Seismic Design

For buildings in regions of low seismic risk—UBC zones 0 and 1—or buildings assigned to SDC A or B, the provisions for the design of elements given in the first 20 chapters of the ACI code are considered sufficient. There are no requirements for special ductile detailing for walls or moment frames. In regions of moderate seismic risk—UBC zones 2A and 2B—or for buildings assigned to SDC C, there is no special ductile detailing required for walls. However, some ductile detailing requirements for moment frames including flat slab-frames are required. In regions of high seismicity—UBC zones 3 and 4 or SDC D, E, and F—almost all structural elements require ductile detailing.

Given the ready availability of computer programs, the analysis of a building is the easy part and, in a broad sense, is the same whether the building is in a high or a low seismic zone. The detailing requirements, particularly at the joints, are the factor that sets the designs apart.

In regions of low seismic risk, it is likely that a building will never experience forces that will result in an inelastic excursion of the building. For these buildings, a safe and economic design is achieved by using an appropriate margin of safety against gravity and lateral overload. This is typically realized in structural steel design by using allowable stress design (ASD), which limits the allowable stress to a percentage of yield stress.

In ultimate strength design, also referred to as strength design, or load resistance factor design (LRFD), the margin of safety is achieved by use of load factors and strength reduction factors. Either of these two methods, ASD or LRFD, results in structures that are believed to have an adequate margin of safety against overloads. Put another way, the probability of yielding of the structure designed by these methods is considered very low. Structural deflections under lateral loads are expected to be elastic and thus fully recoverable. For example, a very tall building, say, at a height of 1400 feet, on a windy day may experience as much as 3 feet of lateral deflection but would not endure any permanent deflection. The elastic design used in the sizing of structural members for these loads assures that after the winds have subsided, the building would come back to its prewind plumbness without any permanent set.

Such is not the case for buildings in moderate-to-high seismic-risk zones. Yes, they too respond elastically under the most severe wind conditions, because the design is meant

Figure 4.14. Full-depth interior brace: (a) plan; (b) schematic section.

to keep the structure elastic under the generally predictable wind loads. However, the lateral loads that we use in seismic design are highly unpredictable. We know this much, as past earthquakes have taught us: The magnitudes of lateral loads experienced by buildings under large earthquakes are so large that an elastic design under these loads is simply not possible. The building designed to perform elastically in a large seismic event will have structural members so large, and costing so much more, that society has accepted the risk of buildings going beyond their elastic limit, with the stipulation that they do not fall down or collapse. In other words, a building may be utterly damaged beyond repair and may never be occupied again, but if it stays up, providing life safety for the building occupants during and after a large earthquake, it is deemed to have performed adequately under present seismic codes.

The collapse of a building is generally preventable if brittle failure of its members and connections is prevented. In other words, the structural elements may bend and twist

Super diagonal brace {Eccentric brace)

Service core, super column

Energy dissipating fuse {Link beam)

Figure 4.15. Eccentric bracing system for super-tall buildings.

Super diagonal brace {Eccentric brace)

Service core, super column

Energy dissipating fuse {Link beam)

Figure 4.15. Eccentric bracing system for super-tall buildings.

to their hearts' content, but may not snap. The intent, then, is to build ductility into the structure so that it will absorb energy, and thus prevent sudden breaking up of members that result in collapse.

Therefore, structures in regions of high seismic risk are detailed to have ductility. The degree of detailing is entirely dependent on the severity of seismic risk. This is the reason that a building in seismic zone 3 or 4, or assigned to SDC D, E, or F, is designed to be more ductile than its counterpart in a less severe seismic zone, or assigned to SDC A, B, or C. The vast difference in design requirements may be appreciated by studying Table 4.1, which gives a comparison of nonseismic and seismic design criteria for moment frames and shear walls.

Seismic design coefficients for concrete buildings as specified in UBC 1997 and IBC-03 are given in Tables 4.2 and 4.3. Since both IBC-03 and NFPA 5000 are based on the same resource document—ASCE 7-02—Table 4.3 is also applicable to NFPA

Seismic design of reinforced concrete buildings entails the following steps:

1. Determination of design earthquake forces, including

• Calculation of base shear corresponding to the computed or estimated fundamental period of vibration of the structure.

• Distribution of the base shear over the height of the building.

2. Analysis of the structure for the lateral forces calculated in step 1, as well as under gravity and wind loads, to obtain member design forces and story drift ratios.

Observe that for certain classes of structures having plan or vertical irregularities, or for structures over 240 feet in height, dynamic analysis is required by most codes. The story shears, moments, drifts, and deflections determined from dynamic analysis are to be adjusted for the static base shear value.

3. Design of members and joints for the most unfavorable combination of gravity and lateral loads, including the design and detailing of members and their connections to ensure their ductile behavior.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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