Chapter 1: Buildings

C: AN INTRODUCTION TO STRUCTURAL CONCEPTS
IN SEISMIC UPGRADE DESIGN

A:  How Buildings Resist Earthquakes

Earthquake forces can act in all directions. Unlike gravity loads that are transferred in a downward direction, earthquake loads start at the supporting soil and are transmitted to the building. The horizontal and vertical earthquake forces travel in different load paths and may result in tension, shear compression, bending or torsion forces. Buildings experience horizontal distortion when subjected to earthquake motion. When these distortions get large, the damage can be catastrophic. Therefore, most buildings are designed with lateral-force-resisting systems (or seismic systems), to resist the effects of earthquake forces. In many cases seismic systems make a building stiffer against horizontal forces, and thus minimize the amount of relative lateral movement and consequently the damage. Seismic systems are usually designed to resist only forces that result from horizontal ground motion, as distinct from vertical ground motion.

The combined action of seismic systems along the width and length of a building can typically resist earthquake motion from any direction. Seismic systems differ from building to building because the type of system is controlled to some extent by the basic layout and structural elements of the building. Basically, seismic systems consist of axial-, shear- and bending-resistant elements.

In wood-frame, stud-wall buildings, plywood siding is typically used to prevent excessive lateral deflection in the plane of the wall. Without the extra strength provided by the plywood, walls would distort excessively or “rack,” resulting in broken windows and stuck doors. In older wood frame houses, this resistance to lateral loads is provided by either wood or steel diagonal bracing.

The earthquake-resisting systems in modern steel buildings take many forms. In moment-resisting steel frames, the connections between the beams and the columns are designed to resist the rotation of the column relative to the beam. Thus, the beam and the column work together and resist lateral movement and lateral displacement by bending. Steel frames sometimes include diagonal bracing configurations, such as single diagonal braces, cross-bracing and “K-bracing.” In braced frames, horizontal loads are resisted through tension and compression forces in the braces with resulting changed forces in the beams and columns. Steel buildings are sometimes constructed with moment-resistant frames in one direction and braced frames in the other.

In concrete structures, shear walls are sometimes used to provide lateral resistance in the plane of the wall, in addition to moment-resisting frames. Ideally, these shear walls are continuous reinforced-concrete walls extending from the foundation to the roof of the building. They can be exterior walls or interior walls. They are interconnected with the rest of the concrete frame, and thus resist the horizontal motion of one floor relative to another. Shear walls can also be constructed of reinforced masonry, using bricks or concrete blocks.

How a building performs in an earthquake depends upon a few key building characteristics described below:

B: Structural Building Elements

The structural elements of a building that comprise the 'skeleton' supporting the rest of the building, includes the foundation, load-bearing walls, beams, columns, floor system and roof system, as well as the connections between these elements. To carry its own weight ("dead load"), live loads, and wind and earthquake forces the building elements and connections are subjected to tension, compression, shear, bending, and torsion. Buildings are primarily designed to resist vertical forces from gravity. The roof and floor systems carry these vertical forces to the supporting beams. The beams carry the forces to the columns and bearing walls, which then carry the forces down to the foundation and the supporting soil. This process of carrying forces from the roof down to the soil is known as a load path. The failure of any building element or connection along the load path can lead to building damage or collapse.

C: Building Materials and Systems Performance Characteristics

• Ductility. Under normal conditions, a building experiences elastic deformations, deforming as force is applied and returning to its original shape when removed. However, extreme earthquake forces may generate inelastic deformations in which the element does not return to its original shape after the force is removed. Ductility is the property of certain elements that have inelastic deformation before failing. Building elements constructed with ductile materials have a "reserve capacity" to resist earthquake overloads. Therefore, buildings constructed of ductile elements, such as steel and adequately reinforced concrete, tend to withstand earthquakes much better than those constructed of brittle materials such as unreinforced masonry.

• Strength and Stiffness. Strength is the property of an element to resist force. Stiffness is the property of an element to resist displacement. When two elements of different stiffnesses are forced to deflect the same amount, the stiffer element will carry more of the total force because it takes more force to deflect it. When stiff concrete and masonry elements are combined with more flexible steel or wood elements, the concrete and masonry take more of the total force.

• Bracing/Seismic Resistant Components. Four basic components provide seismic resistance against lateral forces (Figure 3):

• Braced frames consist of beams, columns, and stiff diagonal braces that perform like shear walls, but use less material.

• Moment resistant frames (generally of steel or reinforced concrete) consist of beams connected to one or more columns to carry multi-dimensional earthquake forces.

• Horizontal Diaphragms are floor and roof deck systems that carry forces across the building to shear walls, braced frames, and/or columns.

• Shear walls are large structural walls placed in a building to carry forces from the roof and floor systems to the supporting foundation, and into the soils.

• Cross walls are interior walls and partitions that are not necessarily continuous to the foundations, but which are attached securely to two floor diaphragms (the top side of a floor diaphragm to the underside of the floor above) and that are stiff and strong enough to resist the independent movement of the two connected diaphragms.

• Connections. Strong building connections allow forces and displacements to be transferred between vertical and horizontal building elements. In addition, strong connections increase the overall structural building strength and stiffness by allowing all of the building elements to act together as a unit. Inadequate connections represent a weak link in the load path of the building and are a common cause of earthquake building damage and collapse.

• Damping. When a tuning fork strikes a surface, it vibrates back and forth at a certain rate - this rate is known as its fundamental period. All objects, including buildings, have their own unique fundamental period of vibration. Ground shaking from an earthquake will cause vibrations in a building. If the ground shaking matches the fundamental period of the building, the building will resonate with the earthquake, causing the building vibrations to greatly increase.  This can lead to extensive building damage. "Damping" diminishes this resonance by pulling the energy out of the system as heat - in the way that a shock absorber in a car dampens a car's vibrations from bumps in the road.  Damping is imparted to a building by the cracking and inelastic movement of its structural elements, and it can also be deliberately added by installing shock absorber-like devices into the building's structure.  In the first case, "controlled" damage at the onset of shaking can reduce the likelihood of catastrophic damage as the shaking intensifies, and in the second case, the damping devices work like vehicle shock absorbers to reduce the response of the structure to a level below that at which post-elastic behavior (and thus damage) will occur.

• Weight Distribution. Buildings that are wide at their base and have most of their weight distributed to their lowest floors generally fare better in earthquakes than tall, top- heavy buildings which act like an inverted pendulum. Inverted pendulum buildings usually experience greater displacements than those shorter and heavier near the base.

• Building Configuration. Square or rectangular buildings with floor plans with symmetrically place lateral force resisting elements tend to perform better in earthquakes than buildings composed of irregular shapes or 'those with large foyers or lobbies that create a soft story condition. Buildings with irregular shapes cannot distribute lateral forces evenly, resulting in torsional response that can increase damage at key points in the building.

• Foundation / Soil Characteristics. The underlying geology of the site can also have a significant effect on the amplitude of the ground motion there. Soft, loose soils tend to amplify the ground motion and in many cases a resonance effect can make it last longer. In such circumstances, building damage can be accentuated. In the San Francisco Earthquake of 1906, damage was greater in the areas where buildings were constructed on loose, natural and manmade fill and less at the tops of the rocky hills. Even more dramatic was the 1985 Mexico City earthquake. This earthquake occurred 250 miles from the city, but very soft soils beneath the city amplified the ground shaking enough to cause weak mid-rise buildings to collapse (see Figure F-5). Resonance (see below) of the building frequency with the amplified ground shaking frequency played a significant role. 

Sites with rock close to or at the surface will be less likely to amplify motion, and with such sites, generally, the farther from the source of an earthquake, the less severe the motion. The type of motion felt also changes with distance from the earthquake. Close to the source the motion tends to be violent rapid shaking, whereas farther away the motion is normally more of a swaying nature. Buildings will respond differently to the rapid shaking than to the swaying motion..  Buildings can be severely damaged when the soils that support the building foundations shift, sink, slide, or liquefy. Optimally, structures should not be located in areas with poor site conditions.  

• Resonance.  Resonance was a major problem in the 1985 Mexico City earthquake, in which the total collapse of many mid-rise buildings (Figure F-5) caused many fatalities. Tall buildings at large distances from the earthquake source have a small, but finite, probability of being subjected to ground motions containing frequencies that can cause resonance. Where taller, more flexible, buildings are susceptible to distant earthquakes (swaying motion) shorter and stiffer buildings are more susceptible to nearby earthquakes (rapid shaking).

• Redundancy.  It is very beneficial for a rehabilitated lateral-force-resisting system to have an appropriate level of redundancy, so that any localized failure of a few elements of the system will not result in local collapse or instability. This should be considered when developing rehabilitation designs.

Proceed to D. RISK ASSESSMENT PROCESS