision for resistance to earthquake forces is not great, and this fact should bo emphasised, as there have been so many loose statements to the contrary made by people who have no knowledge of the subject. POSSIBILITY OF SHOCKS. An eminent American engineer, writing on this subject, says; “The fact that no serious earthquake has happened for a century or two in a certain locality is no proof that none will ever happen. It adds but little to the cost to design, structures which are safe for "all known intensities of, earthquake vibrations, and it is the'duty of the engineering profession to see that this is done.” Another American authority writes: “Time was when the only forethought on earthquakes was that they would come when they migjjt and destroy'what they would. Wo are not content with such resigned fatalism today, we prefer some definite outlook to a blind waiting for future happenings. Our courage as well as our fears have grown, our fears because wo realise how vast are the potentialities of destruction in the crowded and highly complex city life of to-day, our courage because San Francisco, Tokio, and Santa Barbara have shown that man’s constructions can withstand the earthquake shock if rightly built. It is therefore accepted that those who build in the Califronia or Japanese earthquake districts or elsewhere in the unquiet circum-Pacific belt, should build with careful calculation to resist earthquake forces of certain definite intensities.

“It is known from relatively recent experiences that buildings can bo made strong enough to resist even violent earthquakes. It is also known, somewhat less definitely, that the cost of providing protection against moderate earthquakes is not great. In fact—and

this is of great importance—high-grade city buildings have almost as much strength as is needed to resist even fairly violent earthquakes, and if their structure is studied and detailed throughout from the earthquake viewpoint in addition to the conventional load and wind viewpoint, little or no extra cost need be incurred to obtain a very high degree of security.” INCREASE IN COST. The increase in cost to provide resistance to lateral earthquake force depends on the type, height, and weight of the building, and if expressed as a percentage of the total cost it depends'oh the exterior and interior finish. It is obvious that a plain reinforced concrete building will show a greater percentage cost for stiffening the structure against lateral force than an exactly similar building-frame which is stone faced outside and utilises a large amount of marble and other expensive material for its interior finish. Generally speaking, for the better type of office building this increase in cost should lie between 2 per cent, and 5 /per cent, of the total cost/ of the same building in which no provision has been made for earthquake. For a steel frame building the cost of the structural steel work should not be increased by more than 8 per cent, to 12 per cent. An earthquake consists of vibrations both horizontal and vertical in the earth’s crust. The movement is often very complex, but, generally speaking, the horizontal movements are those which are destructive to buildings. The magnitude of an earthquake is rated by its acceleration, this meaning the rate of increase in the earth’s velocity at the point under observation. It was stated that in Yokohama during the great, earthquake the acceleration reached the very high figure of lift a second, the time between successive shock •waves being IJsec, and the maximum amplitude, or horizontal movement in any direction, as much as 15iu. This means roughly that a given spot on the earth’s surface, quiescent to begin with, attained a velocity of from three to four miles an hour, slowed down to quiescence again, moving about 15in in the process, then reversed its direction and repeated the process back to the starting point all in llsec. This horizontal backward and forward movement of the earth’s crust must necessarily be transmitted to the foundations of buildings which move in the same direction and with the same acceleration as the earth surrounding them. The inertia "of tlie, upper portionof the .building tends, - to make it stay in i its. original position- and the walls and columns are suddenly called upon to resist the bending duo to this inertia and make the upper portion of the building follow the movement of the foundation backward and forward with each successive Horizontal movement of the earth. As the walls and columns of each floor have only to do the work of moving tha*t part of the ■ building which is above them, it follows_ that the walls and columns immediately below any floor will he called upon to resist forces greater than those which must be resisted by the walls and columns above that floor, STRENGTH IN WALLS AND COLUMNS. The greater the weight of the floors and roof and the loads carried by them, the greater will he the work done by the ■walls and columns, and therefore the greater the strength required in these parts of the structure. Tlio hending of the columns induces bending in the floor beams, which must also be strengthened at and near their junctions with the columns where the stresses due to earthquake are a maximum. As the stresses caused by earthquake shock at any given intensity vary directly as the weight

of the building, it follows that the floors and other parts of the structure should be made as light as is consistent Avith strength requirements, also that heavy parapets, cornices, and other ornamental details should be omitted. The connection between floor beams and columns should be strong and rigid, and the whole building should have its various parts so securely tied together that they will vibrate ns a unit.

It is also highly important that the footings or foundations from which the earthquake force is transferred to the upper part of the building should be connected to prevent relative movement between their various parts. Timber buildings, the members of which are well tied together, are highly resistant to earthquake shock, mainly becausoof their light weight. _ Buildings with brick-bearing Avails if well constructed and provided Avith concrete bands at floor and roof levels, can be made safe if the height does not exceed three stories and the floors are of Avood. Experience in . all countries subject to earthquakes has proved that it is not safe to construct brick-bearing wall buildings Avith concrete floors. The modern office or factory building is usually constructed of concrete for fire-resisting reasons, and should be provided Avith a "frame of reinforced concrete or structural steel designed to carry all vertical loads, and such bending and sheer forces in members as may bo caused by an earthquake having an acceleration the magnitude of which must be decided upon for any particular country. Reinforced concrete buildings if Avell designed Avith ample column section can be made earthquake resisting, but their Aveak point is the deficiency in sheer strength in the columns. Large concrete columns in the first story of any commercial building occupy . valuable space, and for that reason are objected to, Avith the result that there has been a dangerous tendency to cut the size to a minimum Avbich in many cases is very close to the danger line. A very large measure of security against loss of life or heavy structural damage can be obtained by a relatively small increase in the cost of any building, and no sane man aaTio realises what may happen at any time in any country subject to earthquakes should raise any objection to this small extra expenditure the object of Avhich is to safeguard human life and protect valuable property. /