TRB2003-002234.pdf

Fouad H. Fouad and Elizabeth Calvert 1

Impact Of The New Wind Load Provisions On The Design Of Structural Supports

Submission date: November 15, 2002 Word count: 6725 words (2975+250*15) By

Fouad H. Fouad, Ph.D., P.E., Professor, Department of Civil and Environmental Engineering, The University of Alabama at Birmingham, 1075 13th Street South, Birmingham, AL 35294, ph. 205-934-8430, [email protected].

Elizabeth Calvert, P.E., Consulting Engineer, P.O. Box 1849, West Point, MS 39773, ph. 662-495-2448, [email protected].

Abstract: The AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals (Supports Specifications) have been revised in its entirety through a major research project conducted under the auspices of the National Cooperative Highway Research Program (NCHRP Project 17-10). The new document was approved in 1999 by all state DOTs for adoption by AASHTO and was recently published in 2001. A major part of the revisions included new provisions and criteria for wind loads. These provisions differ considerably from those in previous editions of the specifications. The impact of the new wind load provisions on the design of structural supports from the standpoint of safety and economy has not been studied and was the main goal of this work.

Differences in design wind loads as a result of using different wind speed maps and calculation methods were compared for selected sites in the United States in an effort to ascertain the effect of the new wind provisions on the design of structural supports. Wind load calculations and design examples for various types of structural supports were also performed using both the newly published 2001 AASHTO specifications and the 1994 edition of the specifications. The results are compared and the impact of the 2001 specifications on design of support structures is illustrated.

INTRODUCTION

An examination of the 1994 AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals (1) (hereafter referred to as the Supports Specifications) reveals that the wind loading criteria are based primarily on information and procedures that were first advanced in the early 1960’s and 1970’s. Through the work of NCHRP Project 17-10 (2), significant changes have been introduced in the 2001 Supports Specifications (3), which affect the presentation, terminology, and calculated wind loads. The major changes in the 2001 Supports Specifications are primarily due to an updated wind map. These changes may result in increase or decrease in the magnitude of calculated wind pressure depending on site location. The 2001 Supports Specifications were approved by all state DOTs in 1999 and were published in 2001. The changes in the wind loading criteria provided by the 2001 AASHTO specifications represent a major and fundamental update to the wind loading criteria of 1994 Supports Specifications. These changes, representing over 20 years of progress in the wind technology, update the Supports Specifications to the most current wind methodology. The effects of changing the wind loading criteria and wind map were also reviewed. Wind load calculations in the 2001 Supports Specifications have been revised to be based on a 3-second gust wind speed, rather than a fastest-mile wind speed. The number of maps, representing 10, 25, and 50-year mean recurrence intervals, have been reduced to one 50-year mean recurrence interval map with importance factors used to adjust the intervals. Height factors have also been adjusted for the 3-second gust wind speed. The drag coefficients have been slightly modified. The increase or reduction in calculated wind pressures, which result from the use of the 2001 Supports Specifications, is primarily due to the differences in the 1994 and 2001 wind speed maps. The 2001 Supports Specifications have been updated to reflect currently accepted design procedures to calculate wind loads. The wind loads portion of the 2001 specifications is based on the 1995 version of ASCE 7 (4), and modified specifically for structural supports for highway signs, luminaries, and traffic signals. ASCE 7 Minimum Design Loads for Buildings and Other Structures is considered the most authoritative standard on wind

TRB 2003 Annual Meeting CD-ROM Paper revised from original submittal.


loading in the United States, and most design codes and specifications have generally adopted or referenced ASCE 7.

OBJECTIVE

A major change that was noted in the newly published 2001 Supports Specifications is the use of a new wind map and wind provisions that may result in significant changes in the applied loads on structural supports. As a result, the structures sizes may increase or decrease based on their geographic location. Wind pressures and loads on structures have been studied in NCHRP Project 17-10(2) (5). To this date no studies have been published to investigate the impact of the new 2001 Supports Specifications on the design of structural supports. The main objective of this paper is to report on a study (6) that was performed to evaluate the safety and economy of structural supports for highway signs, luminaires, and traffic signals in the United States that are designed in accordance with the new wind load provisions adopted by AASHTO. Changes in the wind map were identified. These comparisons illustrate the differences between both specifications as related to the wind speeds, height coefficients, gust factors, and mean recurrence intervals (or importance factors). The impact of the new wind provision on design of structural supports was also investigated. Analysis and design were performed on three types of support structures located in selected cities across the United States. Comparisons of structure weights, ground line moments, and shear forces were made between the 1994 and 2001 specifications. Main member sizes and weights were provided to illustrate the magnitude of changes in safety margins and economy of structural supports designed in accordance with the new wind load provisions.

WIND SPEED MAPS

The 1994 Supports Specifications provide three wind maps for the 50, 25, and 10-year mean recurrence intervals. The 50-year mean recurrence interval is generally used for high mast lighting poles and overhead sign structures. For structure types such as street lighting poles and traffic signal poles, the 25-year mean recurrence interval map is normally used. The 10-year mean recurrence interval map is typically used for roadside signs. In the 2001 Supports Specifications, only one map is provided depicting the basic wind speeds for the United States, and importance factors are specified to vary the mean recurrence interval (MRI) depending on structure type. The new wind map as presented in the 2001 Supports Specifications can be divided into five wind speed regions: 1) 90 mph for most of the United States, 2) 100 mph to 150 mph in the hurricane region on the east coast, 3) 85 mph for the west coast, 4) 90 mph to 130 mph for Alaska, and 5) 105 mph for Hawaii. These regions correspond to fastest-mile wind speeds ranging from 50 to 110 mph depending on the site location and the mean recurrence interval as depicted by the three AASHTO 1994 Supports Specifications wind maps. Comparisons to illustrate the differences in wind loads computed according to the two specifications take into consideration the type of structure and wind speeds specified for selected locations in the United States.

DESIGN COMPARISONS FOR 1994 VS. 2001 AASHTO SUPPORTS SPECIFICATIONS

Three structure types were selected so that structure weights, ground line moments, and shear forces could be compared between the 1994 and 2001 specifications. The structure types were a high mast lighting pole, a street lighting pole, and a roadside sign structure, which are recommended to be designed for 50, 25, and 10-year mean recurrence intervals as provided in Section 3.8.3 and Table 3-3 of the 2001 specifications and in Section 1.2.4 of the 1994 specifications. Several cities were selected in order to illustrate the impact of the changes in the specifications on the design of structural supports.

High Mast Lighting Pole

A 160-foot high mast lighting pole was designed for a 50-year mean recurrence interval. The configuration is shown in Figure 1. The structure was designed based on an effective projected area of 50 square feet at the top of the pole and a light fixture dead weight of 1000 pounds. The pole is a hollow tapered steel pole with a hexdecagonal cross section. The yield stress for the steel is 65 ksi. The poles were designed for sites in Birmingham, AL, Atlanta, GA, Lander, WY, and Hartford, CT for wind speeds as shown in Table 1. For the 1994 specifications, group load combination II (dead load plus wind) was applied per Section 2 with an increase in allowable stresses of 1.4. Design considerations for the 1994 specifications include meeting the allowable stresses of Section 4 for a hexdecagonal steel shape. Second-order moments were calculated using the



alternative method of Section 1.3.3.A(2). Deflections were limited to 15%, as recommended in the commentary of Section 1.9.1(B). For the 2001 specifications, group load combination II (dead load plus wind) was applied per Section 3 with an increase in allowable stress of 1.33. Design consideration for the 2001 specifications included meeting the allowable stresses of Section 5 for a hexdecagonal steel shape. Second-order moments were calculated using the detailed method of Section 4.8.2. Deflections were limited to 15%, as required by Section 10.4.2.1. In general, limits on allowable stresses and deflection controlled the design for the 1994 and 2001 specifications. Pole sizes and reactions are provided in Table 2 for the 1994 specifications and in Table 3 for the 2001 specifications. Percent difference in weight, ground line moments, and shear forces are provided in Figure 2. The sites Birmingham, AL, Atlanta, GA, Lander, WY, and Hartford, CT show 8% increase, 7% decrease, 20% decrease, and 33% increase in pole weight, while showing 0% increase, 23% decrease, 41% decrease, and 59% increase in wind loads on the structures, respectively. The sites Birmingham, AL (1994: 70 mph, 2001: 90 mph) and Atlanta, GA (1994: 80 mph, 2001: 90 mph) have wind speeds that are commonly found in the interior region of the United States. The designs indicate that changes in the specifications that are unrelated to wind loads could also influence the design. For the high mast lighting pole, these changes include the following 1. changes in allowable stress equations for hexdecagonal steel sections. 2. changes in the method of calculating the second-order moments (i.e., factor in the detailed method was changed

from 1.38 to 1.45). 3. changes in the increase in allowable stresses under dead load plus wind from 1.4 to 1.33.

Street Lighting Pole Example

The configuration for street lighting pole example is shown in Figure 3. The structure was designed for a double 10-foot mast arm and luminaire. The weight is 75 pounds per mast arm and 50 pounds per luminaire. The effective projected area is 4.4 square feet for each mast arm and 1.4 square feet for each luminaire. The 40-foot pole is a hollow tapered steel pole with a round cross section. The yield stress for the steel is 65 ksi. The poles were designed for sites in Ft. Worth, TX, Dodge City, KS, Mobile, AL, and Wilmington, NC for wind speeds as shown in Table 4. For the 1994 specifications, group load combination II (dead load plus wind) was applied per Section 2 with an increase in allowable stresses of 1.4. Design considerations for the 1994 specifications include meeting the allowable stresses of Section 4 for a round steel shape. Second-order moments were calculated using the alternative method of Section 1.3.3.A(2). Deflections were limited to 15%, as recommended in the commentary of Section 1.9.1(B). Slope limit under dead load as provided in Section 9 is not applicable, since the balanced twin mast arm configuration eliminates the dead load moment at the tip of the pole. For the 2001 specifications, group load combination II (dead load plus wind) was applied per Section 3 with an increase in allowable stress of 1.33. Design consideration for the 2001 specifications included meeting the allowable stresses of Section 5 for a round steel shape. Second-order moments were calculated using the detailed method of Section 4.8.2. Deflections were limited to 15%, as required by Section 10.4.2.1. In general, limits on allowable bending stresses and deflection controlled the design for the 1994 and 2001 specifications. Pole sizes and reactions are provided in Table 5 for the 1994 specifications and in Table 6 for the 2001 specifications. Percent difference in weight, ground line moments, and shear forces are provided in Figure 4. The sites Ft. Worth, TX, Dodge City KS, Mobile, AL, and Wilmington, NC show 1% decrease, 10% decrease, 36% increase, and 6% decrease in pole weight, while showing 7% decrease, 27% decrease, 64% increase, and 11% decrease in wind loads on the structures, respectively. The sites Ft. Worth, TX (1994: 70 mph, 2001: 90 mph) and Dodge City KS (1994: 80 mph, 2001: 90 mph) have wind speeds that are commonly found in the interior region of the United States. The designs indicate that other changes in the specifications that are unrelated to wind loads are influencing the design. For the street lighting pole example, these changes include the following 1. changes in allowable stress equations for round steel sections. 2. changes in the method of calculating the second-order moments (i.e., factor in the detailed method was changed

from 1.38 to 1.45). 3. changes in the increase in allowable stresses for the dead load plus wind load case from 1.4 to 1.33.

Roadside Sign Example

The roadside sign structure shown in Figure 5 was designed for the 10-year mean recurrence interval. The sign had dimensions of 8 feet tall by 16 feet wide and was supported by two steel wide flange posts with yield stress of 36



ksi. The poles were designed for sites in St. Louis MO, Indianapolis, IN, Charleston, SC, and Orlando, FL for wind speeds as shown in Table 7. For the 1994 specifications, group load combination II (dead load plus wind) was applied per Section 2 with an increase in allowable stresses of 1.4. Design considerations for the 1994 specifications include meeting the allowable stresses of Section 4 for a wide-flange steel shape. For the 2001 specifications, group load combination II (dead load plus wind) was applied per Section 3 with an increase in allowable stress of 1.33. Design consideration for the 2001 specifications included meeting the allowable stresses of Section 5 for a wide flange steel shape. The limits on allowable bending stresses controlled the design for the 1994 and 2001 specifications. Post sizes and reactions are provided in Table 8 for the 1994 specifications and in Table 9 for the 2001 specifications. Percent difference in weight, ground line moments, and shear forces are provided in Figure 6. The sites St. Louis, MO, Indianapolis, IN, Charleston, SC, and Orlando, FL show 11% increase, 0% increase, 0% increase, and 67% increase in post weight, while showing 14% increase, 16% decrease, 13% decrease, and 82% increase in wind loads on the structures, respectively. The sites St. Louis, MO (1994: 60 mph, 2001: 90 mph) and Indianapolis, IN (1994: 70 mph, 2001: 90 mph) have wind speeds that are commonly found in the interior region of the United States. Designs were influenced by the fact that the greatest increase for all sites occurred for wind elevations that are less than 15 feet. A significant number of roadside sign structures are in this category. The design modification where the increase in allowable stresses under dead load plus wind changed from 1.4 to 1.33 also influenced the design.

CONCLUSION

In comparing the 1994 versus 2001 wind specifications, it is apparent that changes in wind pressure, either decreasing or increasing, are highly site-specific. Changes are also dependent on wind elevation and structure type (i.e., mean recurrence interval). An increase in wind load of up to 82% was shown for a selected coastal area, whereas a decrease of about 41% was shown for a selected location in an interior region of the United States. However, changes in wind loads for “typical” locations in the interior United States will range from an increase of 14% to a decrease of 27%. These changes in wind loads will obviously impact the design of the structure, including member weight and size, although albeit to a lesser degree. It should also be pointed out that several other changes in the 2001 specifications, which are not directly related to the wind map, may influence the design. These changes are related to the allowable stress equations for steel, the increase in allowable stress for Group II loading, and the calculation of second-order effects. Changes in wind loads applied on the structures varied considerably with site location, along with a slightly less corresponding change in structure weight.

ACKNOWLEDGEMENT

This project was sponsored by The University Transportation Center for Alabama, which is supported by the U.S. Department of Transportation and the Alabama Department of Transportation. Matching funds for the project were provided by the Department of Civil and Environmental Engineering at The University of Alabama at Birmingham.

REFERENCES

1. AASHTO, Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals. Third Edition, American Association of State Highway and Transportation Officials, Washington, D.C. (1994) 78 pp.

2. Fouad, Fouad H.; Calvert, Elizabeth A.; and Nunez, Edgar, “Structural Supports for Highway Signs, Luminaires, and Traffic Signals.” NCHRP Report 411, Transportation Research Board, Washington, D.C. (1998) 114 pp.

3. AASHTO, Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals. Fourth Edition, American Association of State Highway and Transportation Officials, Washington, D.C. (2001) 270 pp.

4. ASCE, Minimum Design Loads for Buildings and Other Structures. ASCE 7-95, American Society of Civil Engineers, Reston, VA (1995) 214 pp.

5. Fouad, Fouad H.; Mehta, Kishor C.; and Calvert, Elizabeth A., Wind Loads Report: Final Draft. NCHRP Project 17-10(2), Prepared for National Cooperative Highway Research Program, Transportation Research



Board, National Research Council, The University of Alabama at Birmingham, Birmingham, AL (Sept. 1999) 168 pp.

6. Fouad, Fouad H.; and Calvert, Elizabeth A., Evaluating the Design Safety of Highway Structural Supports, UTCA Report Number 00218, University Transportation Center for Alabama, Tuscaloosa, AL (July 2001) 65 pp., http://utca.eng.ua.edu/projects/final_reports/00218report.htm, Accessed June 15, 2002.



LIST OF TABLES

TABLE 1 Sites and Wind Speeds for the High Mast Lighting Pole Example TABLE 2 Pole Sizes Using AASHTO 1994 for the High Mast Lighting Pole Example TABLE 3 Pole Sizes Using AASHTO 2001 for the High Mast Lighting Pole Example TABLE 4 Sites and Wind Speeds for the Street Lighting Pole Example TABLE 5 Pole Sizes Using AASHTO 1994 for the Street Lighting Pole Example TABLE 6 Pole Sizes Using AASHTO 2001 for the Street Lighting Pole Example TABLE 7 Sites and Wind Speeds for the Roadside Sign Example TABLE 8 Post Sizes Using AASHTO 1994 for the Roadside Sign Example TABLE 9 Post Sizes Using AASHTO 2001 for the Roadside Sign Example



TABLE 1 Sites and Wind Speeds for the High Mast Lighting Pole Example

AASHTO 1994 AASHTO 2001

Wind Imp. Site Speed Wind Factor

50-year Speed 50-year (mph) (mph)

Birmingham, AL 70 90 1.00 Atlanta, GA 80 90 1.00 Lander, WY 90 90 1.00 Hartford, CT 70 110 1.00



TABLE 2 Pole Sizes Using AASHTO 1994 for the High Mast Lighting Pole Example

AASHTO 1994

Wall Fy = 65 ksi G.L. G.L. Site Tip Base Thickness Shaft Moment Moment G.L. Defl.

Diameter Diameter GL Weight (I) (II) Shear (% of (in) (in) (in) (lb) (lb-ft) (lb-ft) (lb) Height)

Birmingham, AL 7.40 29.80 0.2500 7,955 595,102 681,739 6,393 14% Atlanta, GA 7.60 30.00 0.3125 9,233 774,444 871,157 8,362 15% Lander, WY 8.30 30.70 0.3750 10,799 1,002,455 1,107,184 10,874 15% Hartford, CT 7.40 29.80 0.2500 7,955 595,102 681,739 6,393 14%



TABLE 3 Pole Sizes Using AASHTO 2001 for the High Mast Lighting Pole Example

AASHTO 2001

Wall Fy = 65 ksi G.L. G.L. Site Tip Base Thickness Shaft Moment Moment G.L. Defl.

Diameter Diameter GL Weight (I) (II) Shear (% of (in) (in) (in) (lb) (lb-ft) (lb-ft) (lb) Height)

Birmingham, AL 6.30 28.70 0.3125 8,599 596,375 689,466 6,417 15.0% Atlanta, GA 6.30 28.70 0.3125 8,599 596,375 689,466 6,417 15.0% Lander, WY 6.30 28.70 0.3125 8,599 596,375 689,466 6,417 15.0% Hartford, CT 7.90 30.30 0.3750 10,583 932,641 1,040,237 10,172 14.9%



TABLE 4 Sites and Wind Speeds for the Street Lighting Pole Example




Ft. Worth, TX 70 90 0.87 Dodge City, KS 80 90 0.87

Mobile, AL 70 130 0.8 Wilmington, NC 100 130 0.8



TABLE 5 Pole Sizes Using AASHTO 1994 for the Street Lighting Pole Example

AASHTO 1994

Wall Fy = 65 ksi G.L. G.L. Site Tip G.L. Thickness Shaft Moment Moment G.L.

Diameter Diameter GL Weight (I) (II) Shear (in) (in) (in) (lb) (lb-ft) (lb-ft) (lb)

Ft. Worth, TX 2.1 7.7 0.1196 242 16,933 19,431 584 Dodge City, KS 2.5 8.1 0.1196 265 22,439 24,944 749

Mobile, AL 2.1 7.7 0.1196 242 16,933 19,431 584 Wilmington, NC 4.2 9.79 0.1196 351 33,688 35,491 1,074



TABLE 6 Pole Sizes Using AASHTO 2001 for the Street Lighting Pole Example

AASHTO 2001

Wall Fy = 65 ksi G.L. G.L. Site Tip G.L. Thickness Shaft Moment Moment G.L.

Diameter Diameter GL Weight (I) (II) Shear (in) (in) (in) (lb) (lb-ft) (lb-ft) (lb)

Ft. Worth, TX 2.0 7.6 0.1196 239 15,523 18,050 545 Dodge City, KS 2.0 7.6 0.1196 239 15,523 18,050 545

Mobile, AL 3.8 9.38 0.1196 330 29,855 31,821 961 Wilmington, NC 3.8 9.38 0.1196 330 29,855 31,821 961



TABLE 7 Sites and Wind Speeds for the Roadside Sign Example




St. Louis, MO 60 90 0.71 Indianapolis, IN 70 90 0.71 Charleston, SC 80 120 0.54

Orlando, FL 60 130 0.54



TABLE 8 Post Sizes Using AASHTO 1994 for the Roadside Sign Example

AASHTO 1994

Support G.L. G.L. Site Wide Flange Weight Moment Shear

Size for 2 ea. Posts for 2 ea. Posts for 2 ea. Posts (lb) (lb-ft) (lb)

St. Louis, MO W6x9 320 26,757 2,034 Indianapolis, IN W8x10 355 36,420 2,768 Charleston, SC W8x13 462 47,587 3,619

Orlando, FL W6x9 320 26,757 2,034



TABLE 9 Post Sizes Using AASHTO 2001 for the Roadside Sign Example

AASHTO 2001

Support G.L. G.L. Site Wide Flange Weight Moment Shear

Size for 2 ea. Posts for 2 ea. Posts for 2 ea. Posts (lb) (lb-ft) (lb)

St. Louis, MO W8x10 355 30,596 2,325 Indianapolis, IN W8x10 355 30,596 2,325 Charleston, SC W8x13 462 41,385 3,147

Orlando, FL W8x15 533 48,574 3,695



LIST OF FIGURES

FIGURE 1 High Mast Lighting Pole Example. FIGURE 2 Change in Weight, Moment, and Shear for the High Mast Lighting Pole Example. FIGURE 3 Street Lighting Pole Example. FIGURE 4 Change in Weight, Moment, and Shear for the Street Lighting Pole Example. FIGURE 5 Roadside Sign Example. FIGURE 6 Change in Weight, Moment, and Shear for the Roadside Sign Example.



160'-0"

Luminaire EPA: 50 ft^2 Weight: 1000 lbs

FIGURE 1 High Mast Lighting Pole Example.



8%

-7%

-20%

33%

0%

-23%

-41%

57%

1%

-21%

-38%

53%

0%

-23%

-41%

59%

-60%

-40%

-20%

0%

20%

40%

60%

80%

100%

Weight GL Moment (I) GL Moment (II) GL Shear

Birmingham, AL Hartford, CT

Lander, WYAtlanta, GA

FIGURE 2 Change in Weight, Moment, and Shear for the High Mast Lighting Pole Example.



10'-0"

Luminaire EPA: 1.4 ft^2 Weight: 50 lbsMast Arm EPA: 4.4 ft^2 Weight: 75 lbs

40'-0"

FIGURE 3 Street Lighting Pole Example.



-1%-10%

36%

-6%-8%

-31%

76%

-11%-7%

-28%

64%

-10%-7%

-27%

64%

-11%

-60%

-40%

-20%

0%

20%

40%

60%

80%

100%

Weight GL Moment (I) GL Moment (II) GL Shear

Ft. Worth, TX Dodge City, KS

Mobile, AL

Wilmington, NC

FIGURE 4 Change in Weight, Moment, and Shear for the Street Lighting Pole Example.



16'-0"

8'-0"

9'-9"

17'-9"

FIGURE 5 Roadside Sign Example.



11%

0% 0%

67%

-16% -13%

82%

14%

-16% -13%

82%

14%

-60%

-40%

-20%

0%

20%

40%

60%

80%

100%

Weight GL Moment GL Shear

Orlando, FL

Charleston, SCIndianapolis, IN

St. Louis, MO

FIGURE 6 Change in Weight, Moment, and Shear for the Roadside Sign Example.


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