Architectural Structures

IBC Fig. 1609. Basic wind speed (3-second gust)

Map values are nominal design 3-second gust wind speeds in mph (m/s) at 33 ft (10 m) above ground for Exposure C. Mountains, gorges, ocean promontories, and special wind regions shall be examined for special wind conditions.

Design for Wind

Wind forces are very complex, effected by building form, height, flexibility and openness, as well as topography, environment, wind velocity and direction. Tornadoes are the most destructive. United States tornados occur between Rocky mountains and Appalachian mountains with speeds over 200 mph. However, the probability of any building to be hit is remote. Hurricanes along the Atlantic coast range from 90 to 150 mph. Typhoons, similar to hurricanes, occur in the Pacific, Indian Ocean, Arabian Sea, and Australia.

Wind speed increases with height. Wind load is defined by speed, both constant velocity and dynamic gusts. As air moves along the earth it is slowed near the ground due to friction, which also makes wind more erratic. Wind speed is less in cities but gusts are greater. When wind encounters a building it flows around and over it, causing pressure on the windward side, negative suction on the leeward side and side walls, and pressure or suction on the roof. Flat roofs and leeward slopes are subject to suction, as well as shallow windward slopes, but steep windward roofs have pressure. Wind flow is effected by form; square plans have greater pressure than round ones. Forms like aircraft wings streamline air flow with least resistance. Buildings drift in wind and vibrate in gusts. Wind pressure on edges, ridges and eaves is greater than the mean surface pressure. This is important for cladding and curtain walls.

Wind design methods assume pressure increases with height. However, wind tunnel tests show maximum wind pressure at about 2/3 the building height, reducing toward the top due to air flow over the roof. Regular buildings are designed for wind in two orthogonal directions. Unusual forms may require to investigate other directions, such as 45 degrees. Unusual forms may also require wind tunnel tests. For example wind tunnel tests for the unique Toronto City Hall showed wind load greater than required by code. Structures sensitive to dynamic behavior may also require wind tunnel tests. Wind resistance is effected by type of construction. Buildings of heavy material resist wind better than light buildings. Skeleton buildings with light curtain walls are more vulnerable to wind load than old bearing wall buildings. Flexible structures may sway and vibrate under wind gusts.

Lateral drift is an important issue in tall buildings to assure human comfort and prevent damage or failure of curtain walls, partitions, and other architectural items. IBC tables 1604.3 and 1617.3.1 limit allowable deflection and drift. However, less than code drift may be desirable to prevent damage or failure of curtain walls, partitions and other items and assure occupant comfort. Story drift of h/400 implies about 0.4 inch drift; greater drift could dislodge glass panels. Total drift of h/400 in a 60-story building implies about 2 foot drift each way on top; more drift might cause discomfort for occupants. Given the complexity of computing drift, it is best analyzed by computer.

9-2 DESIGN METHODS Lateral Force Design

Windward pressure (psf)

Interior pressure (psf)

Example: Wood shear walls

Assume: 66’x120’x27’ high, 3 shear walls, L=3x30’=90’, wind speed 90 mph, exposure C, Importance factor I = 1, gust factor G = 0.85 (ASCE 7, 6.5.8 for rigid structures > 1 Hz)

For each level in width direction find: wind pressure P, force F, shear V, shear wall type

Exposure D wind pressure for 10' to 100' height and 85 to 150 mph wind speed

Leeward pressure (psf)

Interior pressure (psf)

Example: CMU shear walls

Assume: Regular flat site

Office building: 6-story, 90’x90’x60’, 30’x30’ core 30’ CMU walls, 8” nominal (7.625”)

Shear wall length L = 2 x (30’– 6’ doors) = 48’ Shear walls resist all lateral load

Roof fabric canopy, 50’x50’x10’, gust factor G = 1.8 Wind speed V = 100 mph

Exposure D Importance factor I = 1

Note:

Wind on lower half of first floor, resisted by footing, has no effect on shear walls

9-7 DESIGN METHODS Lateral Force Design

Amplitude

Period

Richter scale method, courtesy US Geological Survey

Seismic terms

The following seismic terms are important to understanding seismic design:

Period is the time interval of one cycle a wave or a building sways back and forth. A building period resonant with the earthquake period can cause collapse.

Amplitude is the displacement of a wave perpendicular to the direction it moves

Richter scale

The Richter scale was developed in 1935 by Charles Richter of the California Institute of Technology to compare the magnitude of earthquakes. The Richter scale is determined from the logarithm of the amplitude of waves recorded by seismographs; adjusted for the distance to the earthquake epicenter. The Richter Scale gives magnitude in whole numbers and decimal fractions. For example, magnitude 5 is a moderate earthquake, but magnitude 7 is a strong one. Because of the logarithmic scale, each whole number increase represents a tenfold increase in amplitude and force and 31 times more energy.

Velocity is the shaking speed of an earthquake (meter/ second)

Acceleration is velocity increase (or decrease), like car acceleration (meter / second2)Spectral acceleration is what is experienced by a building, modeled by a

particle on a

mass-less vertical rod of the same period as the building

PGA (peak ground acceleration) is experienced by a particle on the ground (without damping)

Body waves

Body waves are seismic waves that move through the interior of the earth, as opposed to surface waves that travel near the earth's surface. P and S waves are body waves. Each type of wave shakes the ground in different ways.

P waves are high-speed seismic body waves, traveling at about 26000 mph (42000 km), that shake the ground in the same and opposite direction as the waves move.

S waves (shear waves) are seismic body waves that shake back and forth perpendicular to the direction the wave is moving.

Surface waves

Surface waves are seismic waves that are trapped near the surface of the earth

Rayleigh waves are seismic surface waves causing the ground to shake in an elliptical motion, with no transverse, or perpendicular, motion.

Love waves are surface waves with a horizontal motion transverse (perpendicular) to the direction the wave is traveling.

Spectral acceleration

Acceleration spectra for four soil types (by Seed)

IBC Design Response Spectrum

Basic concepts

Earthquake ground shaking generates forces on structures. Though these forces act in all direction, the horizontal (lateral) forces are usually most critical. Seismic forces are

f = m a (Force = mass x acceleration ) where

Mass = building dead weight Acceleration

Note:

PGA (Peak ground Acceleration) is experienced by a particle on the ground Spectral Acceleration approximates the acceleration of a building, as modeled by a particle on a mass-less vertical rod of the same period of vibration as the building.

Acceleration Spectra (left)

Based on the 1971 San Fernando and other Earthquakes Seed (1976) developed Acceleration Spectra to correlate time period (X-axis) with acceleration for four soil types. Other studies by Hall, Hayashi, Kuribayashi, and Mohraz demonstrated similar results. Equivalent Lateral Force Analysis is based on Acceleration Spectra, abstracted as

Design Response Spectrum

Design Response Spectrum (left)

The IBC Design Response Spectrum correlate time period T and Spectral Acceleration, defining three zones. Two critical zones are:

T < TS

T > TS where

T = time period of structure (T ~ 0.1 sec. per story - or per ASCE 7 table 1615.1.1) TS = SDS/SD1 (See the following graphs for SDS and SD1)