Plan de Tareas

The goal of seismic design is to determine the proportions of structural elements and their detailing so that the code requirements are fulfilled and the optimal configuration in respect with building function, easy construction and maintenance, labor and mate-rial economy, minimum need of post-seismic intervention, etc is observed. Accordingly, the simple application of codes is not enough. Besides general performance objectives and corresponding requirements provided by codes detailed analysis of each particular features of the future building have to be examined by the designers’ team together with the investor.

6.4.1 P r e l i m i n a r y D e s i g n

From the very early stages of the design process reliable information about destination, general architectural layout and site peculiarities have to be provided to the structural designer. Besides the number of stories, in-plan and elevation building configuration it is important to know if it has a basement or not and, if yes, its scale (number of stories) and destination have to be specified. Information about soil characteristics and about the water table is crucial for selecting the solution for infrastructure.

The most used frame structures have the columns disposed according to the nodes of a rectangular regular mesh. Regularity in plan and elevation, uniform span and bay, advantageous building aspect ratio (total height/total width) are premises for obtaining through structural design a favorable, controlled seismic response.

S e i s m i c J o i n t s

In-plan building asymmetrical layout like T and L shapes generates very unfavorable torsional motions to the building which overload frames situated along the perimeter.

Unfavorable response generates also building in-plan shapes like H or I with large flanges.

Irregularities in elevation (set-backs) are a source of sensitive zones due to the effect of high vibration modes and torsional effects.

When necessary, the unfavorable effects due to irregularities could be diminished or even eliminated by dividing the entire building through seismic joints. The joints’

width is determined from the condition that building units separated by joints are dynamically independent to each other and the joint prevents pounding effects.

Number and location of seismic joints have to be carefully examined. One has to take into account that seismic joints could have a width as large as 15–30 cm or even more.

Accordingly, the architectural and functional conditions are not easy to be fulfilled.

On the other hand, one has to note that, under lateral seismic forces, the building sec-tions behave like vertical cantilevers developing important overturning moments and shear forces. Transfer of these forces to the infrastructure and further to the foundation soil is a hard task. In respect to these issues, it is recommended to observe an aspect ratio (total height/total width) of each unit close to the optimal one. It is recommend-able to observe an aspect ratio of the building (total height/total width) close to three;

ratio up to five can be still accepted.

I n f r a s t r u c t u r e

Preliminary design has to deal with the whole structural system including infrastructure and foundation solution.

Because of cantilever behavior of the superstructure under lateral forces the foundation system has to be considered as a whole for the entire building.

Available solutions are listed and analyzed within the subchapter 6.7. However, some current solutions will be here briefly described.

Medium- and high-rise buildings are normally provided with a basement extended over one or more levels. Current use of basement is for parking but other functions can also be sheltered at this level. Basement always involves a peripheral wall, which is a structural component with high stiffness as compared with the superstructure. When some vertical walls at the basement level are required by the architectural needs they can be treated as structural walls interacting with horizontal slabs (over and within the basement). Peripheral wall interacting with internal walls together with floor slab systems (sometimes with raft, when necessary) acting as horizontal diaphragms consti-tutes a multi-box system extremely rigid and resistant which is the ideal infrastructure.

Even for lower-rise buildings founded on poor soil a multi-box infrastructure is a solution which allows the building to “float’’ on the poor foundation soil avoiding expensive foundation solutions on piles.

O t h e r i s s u e s t o b e t a k e n i n t o a c c o u n t d u r i n g p r e l i m i n a r y d e s i g n

A structural designer has to keep in mind that, in contrast with gravity-dominated buildings, those subjected to high intensity seismic actions behave under lateral forces similar to a cantilever. Any architectural or structural solution which could generate brittle failures or any disadvantageous or uncontrolled effects should be avoided. Some examples are listed below.

Potential pounding between building sections or between adjacent buildings has to be prevented. Similar danger occurs when two neighboring buildings are connected through pedestrian bridges inappropriately treated.

Solutions which generate short columns or short beams with potential brittle shear failure have to be avoided. For example, frame with unequal spans should be preferred for a building with a longitudinal middle corridor instead of frames with a short midspan which lead to a short beam shear dominated (see Fig. 6.3).

Supplementary rules for obtaining good configuration should also be observed as much as possible:

• Try to obtain symmetrical plan shape

• Similar (close) stiffness of the whole system according to principal axes

• Uniform span and bay

• Limit torsional response

• Decouple elevator cages and staircases from the primary structure

• Prefer light-weight deformable partitions for preventing uncontrolled interaction with primary structure

• Keep uniform column cross section over the building height

• Avoid excessive building slenderness (recommendable total height/total width less than 3–5); this is an efficient way to ease the fulfillment of basic requirements S t e p s o f p r e l i m i n a r y d e s i g n

Preliminary design is an iterative trial-and-error process. Its goal is to “tailor’’ the struc-ture so that the basic requirements are easily fulfilled. Thereby the proper structural design becomes a “fine tuning’’ process based on check up of all code requirements.

Best way to achieve a structural solution close to the optimal one is to use, from very early stages, good general computer software to determine the seismic response even though the input data are, initially, rough approximations. Such software allows performing easily modifications of initial model leading towards the desired optimal solution. Recommended steps are the following:

1. Based on architectural layout a structural system model is proposed with approximate sizes of all its components and estimated floor masses

2. Determine the response parameters and check if

• First two modes are translations (not torsional type)

• Vibration period T according two principal axes are close to each other

• Equivalent total masses according first two (translational) modes differ less than 20%

• Inter-story drifts over the whole building height comply with code requirements

3. Modify the structural model and resume step 2 until the above basic requirements are met.

6.4.2 S t e p s o f P r o p e r D e s i g n

Proper design is aimed to validate the sizes of all components of the structural system, to determine their reinforcement amount (flexural and transverse) and detailing.

Criteria to be observed are those of reinforced concrete design rules, as stated by codes, as well as the fulfillment of specific requirements of seismic resistant structures.

As already stated (see Chapter 4), the seismic structural design is governed by the four basic requirements:

1. Strength and stability

2. Ensure favorable dissipating mechanism

3. Control of lateral displacements (or drifts) 4. Ensure local ductility of structure’s members

At this stage of design, proper analysis models (currently 3D) have to be used involv-ing superstructure, infrastructure and foundation soil. Permanent and live loads have to be accurately evaluated; load combinations according to code provisions will be specified as well as the stiffness of all structural components (beams, columns, slabs, infrastructure walls, soil).

Strength and stability is the first requirement to be examined even though, in many cases, the control of inter-story drifts and the capacity design method (corresponding to the second requirement) will lead to changes in members’ sizes.