7/28/2019 Motion CriteriaI n High Rise Buildings

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MOTION CRITERIA IN HIGH RISE BUILDINGSBy Peter Irwin, President

ISSUE NO. 2c

Oscillation of a structure in wind can rapidly lead tostructural failure if the oscillations grow to a largeamplitude. However, certain types of oscillation,while they are not large enough to cause structuralproblems, may cause problems of human discomfortin tall buildings.

Building motions under the action of commonlyoccurring winds need to be kept within comfortablelimits for the building occupants. For lightweightmodem structures, keeping the building motionswithin acceptable limits can be more of a challengethan ensuring that they have sufficient structuralstrength. Acceleration has emerged as the mostcommon index of motion effects. The horizontalforce felt on the human body is directly proportionalto the horizontal acceleration. An acceleration of onethousandth of gravity is called a milli-g. People aresensitive to accelerations as small as a few milli-g.

It is unrealistic to demand that no perceptiblemotions ever occur. Therefore, how much horizontalacceleration is acceptable and how often can it takeplace? The International Standards Organization(ISO) has published guidelines (see graph) in termsof the root mean-square (RMS) value of theacceleration on the top floor that should not be

exceeded more than once in five years on average.For example, for a building with a five secondperiod, the five year acceleration should not exceed18 milli-g. For a building with an eight secondperiod, a higher five year acceleration not exceedin22 milli-g is acceptable. These accelerations will beperceived, but if they only occur once every five toten years, the functioning and commercial viabilityof the building will not be adversely affected.

More recently there has been a trend towards settinthe criteria at a shorter return period of one yearwith values of acceptable acceleration set about 30%lower. This has been prompted by the increasingnumber of buildings being constructed in hurricaneand typhoon areas. Typically, in these areas, there iample warning of the five or ten year winds caused

by these storms. Buildings are usually evacuatedbefore the storm hits, and occupants who stay arenot expecting normal comfortable conditions.Therefore, in such areas, it is more meaningful toconsider the one year wind event.

The criteria discussed above apply primarily to

office buildings since the most data are available fothat situation. Target accelerations for residentialbuildings are often set 20% to 30% lower.

Guideline for 5 Year Acceleration in Buildings

Rowan Williams Davies & Irwin Inc.Consulting engineers specializing in assessing and solving design problems related to:

Wind Engineering Sun / Shade / Glare Acoustics, Noise & Vibration Snow Impacts Pedestrian Comfort Industrial process optimization Air Quality Emergency Planning Hazard & Risk

650 Woodlawn Road West, Guelph, Ontario, Canada N1K 1B8Tel: (519) 823-1311 Fax: (519) 823-1316 Email: info@rwdi.com Website:http://www.rwdi.com