ASCE 7

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Key Changes in ASCE 7-05

Chapter 6: Wind Loads

Parapets

The Combined Net Pressure Coefficient (GC_pn) for parapets have been changed to +1.5 for windward parapet, and -1.0 for leeward parapet.

Design Wind Loads on Open Structures

This section has been expanded to include the following sections.

• Main Wind Force Resisting Systems

This is a new section dedicated to determining design wind loads on the main wind force resisting system for open structures. Fig. 6-18A through 6-18D are new and used for determining the net pressure coefficient.

Fascias for free roofs with an angle less than or equal to 5 degrees shall be considered an inverted parapet.

• Component and Cladding Elements

This is a new section dedicated to determining design wind loads on the components and cladding for open structures. Fig 6-19A through 6-20D are new and used for determining the net pressure coefficient.

• Solid Freestanding Walls and Solid Signs

This is a new section dedicated to determining the design wind loads on solid freestanding walls and solid signs. Fig. 6-20, used to determine the force coefficient (C_f), has been revised to account for more conditions.

ASCE 7 Definitions

Enclosed, Partially Enclosed, Open

Open Defined

A building having each wall at least 80% open. This condition is expressed for each wall by the equation A_O >= 0.8 A_g where

A_O = total area of openings in a wall that receives positive external pressure, in ft² (m²)
A_g = the gross area of that wall in which A_O is identified, in ft² (m²)

Partially Enclosed Defined

There is a lot of discussion about what is a partially enclosed structure. Sometimes the building department criteria or the impact status determines, but the official ASCE 7 description from chapter 6 is as follows:

Building, partially enclosed: A building which complies with both of the following conditions:

1. the total area of openings in a wall that receives positive external pressure exceeds the sum of the areas of openings in the balance of the building envelope (walls and roof) by more than 10%, and

2. the total area of openings in a wall that receives positive external pressure exceeds 4 ft2 (0.37 m2) or 1% of the area of that wall, whichever is smaller, and the percentage of openings in the balance of the building envelope does not exceed 20%. These conditions are expressed by the following equations:

1. Ao > 1.10Aoi

2. Ao > 4 ft2 (0.37 m2) or >0.01Ag, whichever is smaller, and Aoi /Agi # 0.20 where: Ao, Ag are as defined for Open Building Aoi = the sum of the areas of openings in the building envelope (walls and roof) not including Ao, in ft2 (m2) Agi = the sum of the gross surface areas of the building envelope (walls and roof) not including Ag, in ft2 (m2)

see partially enclosed building defined for ASCE 7-05 Florida commentary and the 2007 FBC

Enclosed Defined

..That which doesnt comply with open or partially enclosed...

Directionality Factor Kd

My understanding is that the load combinations listed in the '04 FBC and the ASCE 7 standard are always required to be considered, whether you're using the strength design (LRFD) or the allowable stress design (ASD) method. This being the case, I believe Kd should always be used as specified in the appropriate ASCE 7 table (i.e. Kd=0.85 for components & cladding on buildings).

The way Charlie Everly put it one time, the Kd factor takes into consideration the fact that the load equations in ASCE 7 are based on worst-case conditions exhibited during wind tunnel testing. These "worst-case" pressures usually only developed with winds approaching the models at very particular angles. The more box-like the model, the less common these particular angles of attack occur. Likewise, the more round a structure, the more these specific "worst-case" angles occur, until you have a perfectly round structure (e.g. a flagpole) where every approach angle gives the same wind loads.

That's why the Kd values listed in ASCE 7 get closer to 1.0 as the structure takes on a more rounded shape. In a broad sense, a Kd factor of 0.85 assumes the structure has an 85% chance of having winds approach at one of those very particular "worst-case" angles.

On a separate note, the Kd factor was always considered in the ASCE 7 formulas, only as a hard-coded (non-variable) value. It was simply separated into its own variable and allowed to accommodate different site conditions.

Seems to me, vertical cladding (like shutters), would have only one load in each direction. So while you can argue that you did 'consider' load combinations, the load combination only included one load."

Exposure Categories

ASCE defines building exposure categories as shown in the attached documents.

Exposure Category "B" Defined:

Buildings with 0 - 30 ft Mean Roof Height: Urban & suburban areas, wooded areas or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger for a distance of 1500 ft (0.28 miles) or more in any* direction. Buildings with Mean Roof Height > 30 ft:

Urban & suburban areas, wooded areas or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger for a distance of 2630 ft (~1/2 mile) or more in any* direction.

Note that the 2006 Supplement to the 2004 Florida Building Code states that Exposure Categories shall be determined on a site-specific basis, i.e. not blanketly applied on a county-wide or other basis. This means that, for example, a job in Palm Beach County should not be considered Exposure "B" by default. This office will consider all projects in Florida as having Exposure Category "C", unless proof of Exposure "B" site conditions can be demonstrated.

Zone 4 - 5 Weighted Average

For components and Cladding pressuers on openigns that fall both in zones 4 and 5, you are permitted to take the weighted average up to 50% in zone 5.

Upon searching for a formal publication that stipulates this well known and widely used rule, I received information from Charles Everly, P.E., one of the technical editors of the ASCE-7 wind committee. On February 26, 2007, he writes to me:

[Charles Everly] Idon't know what window type youre dealing with but whatever it is it isn't too hard to figure what the moment would be on the spanning elements such as meeting rails and reverse engineer the capacity of these elements from the allowable load for the window. For the glass I see no problem using a weighted average but I can't give you explicet ASCE7 language that says it's OK. I think that is a matter of engineering judgement. We have a meeting of the ASCE7 Wind Load Committee Mar 27-28. I'll pose that question informally then but I'm sure they would all answer exercize your judgement)

The search will continue for a formal interpretation, but for now this is the long used industry standard.