Structural Design Basics

Structural design basis starts with the type of structure to be designed. For any structural design to commence, we require certain data. These data includes information about type of structure, site conditions, loading conditions, type of environmental exposure, earthquake zone and wind zone.

Site Conditions:

Structural design of foundation requires soil geotechnical (soil) investigation data i.e. safe bearing capacity of soil, depth of water level below the ground.

Loads on the structures:

1. Dead Loads:

Dead Loads are loads from the self weight of the structural members such as beams, columns, slabs, wall, finishing, plastering etc. Any stationary elements or equipments which may be permanently positioned on the structure shall also be considered as dead load. Dead loads are also called as self weight and is calculated as volume multiplied by its unit weight. Unit of different materials are provided by the standard codes.

2) Live Loads:

Live loads also called as imposed loads are probable loads that the structure may be subjected to during occupancy. These are loads which are moving or dynamic in nature and may or may not be present on the structure during intended use of the structure. For example, for an industrial structure, loads from people, maintenance tools etc. can be called as live loads, while loads from equipments which are stationary at a location is considered as the dead load.

Live loads are different for different types of structures and varies with type of occupancy. For example, for a residential building, live load on floor is considered as 3 kN/m² while for industrial structures or business centers live loads can be taken as 4 or 5kN/m². These loads vary for different structures based on intended use.

Minimum live loads to be considered for design purpose are obtained from respective standard codes based on country or 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.

For design of any structure, in live loads should be considered based on future expansion plans and all the probabilities of loads which the structure may experience during its lifetime, should be considered.

3. Wind Loads:

Wind loads are horizontal loads on the building which are exerted on the surface area of the building on windward side. This load is calculated based on the wind zone which provides the maximum wind speed in the given zone. This can be obtained from the wind map of the location. This wind speed is converted into force based on the surface area and orientation of building w.r.t. wind direction. Shape of the building is or structural member is also considered for calculation. Wind loads are considered only on those structural members are exposed to the wind or which resists the wind. The guidelines for calculation of wind force on 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.

4. 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.

Design of Structures:

Structural design of buildings or other structures should be carried out as per the relevent code of practice.

Structural concrete Design shall conform to ACI 318-95 / 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. Working stress method, Limit State Method, Load Resistance Factor Design method. These are the methods used for the design of structural members and are guided by the relevant standard code of practice.

Following factors to be considered for 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 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 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 easy escape of occupants during emergency situations such as fire.

These are only 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.