September 2, 2017

January 10, 2015

Please reload

Recent Posts

Revenge of the M-Word

June 1, 1996

1/1
Please reload

Featured Posts

ASCE 7 Revised: New Emphasis on the Effect of the Building Form on Envelope Design for Wind Resistance

 

Introduction

Continuity and quality assurance in the load path from each exterior component to the ground is the key to creating buildings and building elements that withstand wind loads. With the release this year of

ASCE 7-95 Minimum Design Loads for Buildings and Other Structures, the American Society of Civil

Engineers (ASCE) has placed new emphasis on the effect of the building form on pressures placed on

the exterior envelope in wind-load analysis and design. This revision to ASCE 7-93 responds to the

studies of the 1993 ASCE symposium, “The Hurricanes of 1992” which had been organized to learn

from that year’s turbulent Hurricanes Andrew, Iniki, and Omar [Heineman, p.32].

 

The Envelope: Cladding and Components

Those hurricanes reiterated the importance of envelope resistance to wind loads. Wind exerts positive

pressure on windward walls and highly sloped roofs, and negative pressure (also called suction) on flat

roofs and leeward walls. In addition, different portions of these surfaces respond differently to the wind,

with the greatest pressures occurring near corners, sharp edges, and protrusions. These high-pressure

zones can be as large as 1/10 to 1/8 of the surface’s width [CTBUH, p. 66]. Therefore, in addressing

cladding and components, the revised ASCE-7 has expanded its categories of building types to include

more varied building profiles.

 

Where ASCE 7-93 gave component and cladding pressure coefficients for:

1. Buildings with mean roof height less than or equal to 60 feet. Loads at:

a. Walls.

b. Roofs.

2. Buildings with mean roof height greater than 60 feet. Loads at:

a. Walls.

b. Roofs.

 

The ASCE 7-95 categories include:

1. Buildings with mean roof heights less than or equal to 60 feet. Loads at:

a. Walls

b. Gabled and Hipped Roofs (with or without roof overhangs)

c. Stepped Roofs

d. Multispan gabled roofs

e. Monoslope and Sawtooth roofs

f. Sawtooth roofs - two or more spans

2. Buildings with mean roof height greater than 60 feet. Loads at:

a. Walls.

b. Roofs.

 

The Envelope: Openings

 Building forms that which cup the wind can be greatly affected by the total wind force on the building

(Figure 1). Openings in the windward building faces can allow positive pressures to reach the leeward

surfaces which are also experiencing aerodynamic suction. Cladding failures that cause unplanned

openings in the building skin can be especially catastrophic during hurricanes, as an unexpected hole

amounting to less than 5 percent of the windward surface area can cause a building to collapse

[Heineman, p. 36]. In response to these failure modes, a three-pronged approach to envelope design

is appropriate: Protect openings, prevent unplanned openings, and design for internal pressure

coefficients (Figure 2). ASCE 7-93 and ASCE 7-95 provide adjustment of design factors by a hurricane

importance factor for buildings and structures in affected regions. While glazed areas are not

considered openings from the wind engineer’s point of view, an unprotected glazed area in a hurricane

region could be a failure waiting to happen. As a result, ASCE 7-95 addresses unprotected glazed

openings in hurricane regions as if they were unglazed.

 

 In ASCE 7-93, internal pressure coefficients were provided for two types of envelopes:

1. Open buildings

2. All other buildings

ASCE 7-95 addresses internal pressure coefficients for:

1. Open buildings

2. Partially enclosed buildings

3. Buildings sited in hurricane regions and having glazed openings not protected from windborne

debris.

4. All other buildings

 

The Primary Structural System

 Sustained wind speeds are usually less of a problem than gusts. A gust is a brief increase, or surge,

in the wind velocity. Due to its higher velocity and slamming effect, the gust actually represents the

most critical wind effect in most cases. Gusts can be 20 to 30 percent faster than the prevailing wind

speed, and cladding and components can fail within 1 to 3 seconds of a gust [Cook 1993, p. 30]. Wind-related failure of the main structural system is much more rare. Compared to cladding, the main

structural system is usually much heavier and slower to move, and is less affected by gusts which

usually last only a fraction of a second.

 

Wind-related failure of the primary structural system is more likely to occur if it depends on the cladding

system for bracing and the cladding system fails. There were some examples of this in Dade County,

Florida, during Hurricane Andrew. Precast concrete double-Tee decking failed in several cases [Mays,

p. 112]. This type of construction is used for such partially open buildings as parking garages, and the

concrete Tee is typically not designed to take lateral loads, or uplift loads greater than the counteracting

dead load. In cases where the double-Tee decking was the only thing providing bracing to the columns,

entire buildings failed. Similarly, the loss of roof sheathing in gabled stick-built construction caused

truss failure during Andrew, in cases where the sheathing was the only thing providing bracing for the

trusses [Mays, p. 112]. Due to their very different responses to wind loads, the type and amount of

dependance between a building’s cladding and it’s primary structure can become an issue.

 

Other ASCE-7 95 Revisions

 While the major revision in ASCE 7-95 is the greatly expanded section on wind loads as summarized

above, new requirements have also been added in ASCE 7-95 for flood loads and ice loads. There is

also a new appendix providing guidance for the design of building serviceability and the comfort of

building occupants as affected by deflection, vibration, drift, and other environmental effects.

 

The ASCE-7 Standard in Baltimore

ASCE 7-93 has been adopted by all major building codes (BOCA National Building Code [NBC], SBCCI

Standard Building Code [SBC], and ICBO Uniform Building Code [UBC]). ASCE 7-95 can be expected

to be adopted across the nation as codes are updated. The Baltimore City Building Code uses NBC

1993. Other jurisdictions in Maryland already use, or will be adopting, NBC 1993 as a requirement of

state building code legislation enacted in 1995. NBC is updated every three years, and the new

NBC1996 has already been released. NBC 1996 however, still incorporates ASCE 7-93. Therefore,

even when Baltimore City and other Maryland jurisdictions update to NBC 1996, ASCE 7-95

requirements will not be included, unless separate local amendments are made.

 

The Pedestrian Environment

 It is important to remember that high winds affect not only buildings, but the spaces around them.

Terraces, plazas, park areas, and other pedestrian environments next to tall buildings can have

unacceptable wind effects [CTBUH, p 52]. ASCE 7 addresses the effect of environmental forces on

structures, but not the sometimes significant effect of structures on the environmental forces.

 

Conclusion

Come let us mock at the great

That had such burdens on the mind

And toiled so hard and late

To leave some monument behind,

Nor thought of the leveling wind.

–W.B. Yeats

ASCE 7 is a minimum standard. It specifically advises the use of wind-tunnel testing, instead of its

guidelines, for determining the wind-loading and structural response of structures with unusual

geometries, response characteristics, or site location.

 

Selected Bibliography

Ambrose, James and Dimitry Vergun. Design for Lateral Forces, 1987, John Wiley & Sons, NY.

Cook, Richard L. “Gone with the Wind” The Construction Specifier, November 1993, pp. 28-39. “Hugo:

Lessons Learned.” The Construction Specifier, November 1990, pp. 41-43.

Council on Tall Buildings and Urban Habitat (CTBUH). “Design Issues for Wind Loads” in Cladding,

1992, pp. 49-70.

 

Easter, R. Lee. “Can Glass and Hurricanes Mix?” The Construction Specifier, May 1985, pp. 46-50.

Gilmer, William B. “Effects of Wind and Snow on Skylight Framing Systems.” The Construction

Specifier, March 1991, pp. 54-72.

 

Harpole, Tom. “Hugo Observations, Wind Effects, and Subsequent Studies.” The Construction

Specifier, March 1991, pp. 74-82.

 

Heineman, Tom. “Protecting the Envelope.” The Construction Specifier, November 1994, pp. 30-46.

Mays, Vernon. “After Andrew.” Architecture, April 1993, pp. 110-113.

 

Minor, Joseph E. “Accommodating Wind Forces in Glazing Design.” The Construction Specifier,

August 1987, pp. 25-26.

 

Moskowitz, Steve. “Putting the Wind to Work.” The Construction Specifier, November 1993, pp. 88-93.

Smith, William D. “Missile Impact Standards are Coming Your Way.” The Construction Specifier,

November 1994., pp. 36-38.

 

Schroeder, Edward K. “Regarding Roof Edges.” The Construction Specifier, November 1993, pp. 40-

54.

 

Schroter, Richard C. “The Load Path.” The Construction Specifier, November 1994, pp. 67-77.

UL 580, Tests for Wind-Uplift Resistance of Roof Assemblies, Underwriters Laboratories, Inc.,

Northbrook, IL.

Vonier, Thomas. “Blown Away.” Progressive Architecture, June 1993.

 

 

 

Please reload

Follow Us
Please reload

Search By Tags
Please reload

Archive
  • Facebook Basic Square
  • Twitter Basic Square
  • Google+ Basic Square

© 2017 by CSIBaltimore.org. Created by JenniferPearre.com using Wix.