DYNAMIC WIND ANALYSIS FOR HIGH RISE BUILDING – TYPICAL OBSERVATIONS

In the high rise building, wind is critical load and needs to be considered for safety and serviceability of structures. There is also need to understand critical effects and assess dynamic behaviour of structures as per the provisions of established standards. The Indian standard IS 875(part 3) is revised in 2015, many modifications are carried out in sections related to dynamic wind effects, very few research papers are available in relevance to these effects. Thus there is great need to understand dynamic wind effects in high rise structures, and prepare database for various sub sections and quantities mentioned in the standard. In this paper, along wind effect on both directions i.e. along shorter as well as longer direction located in category IV having varying base dimensions with same width along shorter direction with varying height of building from 42 to 60 m have been computed as per Indian Standard code IS 875(part 3): 2015 by the Gust Factor Method by developing excel sheets. Base dimension plays critical role, with the increase in dimension in the direction of wind, the response of building reduces.


Introduction
One of the principal loads acting on above-ground structures is due to the wind. Accurate determination of design wind loads is important in achieving safety that is consistent with economy of construction. In professional practice throughout the world, design wind loads for a majority of structures are evaluated on the basis of wind load provisions specified in standards and codes. In the present day, advance changes in building construction technique have tended to make tall and more flexible structure for action of wind. So that wind loading are more significant along with other force acting on the structure which considered in design of low as well as flexible structure. We need to analyse the high rise building with considering all parameter of wind which are given by IS 875(Part 3):2015.
Static wind effect primarily causes elastic bending and twisting of structure. For tall, long span and slender structures dynamic analysis of the structure is essential, Wind gusts cause fluctuating forces on the structure which induce large dynamic motions, including oscillations [1]. Several failures of structures have occurred in India due to wind. [2].
Lateral loads due to wind which acting on multi story building can cause shake in the upper stories. This could be effect caused due to wind at upper stories as the wind intensity is increasing with graduating heights [3]. The wind pressures are fluctuating the nature and this is illustrated by the wind spectrum. There is a possibility of the fundamental frequency of the tall building structure machine with the wind frequency [4]. Tall buildings being flexible when subjected to randomly varying wind will experience wind forces which acts in the direction of wind known as along wind component resulting from buffeting effects caused by turbulence [5]. Wind loads and response to wind are important design parameters for many structures including bridges, high rise buildings, tall towers etc.; it is nowadays essential to ensure that such structures can survive the high winds and gusts likely to be encountered [6]. It is necessary for us to study it especially for Tall buildings that are prone to windinduced oscillations [7]. Different codes have different approach to estimate wind load such as gust factor [8].
In this paper, different base dimensions are considered with varying height of storey. We have to apply static and dynamic wind effect according to their storey height because wind effect can vary with height. We have to find out whether wind effect varies with base dimension of building or not for better performance of building along both dimensions of buildings. We have to also know effect of static force over dynamic effect on the high rise building. Firstly, the effect of static and dynamic force with respect to base dimension and height of building with all considering parameter of wind as per IS 875(Part3): 2015 is investigated.
The paper consists of Relevant Codal Provisions, Basic assumptions made in analysis, Rsults and discussion and conclusion. Where, H = total height of the main structure of the building in metres, d = maximum base dimension of building in meters in a direction parallel to the applied wind force.
The design peak along wind base bending MOMENT, (Ma) shall be obtained by summing the moments resulting from design peak along wind loads acting at different heights z , along the height of the building/ structure and can be obtained from,

Basic Assumption Made in Analysis
High rise building of different base dimension of varying aspect ratio and height of buildings were analysed for along wind load calculation on the both direction of buildings. RC frame high rise structural parameters are listed in following tables. The wind loads were estimated considering the effect on both base direction of building. first one is the along shorter wind load calculation is assumed to act parallel to the shorter dimension and other one is along longer wind load calculation is assumed to act parallel to longer dimension. Grid size 5m

Results and Discussions
The along wind loads acting on all the buildings considered are computed as per IS 875 (Part 3): 2015. The variation of the natural frequency with respect to aspect ratio of the buildings is shown in Fig.1 It was observed that the natural frequency decreased with increase in the building aspect ratio.

Conclusions
The following conclusions can be drawn.

1.
Along wind induced response of high rise building is increasing with increasing aspect ratio i.e. h/b ratio due to this also drag force coefficient (Cf.z) increases with height of height of building structure.

2.
From the observation of turbulence intensity for different terrain category, alternative equations developed gives turbulence intensity directly for various heights of structure.

3.
Wind dynamic varies with increasing height of building structure, but it slightly varies with base dimensions also.

4.
Wind dynamic effect is more along shorter dimension i.e. when we consider same width for all buildings than along longer direction where width is different for all building. Hence shorter dimension gives maximum along wind induced response as compared to longer direction.