BRE DG436/3 PDF

Introduction

The wind-induced dynamic response of a building can be broken down into alongwind response, crosswind response and torsional response. These responses can be analysed independently and the overall response determined by combining together each of the components. The alongwind dynamic response is dependent on the scale, magnitude and frequency of the turbulence in the approaching wind. The overall alongwind response includes the static (or quasi-static) response due to the approaching mean wind speed. The crosswind dynamic response is more complex and can include a number of excitation mechanisms such as vortex shedding and aeroelastic instabilities such as galloping and flutter (Box 2), as well as crosswind buffeting. The incident wind can also generate crosswind (or lift) forces depending on the approaching wind angle and the cross-sectional shape of the building. Torsional dynamic response can be amplified by the centre of twist of the building being offset from its centre of mass, ie a rectangular building with the lifts and stairwells concentrated at one end of the building. It can also be caused by non-symmetrical wind forces, or where an upwind building shelters one-half of the building. Non-symmetrical wind forces can also be produced if there is a nearby adjacent building at one end that creates a localised region of accelerated or sheltered flow.

The EN provides calculation procedures for the alongwind dynamic response of the first fundamental mode of bending and for vortex shedding response. It does not consider torsional dynamic response, crosswind buffeting response or response from higher modes of vibration (Box 3). For particular designs of structures, the crosswind response can govern the structural design. The torsional response often governs the serviceability design (for occupant comfort) due to high acceleration levels at the outermost corners of buildings with a large moment arm from the centre of twist. Assessment of occupant comfort due to dynamic building response is not included in the EN; for further information on this topic see BRE IP 21/12^[4].

This part of the Digest demonstrates the calculation procedure for determining wind loads on a 120 m tall building using the full dynamic approach given in Annex B of the EN for alongwind response. Advice on torsional and crosswind dynamic response is included at the end of this part of the Digest.

The example calculation in this part uses the same building as used in the example in Part 2 of this Digest. Details of the building and site are given below. The building is a tower 120 m tall on an 8 m podium. The dimensions of the building are shown in Figure 1 (page 4). The building is assumed to be steel-frame with an average building density of 150 kg/m3. The logarithmic decrement of damping δs has been taken as 0.05 from EN Table F.2. Only orthogonal winds from the worst-case NW and SW wind directions are considered in the example.

The podium is not a dynamic structure and therefore the dynamic factor cd may be taken as 1.0 (NA Figure NA.9 Note 4). The wind loads on the podium are not considered in this example; see Part 2 of this Digest for this calculation.