BAS Multi Hazard

Question 1

the building codes require buildings to resist winds of at least…

Answer 1

… 70 MPH

Question 2

The probability of occurrence of both earthquakes and hurricanes is com- monly expressed by use of the term …

Answer 2

…“return period.”

This is defined as the average or mean time in years between the expected occurrence of an event of specified intensity.

For example, until recently earthquake codes used as a basis of severity a level of shaking (an acceleration value) that corresponded to a 10 percent probabil- ity of exceedance in 50 years - or a probability that it would be exceeded one time in about 500 years: a 500-year return period.

Question 3

Values for high winds are commonly expressed in codes as a –1–cause their incidence is much more frequent. Floods are expressed as a –2–return period - the “–2–year flood.”

Answer 3

1) 50 year
2) 100

Question 4

Each hazard has its own methods of measurement and its own scales for comparing past and future events.

1) For earthquakes,
2) for wind
3) and for floods

Answer 4

acceleration is the key characteristic for design, combined with duration and the nature of the ground;

2) the key element is wind speed;
3) the critical parameters are the depth of water and velocity of flow.

Question 5

Richter and Mercalli scale measure what?

Answer 5

earthquakes

Mercalli scale measures intensity, which describes the effect of the earth on people, land, objects, and buildings.

Richeter measure magnitude

Question 6

The Saffir-Simpson Hurricane Scale is currently the preferred scale for measuring …

Answer 6

hurricanes and is used by National Oceanic and Atmospheric Administration forecasters. This scale can be used to give an estimate of the potential property damage and flooding expected along the coast from a hurricane. The Saffir-Simpson Scale is subjective, based on the judgment of a trained observer. The criteria for each of the five categories in the scale are shown in Figure 2.2.

Question 7

The Fujita Scale is used in measuring…

Answer 7

… tornadoes. To assist observers in making equivalent values the Fujita scale provides a set of damage photographs as a common reference to illustrate the differing scales as shown in Figure 2.3.

Also subjective

Question 8

For floods, this is used as a key reference measurement.

Answer 8

the Base Flood Elevation (BFE) is a key reference measurement. The BFE is the flood elevation that has a 1 percent chance of occurring in any given year: it is often referred to as the “hundred-year flood.” The 100-year floodplain - the land area likely to be inundated by a hundred-year flood - is known by FEMA as the Special Flood Hazard Area. BFEs and Special Flood Hazard Areas are key pieces of information provided on national “Flood Insurance Rate Maps.”

Question 9

Earthquakes attack building by

Answer 9

creating inertial forces: forces that are produced through- out the building as it resists the vibration of the ground. These forces are predominantly horizontal but also vertical (up and down).

Question 10

Winds attack building by…

Answer 10

creating pressure and suction on the face of the building and also internal pressures on walls and roofs if the building has significant openings. Forces are predominantly horizontal but may also be vertical (up and down).

Question 11

Floods attack building by…

Answer 11

• creating hydrostatic and hydrodynamic pressures on the building which are predominantly horizontal but also may be vertically up- wards (buoyancy).

Question 12

Wind and seismic, being predominantly what type of forces?

Answer 12

lateral forces, share a number of characteristics

Question 13

True or False. Wind has a vibratory element with frequency content, like earthquakes, and can result in resonance effects if the period of building coincides with the period of wind vibrations, resulting in amplification of the forces on the building.

Answer 13

True

Question 15

Describe period and frequency

Answer 15

Period is the inverse of frequency: where frequency is generally measured as number of vibrations per second, period measures the length of a single cycle of vibration.)

Question 16

Resonance and considerable amplification of response can occur when what two factors coincide?

Answer 16

building period and the ground motion period coincide.

Question 17

base isolation (in which the buildings sits on flexible or pendulum- like bearings that partially isolate the building from ground motion) works by…

Answer 17

… increasing the period of a short period building so that it responds to the lower accelerations in the long period area of the response spectrum.

Question 18

Wind forces impact buildings differently. While a building’s weight may increase its ability to resist wind forces, it does not directly affect the amount of force the building “sees” and is subjected to. This is a function of the building’s …

Answer 18

surface area and,

in general, a building with a large surface area relative to its enclosed volume will “see” and need to resist larger wind forces than a similar building with less surface area. Further, the larger the surface area the greater the opportunity for wind to damage and/or penetrate glazing, cladding, and roofing. With their relatively large surface areas, wood-frame houses, ifnot properly tied together and anchored to the ground, are extremely vulnerable.

Question 19

Lightweight cladding is beneficial in earthquakes because it …

Answer 19

…reduces building weight, and hence response, but it has proven unsatisfactory in winds, unless very carefully installed, because of leakage and consequent severe internal water damage.

Question 20

that weight is a disadvantage for seismic design and an advantage for …

Answer 20

…winds and floods. This is because the inertial earthquake forces increase with the mass, or weight, of the building, as noted earlier.

Question 21

A strong connection to the ground is a prerequisite for seismic and wind design, although a major new development in seismic design is the use of –1–techniques that detach the building from the ground and reduce the transmission of ground motion through the building.

Answer 21

1)base isolation

This is a true conceptual breakthrough, unique to seismic design, because it modifies and reduces the earthquake forces instead of accepting them and trying to resist them. Nothing approaching this has been developed for the other hazards.

Question 22

While security of the building envelope is necessary for both wind and earthquake design, the requirements are somewhat different. For earthquakes –1–: for wind,–2–

Answer 22

1) he need is to ensure that nonstructural cladding and glazing does not fall from the building
2) the integrity of the wall and roof finishes must be maintained to ensure protection from water and the possibility of destructive internal forces.

Question 23

Base isolation works by …

Answer 23

…changing the period of a building so that it moves into the range of greatly reduced forces towards the right hand side ofthe response spectrum (see Figure 3.1). This assumes a fairly stiff, short period building and short period ground motion. Base isolation shifts the building period into the 2 second range: if the ground also has a 2 second period (not likely, but possible) then base isolation will not work because of the possibihty of resonance and amplification.

Question 24

For active systems to be applied to the earthquake problem, what is the primary problem that must be surmounted?

Answer 24

some difficult problems of reliability of the electronic control and mechanical systems must be surmounted: a system might be inert for 30 years before being called upon to respond in a fraction of a second.

Question 25

Structural integrity in winds is dependent on…

Answer 25

… a clear and direct load path by which the wind forces can be transmitted to the resisting structure and brought down to the foundations. This is particularly important for light wood-frame structures such as houses, and large light structures with a big surface area such as warehouses, industrial, and retail buildings.

Question 26

In order to resist winds, design methods include:

Answer 26

Secure attachment of roofs, cladding and appendages.

Reduceoreliminatewindowsordoors(toreducetheeffectsofopenings).

Control potential wind-borne missiles.

Provide protection for openings.

Question 27

In order to resist floods, the possible methods are:

Answer 27

Elevation above the Base Flood Elevation.

Levees

Floodwalls: a kind of private, small-scale levee

Question 28

The only really effective methods of dealing with floods are …

Answer 28

by correct siting (avoid floodplains and vulnerable coastal areas) and the use of well designed and constructed elevation well above the Base Flood Elevation.