# Earthquake Analysis on a Structure – A Simplified Analysis Approach

Earthquake load is one of the least talked about effects in structural engineering classes in Nigeria and probably many other countries in Africa. Although there are codes of practice which provide standards and recommendations, this aspect still remains a mystery to a number of civil engineers. This is due in part to the fact that Africa hardly has any earthquakes neither is there sufficient data in the public space on the characteristics of tremors in the past. For a civil engineer to be indeed a global engineer, there is need for these calculations to be included in calculations, albeit for demonstration purposes.

The following are some codes for earthquake Analysis:

1. BS EN 1988-1:2004 – Eurocode 8: Design of Structures for Earthquake Resistance
2. IS 1893-2002/2005 – Criteria for Earthquake Resistant Design of Structures
3. ASCE 7-05 – Minimum Design Load for Buildings and other Structures

During the analysis of a structure for earthquake, there are guidelines used to obtain two parameters:

SDS = 2/3 SMS

SD1 = 2/3 SM1

Where;

SDS – Design earthquake spectral response acceleration at short periods

SD1 – Design earthquake spectral response acceleration at a 1-second period

SMS – Maximum considered earthquake spectral response acceleration at short periods

SM1 – Maximum considered earthquake spectral response acceleration at a 1-second period

The above design earthquake spectral response acceleration can be used to estimate the resulting moment, displacement and other impact on a structure. The spectral acceleration is applied at the centre of gravity of the structure in question. For small components which are attached to structures, the following is the formulae to estimate the force on the structural element which acts at the centre of gravity of the said structure. Such structures as isolated footings, electrical, mechanical and architectural component systems can have their earthquake load estimated thus:

Where force = mass X acceleration = Wp X acceleration

According to API 650, spectral response accelerations less than 0.05g (where g is acceleration due to gravity) or 0.491 m/s2, no earthquake check is necessary. Fortunately for African countries, most spectral acceleration fall well below this value.

When it comes to large multistory buildings and other such structures, some more rigorous analysis is required and the above formulae may be difficult to interpret. With the aid of STAAD Pro, these analysis can be simplified.