Structural design in buildings: theoretical principles and good practices

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Plan - Structural design | ACCA software

A) Plan

B) 3D

Structural model - Structural design | ACCA software

C) Structural model

Structural details - Structural design | ACCA software

D) Structural details

Structural planning and phases

Structural design can be translated into the application of theoretical and good practice principles, as well as specific rules designed to guarantee the essential requirements of mechanical strength, stability and durability.

The current design paradigm provides that the works must guarantee certain levels of performance in accordance with the Limit States, throughout the nominal life of the project.

The achievement of certain performance levels allows to confer the ability to avoid collapses, loss of balance but also the ability to maintain a certain level of mechanical resistance in fire conditions.

The structural and building design process, which leads to the achievement of certain levels of performance, can be divided into the following phases:

Structure planning
Actions and load evaluation
Analysis methods
Structural members design
Details, drawings and activities preparation

AC frame

Structural planning

Structural planning is strongly influenced by the technological choice made, ie by the material and the technology associated with it.

The technological choice may depend on several factors, such as:

Cost
Running time
Environmental requirements

A reinforced concrete structure could have a lower cost but longer construction times than a similar one built through prefabrication technology. Therefore, the choice should always be made following a cost-benefit analysis.

In addition to these aspects, however, the technological choice in turn influences the structural planning, which must clash with other aspects, such as the dangerousness of the site, the category of the building from which the static loads derive and the needs of the client.

Considering the case of a reinforced concrete frame structure, the structural planning process can be divided into the following phases:

Study of the hazard of the site and definition of the design model
Positioning, orientation and compliance with the limitations on the columns;
Positioning and compliance with the limitations on the beams
Definition of the horizons
Choice of the type of foundation

Correct orientation example

Non-beneficial orientation to structural checks example - structural design example | ACCA software

Example of non-beneficial orientation towards satisfying structural checks

Study of the hazard of the site and definition of the design model

In light of the past and recent seismic events that have affected our territory, and therefore of the danger that has been deduced from this knowledge, the designer cannot ignore the application of design models capable of facing the events, which in all probability are bound to occur.

The normative translation defines, for structures subject to seismic actions and not equipped with isolation and / or dissipative devices, two behaviors for the structures:

dissipative structural behavior
non-dissipative structural behavior

Where, for structures with dissipative behaviour, the ductility class to be associated must be chosen, which will affect the various prescriptions in order to regulate the dissipative capacity both locally and globally.

According to this approach, the designer will be asked to make choices about the behavior he wants to obtain, also based on the danger of the site, which in particularly complex cases could also be deduced from a local seismic response study.

Positioning and compliance with the limitations on the beams

Beams allow loads transfer to the columns and in contexts of exceptional loads, they must allow the activation of the plastic hinges so as to favour mechanisms as ductile as possible.

As described for the columns, the beams also need to be addressed in the following aspects:

Positioning: In fact, the beams should, unless there are particular architectural requirements or types of structural schemes, be placed between the columns in order to create a three-dimensional spatial frame. This contributes to making the building regular in height and also to limiting inter-storey displacements.
Regulatory Limitations Several limitations are defined on the geometry but in addition to these, we can mention the good rule:

h_{_beam} ≤ h_{_column}-5cm

to be taken into consideration when pre-dimensioning in order to comply with the principle of Hierarchy of Beam-Column Resistances, however, to be verified following the structural analysis.

Separation joint useful for structural improvement example - Structural design | ACCA software

Example of separation joint useful for structural improvement

Definition of the horizons

From the structural point of view, the horizontals, or floors, can be single or double frame. Single frame floors are ideally designed considering a unitary strip free from any transversal connection, therefore under load ideally they should exhibit a cylindrical deformation, but this almost never happens.

This behavior highlights the need to carefully establish the structural design parameters. Simple rules suggest keeping the ratio between the free span of the floor and its thickness in the range 18-20 in order to limit deformability. By limiting the deformability it is possible to avoid stress concentrations produced by non-structural elements and therefore the onset of crack patterns.

In single-frame floors, the interaction at the edges is an aspect that should definitely be considered. In fact, the structures parallel to the warping, therefore which do not constitute direct support, constitute a real transversal constraint, which gives rise to internal stresses and therefore possible cracking states. The problem can be solved by forming ribs or in cases of high stresses by creating a structural joint along these directions.

The warping direction is certainly an aspect of interest to the designer. Generally there is a tendency to alternate the warping direction in order to maintain a uniform distribution of the load on the various floors.

Design of foundation works by investigating the tensions generated in the ground as result of superstructure - Structural design | ACCA software

Design of the foundation works by investigating the tensions generated in the ground as a result of the superstructure

Choice of the type of foundation

In the structural planning process, in addition to the aspects already defined and others not mentioned, the choice of the type of foundation is an aspect of great importance. The foundation is that part of the work that has the task of transferring the loads deriving from the superstructures to the ground, in static and dynamic conditions.

The geotechnical investigations in this phase can help to investigate the properties of the soil, also providing useful information regarding the presence of any aquifers and variations in their level.

There are different types of surveys and clearly they are to be commensurate with the nature of the work and / or intervention to be carried out, but it should be emphasized that the designer is responsible for defining the survey plan, characterization and geotechnical modeling, even if such study is always affected by an intrinsic uncertainty.

Therefore, the choice of the type of foundation must never be made a priori, but it is always necessary to investigate and therefore define a geotechnical model of the ground in order to make the best choice, for the entire nominal life of the project.

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