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The structural design process of a complete construction project is divided into three phases Planning, Design, and Construction.

The planning phase will consider the various factors that affect the layout and the dimension of the structure. Here, the decision is made based on the function of the building. Next factor under consideration will be aesthetics, sociology, the economy and the environment.

In the design phase, the structural design details of every building member are determined. Here, the dimensions, proportions and connection details of all structural members are determined.

The construction phase of a structural design project will involve the procurement of materials and equipment, labor movement and the erection process. Practical difficulties will demand for the redesign of the particular work.

General guidelines in structural design are explained briefly in the below section.

Structural Design Basis

The structural design basis of any structure will depend on the type of structure that is to be designed. The following information is necessary in order to proceed with the design of a structure.

  1. Site Conditions
  2. The loads Acting on the Structure
  3. Design of Structure

Other than the major information mentioned above, data regarding the type of structure, the environmental exposures and the zones (earthquake or wind zones) of the structure are also required.

1. Site Conditions

A complete study of site conditions will let the designer decide about the:

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  1. Bearing capacity of the soil
  2. Maximum load capacity of the soil
  3. The foundation to be chosen for the soil
  4. Requirements of soil improvements if any
  5. The water table level of the site

2. Loads on Structures

The structure constructed will be subjected to various types of loads. Different types of loads that are to be considered in the design of structures are described below.

Loads Acting on Structures

Fig.1: Loads Acting on Structures

Dead Loads

The load due to the self -weight of the structural members forms the dead load. The structural members are columns, beams, loads due to plastering and finishing, wall loads, slab loads etc. If any element that is stationary and placed permanently on the structure it will be also included as dead load. The dead load or the self -weight of any member can be calculated as the product of its volume and its self-weight.

Live Loads

The imposed loads the structures are subjected during the occupancy period are called as live loads. These loads can be either static or dynamic in nature. Sometimes these loads may or may not be present during the use. This situation is common in the industrial buildings and structures, where live loads are from the people, maintenance tools etc.

For different structures, the live loads are different. For a residential building the live load value taken will be 3 kN/m­­­2 while for commercial buildings, the value of live load can be 4 to 5kN/ m­­­2.

The minimum load values for different structure types will be recommended in the standard codes of the respective region. ASCE 7 Provides live minimum live loads for buildings and other structures in USA. While in India, IS875 Part-2 provides guidelines for minimum live loads.

While considering live loads in the design,loads that can be formed if there is a possibility of future expansion of the structure must be considered. So load probabilities during its lifetime must be considered while designing for live loads.

Wind Loads

Wind loads act horizontally on the surface area of the building on its windward site. Every region or site under consideration comes under a wind zone. Based on the wind zone, the maximum wind speed in the area is calculated. The wind map of the location will give all these data.

Based on the surface area and the building orientation, the wind speed is converted into force. The wind force is calculated with respect to the wind direction. While calculating the wind loads, there is no need to take into consideration the shape of the building or the structural member.

The wind loads are calculated only for structures subjected to wind or for those structures intended to resist wind.

The guidelines for calculation of wind force on the structure is provided by ASCE 7-95 / UBC -1997 /IS 875 : 1987 (Part 3) / BS CP3 : Chapter V : Part2 : 1972 or whichever applicable codes shall be considered based on the location of the building or structure.

Seismic / Earthquake Loads

Design for seismic loads shall be carried out as per ASCE 7/ UBC/ IS 1893 or whichever standard code is applicable. The guidelines provided by these applicable codes shall be followed for calculation of earthquake forces.

Read More: Loads on Structures

3. Design of Structures

The structural design of buildings or other structures should be carried out as per the relevant code of practice.

Structural concrete Design shall conform to ACI 318-14 / IS 456: 2000 / BS 8110: Part 1: 1985 or other whichever code is applicable.

Structural steel design and fabrication shall conform to AISC-ASD (9th Edition) / IS 800: 1984 / BS 5950: Part 1: 1990.

Structural design methods are selected based on the local practices. The three design methods are:

  1. Working stress method
  2. Limit State Method
  3. Load Resistance Factor Design method.

Read More: Methods of Structural Design

These are the methods used for the design of structural members and are guided by the relevant standard code of practice.

Factors Considered in Design of Structures

Following factors to be considered for the design of buildings or other structures and shall conform to the standard codes:

  1. Maximum allowable settlement of foundation/structure.
  2. Vertical and lateral deflections of buildings, structures as a whole and other structural members.
  3. Sliding and overturning of buildings or structures should be checked and prevented by design.
  4. Standard detailing guidelines should be followed in the drawing.
  5. All engineering and design shall comply with relevant and applicable codes of practices, local bye-laws, and rules as per Directorate of industries and factories & as listed in Project Design Basis.
  6. Environmental exposure conditions should be considered in the design and respective factors must be applied in structural member design.
  7. Types of construction materials and structural members and their properties should be used during design.
  8. Special care should be taken to provide an easy escape of occupants during emergency situations such as fire.

These are only a few points, many other factors should also be considered which may be relevant for the design. A checklist for different types of structural design should be maintained and followed to prevent any error during design and detailing for buildings and other structures.

Read More: Structural Design

 

 

Gopal Mishra

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