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Structural Systems > FEMA > Flashcards

FEMA Flashcards

1
Q

A site response spectrum is…

A

…a graph that plots the maximum response values of acceleration, velocity, and displacement against period (and frequency).

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2
Q

as the period of a building lengthens, accelerations –1– and displacement –2–.

A

1) decrease
2) increase

On the other hand, one- or two-story buildings with short periods undergo higher accelerations but smaller displace- ments.

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3
Q

In general, does a more flexible longer period design may expect to experience proportionately lesser or more accelerations than a stiffer building.

A

lesser

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4
Q

There is an exchange, in that the lower accelerations in the more flexible design come at the expense of more motion. What is this exchange

A

This increased motion may be such that the building may suffer considerable damage to its nonstructural components, such as ceilings and partitions, in even a modest earthquake.

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5
Q

If a structure is made to vibrate, the amplitude of the vibration will decay over time and eventually cease. This is called…

A

Damping

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6
Q

critical damping.

A

This is the least amount of damping that will allow the structure to return to its original position without any continued vibra- tion.

• When damped, buildings are inefficient in their vibration and when set in
motion, return to their starting position quickly.

For typical structures, engineers generally use a value of 5 % critical.

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7
Q
A

The response spectra show that the peak acceleration is about 3.2g for a damping value of 0 %, 0.8g for a damping value of 2 % and a value of about 0.65g for a value of 10 %.

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8
Q

The increase of the structural movement over that of the ground motion is commonly referred to as

A

dynamic amplification.

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9
Q

Difference between Ductility and reserve capacity

A

are closely related

ductile mate- rials can take further loading before complete failure. In addition, the member proportions, end conditions, and connection details will also affect ductility. Reserve capacity is the ability of a complete structure to resist overload, and is dependent on the ductility of its individual mem- bers. The only reason for not requiring ductility is to provide so much resistance that members would never exceed elastic limits.

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10
Q

Strength

A

Sufficient strength is necessary to ensure that a structure can support imposed loads without exceeding certain stress values. Stress refers to the internal forces within a material or member that are created as the structural member resists the applied load. Stress is expressed in force per unit area (for example, pounds per square inch).

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11
Q

Stiffness

A

Stiffness is measured by deflection, the extent to which a structural member, such as a floor, roof, or wall structure, bends when loaded. De- flection is generally expressed as a fraction of length of the member or assembly.

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12
Q

n seismic design, deflection of vertical structural members, such as columns and walls, is termed

A

drift

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13
Q

Thus the IBC requires that drift be limited in typical buildings to between …

A

…0.02 and 0.01 times the building height, depending on the occupancy of the building.

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14
Q

For a building that is 30 feet high, drift would be limited to between…

A

… 3.6 inches and 7.2 inches depending on the building type.

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15
Q

If two elements (two frames, walls, braces, or any combination) are forced to deflect the same amount, and if one is stiffer,which one will take more of the load?

A

the applied forces are “attracted to” and concentrated at the stiffer elements of the building.

Only if the stiffnesses are identical can it be assumed that they share the load equally.

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16
Q

What is a short-column condition, and how is it inadvertently created after the building is occupied?

A

For example, the space between columns may be filled in by a rigid wall, leaving a short space for a clerestory window. Such a simple act of remodeling may not seem to require engineering analysis, and a contractor may be hired to do the work: often such work is not subject to building department reviews and inspection.

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17
Q

How are torsional forces created in a building?

A

Torsional forces are created in a building by a lack of balance between the location of the resisting elements and the arrangement of the building mass.

Engineers refer to this as eccentricity between the center of mass and the center of resistance

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18
Q

Shear walls

A

Shear walls are designed to receive lateral forces from diaphragms and transmit them to the ground. The forces in these walls are predominantly shear forces in which the material fibers within the wall try to slide past one another. To be effective, shear walls must run from the top of the building to the foundation with no offsets and a minimum of openings.

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19
Q

In plan, a configuration that is most likely to produce stress concentrations features…

A

… re-entrant corners:

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20
Q

how can you reinforce soft story shown?

A
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21
Q

The soft first story failure mechanism.

A
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22
Q

Moment-resistant frames

A

A moment resistant frame is the engineering term for a frame structure with no diagonal bracing in which the lateral forces are resisted primarily by bending in the beams and columns mobilized by strong joints between columns and beams. Moment-resistant frames provide the most architectural design freedom.

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23
Q

Braced frames

A

Braced frames act in the same way as shear walls; however, they generally provide less resistance but better ductility depending on their detailed design.

conventional concentric and eccentric

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24
Q

In the eccentric braced frame, the braces are deliberately designed to meet the beam some distance apart from one another: the short piece of beam between the ends of the braces is called …

A

… a link beam.

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25
Q

The purpose of the link beam is to …

A

…provide ductility to the system: under heavy seismic forces, the link beam will distort and dissipate the energy of the earthquake in a controlled way, thus protecting the remainder of the structure

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26
Q

A diaphragm that forms part of a resistant system may act either in a flexible or rigid manner,depending partly on…

A

… its size (the area between enclosing resistance elements or stiffening beams) and also on its mate- rial.

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27
Q

These are diaphragm framing mem- bers that “collect” or “drag” diaphragm shear forces from laterally unsupported areas to vertical resisting elements

A

Collectors, also called drag struts or ties

28
Q

Building attributes for optimum seismic performance

A

Continuous load pat: Uniform loading of structural elements and no stress concentrations.

Low height-to base ratio:Minimizes tendency to overturn.
Equal floor heights:

Equalizes column or wall stiffness, no stress concentrations.

Symmetrical plan shape:Minimizes torsion.

Identical resistance on both axes: Eliminates eccentricity between the centers of mass and resistance and provides balanced resistance in all directions, thus minimizing torsion.

Identical vertical resistance: No concentrations of strength or weakness.

Uniform section and elevations:Minimizes stress concentrations.

Seismic resisting elements at perimeter:Maximum torsional resistance.

Short spans: Low unit stress in members, multiple columns provide redundancy -loads can be redistributed if some columns are lost.

No cantilevers:Reduced vulnerability to vertical accelerations.

No openings in diaphragms(floors and roof): Ensures direct transfer of lateral forces to the resistant elements.

29
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30
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31
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32
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33
Q
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34
Q

Three typical conditions create a soft first story. What are they?

A

flexible first story

doscontinuity

heavy super structure

35
Q

THe upper dot ir Center of resistinance the lower dot is the center of mass. How would the building react to a lateral force?

A
36
Q

What solution could an architect propose to optimize this plan’s seismic performance?

A
37
Q

The large imbalance in perimeter strength and stiffness results in…

A

… large torsional forces.

38
Q

The solution to torsional forces is …

A

…to reduce the possibility of torsion by en- deavoring to balance the resistance around the perimeter.

39
Q

3 strategies to counteract torsional forces

A

The first strategy is to design a frame structure of approximately equal strength and stiffness for the entire perimeter.

A second approach is to increase the stiffness of the open facades by adding sufficient shear walls, at or near the open face, designed to ap- proach the resistance provided by the other walls

A third solution is to use a strong moment resisting or braced frame at the open front, which approaches the solid wall in stiffness.

40
Q

The re-entrant corner is the common characteristic of building forms that, in plan, assume the shape of an L, T, H, etc., There are two problems created by these shapes. What are they?

A

1) The first is that they tend to produce differential motions between different wings of the building that, because of stiff elements that tend to be located in this region, result in local stress concentrations at the re-entrant corner, or “notch”.
2) The second problem of this form is torsion. Which is caused because the center of mass and the center of rigidity in this form cannot geo- metrically coincide for all possible earthquake directions.

41
Q

What does the torional diagram look like?

A
42
Q

There are two basic alternative approaches to the problem of re-entrant- corner forms:

A

structurally to separate the building into simpler shapes, or to tie the building together more strongly with elements positioned to provide a more balanced resistance

43
Q

The use of –1– rather than right angle re-entrant corners lessens the stress concentration at the notch

A

1) splayed

44
Q

Vertical members such as columns or walls may fail by buckling when the mass of the building exerts its gravity force on a member distorted or moved out of plumb by the lateral forces. This phenomenon is known by engineers as …

A

…the P-e or P-delta effect, where P is the gravity force or weight, and “e” or “delta” is the eccentricity or the extent to which the force is offset. All objects that overturn do so as a result of this phenom- enon

45
Q

The International Style often had a number of characteristics not present in earlier frame and masonry buildings that led to poor seismic performance. These were:

A

1) Elevation of the building on stilts or pilotis:designers often created soft and weak stories.
2) The free plan and elimination of interior-load bearing walls:the replacement of masonry and tile partitions by frame and gypsum board greatly reduced the energy absorption capability of the building and increased its drift, leading to greater nonstructural damage and possible structural failure.
3) The great increase in exterior glazing and the invention of the light- weight curtain wall: Like free interior planning, the light exterior cladding greatly reduced the energy-absorption capability of the building and increased its drift.

46
Q

Only two irregularities are banned outright:

A

extreme soft stories and ex- treme torsion in essential buildings in high seismic zones.

47
Q

A design that has attributes of the ideal configuration should be used when:

A
  • The most economical design and construction is needed, including design and analysis for code conformance, simplicity of seismic detailing, and repetition of structural component sizes and placement conditions.
  • When best seismic performance for lowest cost is needed.
  • When maximum predictability of seismic performance is desired.
48
Q

Seismic codes in this country did not develop at all until the 1920s,
and at that time they were used voluntarily. A mandatory code was not enforced in California…

A

… until 1933.

Unreinforced masonry (URM) build- ings, for example, a popular building type early in the twentieth century and now recognized as perhaps the worst seismic performer as a class, were not outlawed in the zones of high seismicity until the 1933 code, and continued to be built in much of the country with no significant seismic design provisions until quite recently.

49
Q

the required lateral strength of a seismic system is generally traded off with the…

A

… ductility (the ability to deform inelastically—normally controlled by the type of detailing of thecomponents and connections) of the system.

Higher strength requires lower ductility and vice versa.

50
Q

torsion in a building is a…

A

…twisting in plane caused by an imbalance in the location of the mass and resisting elements.

51
Q

As the conceptual framework of evaluation and retrofit developed, legal and code requirements were also created. These policies and regulations can be described in three categories…

A

1) active:Active policies require that a defined set of buildings meet given seismic criteria in a certain time frame—without any triggering action by the owner. For example, all bearing-wall masonry buildings in the commu- nity must meet the local seismic safety criteria within ten years.
2) passive:Passive policies require minimum seismic standards in existing buildings only when the owner “triggers” compliance by some action—usually extensive remodeling, reconstruction, or addition.
3) post-earthquake: Post-earthquake policies devel- oped by necessity after several damaging earthquakes, when it became obvious that repairing an obviously seismically poor building to its pre- earthquake condition was a waste of money. It then became necessary to develop triggers to determine when a building could simply be repaired and when it had to be repaired and retrofitted as well.

52
Q

Seismic “renewal” was defined …

A

…as providing life safety, but not necessarily reaching the performance expected from a new building.

53
Q

A second kind of trigger—that could be termed “trigger of opportunity”— has also been used in some communities. Examples?

A

These policies try to take advantage of certain conditions that make seismic improvements more palatable to an owner, such as retrofit of single-family dwellings at point of sale or requiring roof diaphragm upgrades at the time of re-roofing.

54
Q

Triggers based on alterations to the building are by far the most common, but they created two difficult socio- economic-technical issues that have never been universally resolved. What are they?

A

The first is the definition of what level of building renewal or increase in oc- cupancy-risk triggers seismic upgrading.

The second is to establish the acceptable level of seismic upgrading.

55
Q

Events or actions that require owners to seismically retrofit their buildings are commonly called …

A

…triggers. For example, in many communities, if an owner increases the occupancy risk (as measured by number of occupants, or by use of the building), they must perform many life-safety upgrades, including seismic ones.

56
Q

The second program, created by SB 1953 in 1994 following the North- ridge earthquake, gave California hospital owners until 2030 to upgrade or replace their hospitals to comply with state law governing new hospital buildings. What was the law’s intention?

A

The program’s intention is to enable buildings to be func- tional following an earthquake. This law affected over 500 hospitals and over 2,000 buildings (Holmes, 2002).

57
Q

Traditionally, communities (building departments) have used color codes for several or all of the following categories of buildings following an earthquake:

A

A. Undamaged; no action required. If inspected at all, these buildings will be Green-tagged.

B. Damaged to a slight extent that will only require repair of the damage to the original condition. These buildings will generally be Green-tagged, but the category could also include some Yellow-tags.

C. Damaged to a greater extent that suggests such seismic weaknesses in the building that the overall building should be checked for compliance with minimum seismic standards. This will often require overall retrofit of the building. These buildings will generally be Red-tagged, but the category could also include some Yellow tags.

C1. (A subcategory of C). Damaged to an extent that the building creates a public risk that requires immediate mitigation, either temporary shoring or demolition. The ultimate disposition of these buildings may not be determined for several months. These buildings will all be Red-tagged.

58
Q

What does FEMA stand for?

A

Federal Emergency Management Agency (FEMA)

59
Q

In 1990, when the development of HAZUS began, the primary
goals were …

A

…to raise awareness of potential local earthquake risks, to provide local emergency responders with reasonable descriptions of post-earthquake conditions for planning purposes, and to provide consistently created loss estimates in various regions to allow valid comparison and analysis.

60
Q

t is nat- ural to catalogue damage after an earthquake by buildings with common characteristics, the 2 most obvious characteristics being …

A

…the construction type, and the secondary characteristic being the configuration.

61
Q

HAZUS

A

Starting in 1991, FEMA began a major program to develop a standard way of performing such loss estimations to facilitate comparative loss estimates in various parts of the country. This program resulted in a computer program, HAZUS

62
Q

Initial Evaluation (ASCE 31 Tier 1)

A

The ASCE Tier 1 evaluation is similar to FEMA’s Rapid Evaluation in that it is based on the model building type and certain characteristics
of the building.

he significant difference is that structural drawings,
or data equivalent to structural drawings, are required to complete the evaluation, and the evaluation will take several days rather than several hours.

63
Q

FEMA’s Rapid Evaluation

A

three categories: obviously hazardous, obviously acceptable, and uncertain. -

The intent was to spend less than two hours per building for this rapid evaluation.

-Under the plan, the uncertain group would then be evaluated by more detailed methods.

64
Q

Intermediate Evaluation (ASCE 31 Tier 2)

A

Normally, an analysis of the whole building is performed and the equivalents of stress checks are made on important lateral force-resisting components.

65
Q

Detailed Evaluation (ASCE 31 Tier 3)

A
  • The most common method used in Tier 3 is a performance evaluation using FEMA 356, Pres- tandard and Commentary for the Seismic Rehabilitation of Buildings (FEMA, 2000)
  • This method approximates the maximum lateral de- formation that the building will suffer in a design event, considering the nonlinear behavior created by yielding and damage to components.

Structural Systems (12 decks)

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