© L. Prieto-Portar - 2008

EGNEGN--54395439 The Design of Tall BuildingsThe Design of Tall Buildings

Lecture 09Lecture 09

ASCE 7ASCE 7--02 Solved Problem #2:02 Solved Problem #2:

Analytical Method 1 Analytical Method 1 –– The Simplified Procedure.The Simplified Procedure.

In order to illustrate the calculations of the wind design pressures using ASCE 7-02, Method 1, The Simplified Procedure, we will use the same office-warehouse building of Example #1 shown in Lecture 08.

Example #2 also uses the warehouse building 50 feet wide, 100 feet long and 20 feet high, located in downtown Tampa.

The text for the Simplified Procedure is given in ASCE 7-02, Section 6.4.1, page 26.

SECTION 6.4. METHOD 1—SIMPLIFIED PROCEDURE

6.4.1 Scope.

A building whose design wind loads are determined in accordance with this Section shall meet all the conditions of 6.4.1.1 or 6.4.1.2. If a building qualifies only under 6.4.1.2 for design of its components and cladding, then its main wind force-resisting system shall be designed by Method 2 or Method 3.

6.4.1.1 Main Wind Force-Resisting Systems.

For the design of main wind force-resisting systems the building must meet all of the following conditions:

1. The building is a simple diaphragm building as defined in Section 6.2;2. The building is a low-rise building as defined in Section 6.2;3. The building is enclosed as defined in Section 6.2 and conforms to the wind-borne debris provisions of Section 6.5.9.3;4. The building is a regular shaped building or structure as defined in Section 6.2;5. The building is not classified as a flexible building as defined in Section 6.2;6. The building does not have response characteristics making it subject to across-wind loading, vortex shedding, instability due to galloping or flutter; and does not have a site location for which channeling effects or buffeting in the wake of upwind obstructions warrant special consideration;7. The building structure has no expansion joints or separations; 8. The building is not subject to the topographic effects of 6.5.7 (i.e., Kzt = 1.0);9. The building has an approximately symmetrical cross section in each direction with either a flat roof, or a gable or hip roof with � � 45 degrees.

6.4.1.2 Components and Cladding.

For the design of components and cladding the building must meet all the following conditions:

1. The mean roof height h � 60 feet;2. The building is enclosed as defined in Section 6.2 and conforms to the wind-borne debris provisions of Section 6.5.9.3;3. The building is a regular shaped building or structure as defined in Section 6.2;4. The building does not have response characteristics making it subject to across-wind loading, vortex shedding, instability due to galloping or flutter; and does not have a site location for which channeling effects or buffeting in the wake of upwind obstructions warrant special consideration;5. The building is not subject to the topographic effects of Section 6.5.7 (i.e., Kzt = 1.0);6. the building has either a flat roof, or a gable roof with � � 45 degrees, or a hip roof with� � 27 degrees.

Method 1’s Basic Equation for the MWFRS cases is very simple,

Let us now consider each of the parameters.

where � is the Adjustment Factor for the building height and exposure, and is based on the building’s Exposure Category (ASCE 7-02, Figure 6-2, page 43).

30S sp Ipλ=

30S sp Ipλ=

6.5.6.3 Exposure Categories. This Example #2:

Exposure B: Exposure B shall apply where the ground surface roughness condition, as defined by Surface Roughness B, prevails in the upwind direction for a distance of at least 2,630 ft (800 m) or 10 times the height of the building, whichever is greater.

Exception: For buildings whose mean roof height is less than or equal to 30 feet (9.1 m), the upwind distance may be reduced to 1,500 ft (457 m).

Exposure C: Exposure C shall apply for all cases where exposures B or D do not apply.

Exposure D: Exposure D shall apply where the ground surface roughness, as defined by surface roughness D, prevails in the upwind direction for a distance at least 5,000 feet (1,524 m) or 10 times the building height, whichever is greater. Exposure D shall extend inland from the shoreline for a distance of 660 feet (200 m) or 10 times the height of the building, whichever is greater. For a site located in the transition zone between exposure categories, the category resulting in the largest wind forces shall be used.

Exception: An intermediate exposure between the above categories is permitted in a transition zone provided that it is determined by a rational analysis method defined in the recognized literature.

This Example #2’s Exposure

Our bldg’s height

This Adjustment Factor Table unites the data from both the height z and the exposure.

Continuing with the Basic Equation,

where I is the Importance Factor, and is based on the use of the structure as well as the Nature of Occupancy (ASCE 7-02, Table 6-1, page 73).

( ) 301.0S sp Ip=

This Example

V=120 mph

This ExampleI = 1.00

( ) ( ) 301.0 1.00S sp p=

Continuing with the Basic Formula,

where ps30 is the Simplified Design Pressure for Exposure B with a building height h = 30 feet (that is the reason why the notation ps30) and an importance factor I = 1.0 and is based on the Basic Wind Speed for the location of the structure (ASCE 7-02, Figure 6-2, pages 41 to 43).

Notice that our building is only 20 feet. Does that mean that our coefficients will be reduced because of the reduced height?

No. The Code writers assume that the value at h = 30 feet is the minimum, and will not be reduced for lower heights. However, above the 30 feet, the coefficients will be gradually increased.

In fact, the FBC 1609.1.2 states that for the MWFRS case, the design pressures can never be below 10 psf, regardless of a lower calculated value.

The Basic Wind Speed of ASCE 7-02 is found in Figure 6-1b, pages 36 to 40.

Within the State of Florida the wind speeds are obtained from the local county where the project is located through the county’s wind maps, through,

www.dca.state.fl.us/fbc/maps/2_maps.htm

Some counties allow interpolation between wind speed lines, whilst others do not.

To obtain a wind map of this specific example in downtown Tampa (Hillsborough County), use this address,

www.dca.state.fl.us/fbc/index_page/maps/county_maps/hillsborough2.pdf

Example #2 will use the same location in downtown Tampa as in Example #1.This Example’s site.Use V = 120 mph.

In order to determine the last parameter, the simplified design pressure ps30, the first case is the MWFRS, using ASCE 7-02, Figure 6-2, pages 41 to 43.

For the MWFRS case, notice that the table does not identify the For the MWFRS case, notice that the table does not identify the ten building zones ten building zones by numbers (1 by numbers (1 –– 10) like Method 2, but rather uses the ten letters A through H,10) like Method 2, but rather uses the ten letters A through H, plus plus EOH and GOH for the overhangs.EOH and GOH for the overhangs.

This Example #2: V = 120 mph and � = 0° Design pressures ps30

The design pressures at The design pressures at VV = 120 mph are shown below; the building diagram identifies = 120 mph are shown below; the building diagram identifies the ten different surfaces (zones).the ten different surfaces (zones).

-12.10H

-19.10G

-15.60F

-27.40E

-7.00D

15.10C

-11.90B

22.80A

psf

pressuresurface

DesignBuilding

Compare these design pressures with the ones found using Method 2 (Lecture #08):

NAGOH

NAEOH

-7.00D-11.90B

NA-13.404E-15.60F-15.503E-27.40E-27.402E22.80A17.301ENA-13.806NA-13.805NA-10.304

-12.10H-12.003-19.10G-19.10215.10C12.701

psfpsf

pressuresurfacepressurezone

DesignBuildingDesignBuilding

Method 1: SimplifiedMethod 2: Analytical

Consider now what happens to the pressures when the roof’s pitch is increased from � = 0° to a pitch of � = 20°, which is approximately a roof pitch of 5:12 (V/H).

The design pressures on a modified roof pitch of � = 20° or roughly 5:12.

-14.50-12.10H

-19.10-19.10G

-19.10-15.60F

-27.40-27.10E

-4.60-7.00D

21.1015.10C

-8.30-11.90B

31.6022.80A

� = 20°� = 0°surface

Design pressures (psf)Building

Notice the increased design pressures due to the increased roof pitch:

Method 1’s Basic Equation for the C&C case is,

Let us now consider each of the parameters.

where � is still the Adjustment Factor for the building height and exposure, but the C&C factor is different from the one we used for the MWFRS case. This new factor is found under ASCE 7-02, Figure 6-3, page 46.

30S netp Ipλ=

30S netp Ipλ=

For the C&C case, ASCE 7-02, Figure 6-3, page 46:

This Example #2� = 1.00

Continuing with the Basic Formula,

where I is the Importance Factor, found in ASCE 7-02, Table 6-1, page 73,

( ) 301.00S netIp p=

This Example #2I = 1.00

Finally, the remaining factor of the Basic Formula for the C&C case,

where pnet30 is the Simplified Design Pressure for Exposure B with a building height of h = 30 feet and an Importance Factor of I = 1.00, and is based on the Basic Wind Speed for the location of the structure. The value is found in ASCE 7-02, Figure 6-3, pages 44 to 46.

( )( ) 301.00 1.00S netp p=

Consider the design pressures for the two effective areas of 10 SF and 100 SF:

This Example #2

The C&C design pressures are given below. Notice that the end arThe C&C design pressures are given below. Notice that the end areas are still 5 feet eas are still 5 feet (versus 10 feet for the MWFRS cases).(versus 10 feet for the MWFRS cases).

-28.108.301003

-65.4010.50103

-28.108.301002

-43.5010.50102

-23.708.301001

-25.9010.50101

( - )(+)(square feet)

(psf)Areasurface

Design pressuresEffectiveBuilding

Complex Shaped Buildings.

Some buildings have complex shapes, due to strange site constraints, “creative”architecture, etc. How are these buildings handled?

For example, instead of a simple rectangular floor plan, a building could have an “L” shaped floor plan.

One procedure is simply to divide the “L” in two separate rectangular buildings, one for each leg. Then, each building is analyzed separately and the results are superimposed.

Similarly, a “U” shaped building would be divided into three rectangular smallerbuildings, etc.

An alternative procedure, is to enclose the complex shaped building by an imaginary larger “box” that encloses the whole smaller complex structure. The imaginary box is analyzed, and the highest positive and negative pressures are used to calculate the lateral forces on the structural skeleton frame.

References.References.

1. American Society of Civil Engineers, Publication ASCE 7-02, “Minimum Design Loads for Buildings and Other Structures”, Washington DC, 2002;

2. W. C. Bracken PE, “Wind Load Design”, Florida Engineering Society, Tallahassee, 2007;

3. K.C. Mehta, J.M Delahey, “Guide to the Use of the Wind Load Provisions of ASCE 7-02” ASCE Press, Washington DC, 2003.