Report on
Wind Resistance of Signs supported by
Glass Fiber Reinforced Concrete (GFRC) Pillars
Prepared for
US Sign and Fabrication Corporation
January, 2006
Wind Capacity of GFRC Pillars
January, 2006
US Sign and Fabrication Corp.
_______________________________________________________________________________________________
SUMMARY
This study found the attachment of the signs to the GFRC pillars and the anchorage of the
pillars to the concrete footings to be adequate for gust wind speeds of 150 mph. This
value is the highest listed speed for all 50 states and territories in the US, with the
exception of Guam. However, locations within special wind zones which are identified
on the wind speed maps can be subjected to higher wind speeds; these locations must be
investigated with local knowledge.
The critical element in the stability of the signs is the concrete footing. Analysis of the
footings for failure in the soil found the following:
•
The 2.0 feet deep footing should be used only in areas with peak 3-second wind
gust speeds less than 100 mph, as shown in the map given in either AASHTO or
ANSI 7-02.
•
The 3.0 feet deep footing can be used in most locations with gust wind speeds
listed as being less than 135 mph on the map.
•
The 3.5 feet deep footing can be used anywhere, and with care in special wind
zones.
•
The 4.0 feet deep footing can be used anywhere.
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Wind Capacity of GFRC Pillars
January, 2006
US Sign and Fabrication Corp.
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INTRODUCTION
US Sign and Fabrication Corporation (USSFC) manufactures monument sign
columns that are used to mount billboard type signs. The USSFC columns are replicas of
custom-designed, hand-made pillars that would normally be constructed of stone or
masonry. They are installed in locations where an upscale and elegant touch is required.
The USSFC pillars are constructed of Glass Fiber Reinforced Concrete (GFRC) that is
cast over a galvanized steel frame. The bottom of the frame is attached to anchor bolts
projecting from the top of a concrete footing that is buried in the earth, so that the pillars
appear to be standing on the ground.
The concrete footing is unreinforced; it measures 12 inches in diameter and 2 feet
deep. Four 8-inch long “J” bolts are embedded 7 inches deep in the upper part of the
footing. The remaining 1 inch projects above the top of the footing, and are used to
attach the frame inside the pillar to the footing. The bolts are 3/8 inch diameter threaded
rods, and have a 90 degree bend at their bottom end so that they look like the letter J in
profile.
This study examines the capacity of the pillars-sign combination to safely resist
loads applied to them by high winds. The signs are primarily rectangular panels made of
either wood or sheet metal, and can be as large as 8 feet wide and 5 feet tall. They are
fastened to the pillars at four points, two each on a vertical edge. A ¼ inch diameter
stainless steel stud (threaded rod) connects the mounting hardware to the pillar. A 2 inch
wide and 1/8 inch thick galvanized steel plate is provided inside the pillar wall to
distribute the applied loads and to prevent localized failures in the pillar walls.
This study assumes that the sign panel is properly designed for applicable winds by
the sign maker. The integrity of the panel is of interest only insofar that the wind load
applied to its surface is transferred to the two supporting pillars.
Section 1: DESIGN WIND PRESSURES AND FORCES
The study examines the stresses experienced by the various parts of the pillars and
the anchorage when loaded by high winds, under real world conditions. The last phrase
refers to the peak wind conditions that may be obtained anywhere in the United States.
The values for the maximum expected wind speeds are given in several references
that are used to design all structures, including buildings, highway signs and luminaires.
The two most common references are “Standard Specifications for Structural Supports
for Highway Signs, Luminaires and Traffic Signals (4th edition, 2001)” published by the
American Association of State Highway and Transportation Officials (AASHTO), and
the “Minimum Design Loads for Buildings and Other Structures, ASCE 7-02” published
by the American Society of Civil Engineers (ASCE). Of the two, the AASHTO
publication is considered more applicable to this study and is therefore used exclusively
as the reference.
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Wind Capacity of GFRC Pillars
January, 2006
US Sign and Fabrication Corp.
_______________________________________________________________________________________________
The basic wind speeds are based upon the peak 3-second gust speed measured at
485 weather stations across the US and predictions of hurricane speeds on the Gulf and
Atlantic coasts. The 3-second gust wind speeds are associated with a 50-year mean
recurrence interval which is equivalent to an annual probability of 0.02 (2%) that the
listed values will be equaled or exceeded. The wind speeds are given as contours on a
map of the US in both references (AASHTO Fig. 3-2). The following table shows the
basic wind speeds for selected locations.
Location
Cape Cod, Massachusetts
New York City & Connecticut coastline
Outer Banks, North Carolina
Florida Keys, Miami
Alabama & Louisiana Coastline
Texas Coastline
Pacific Coastline
Basic Wind Speed (miles per hour)
130
120
140
150
150
140
85
The only location with a higher wind speed is Guam with a value of 170 mph,
however such speed is not obtained in the continental US anywhere. The Basic Wind
Speed for the analysis of the anchorage of GFRC pillars is therefore selected as 150 mph.
The pressure applied by the wind on any structure is obtained using the following
equation:
P = 0.00256KzGV2IrCd (AASHTO Eq. 3-1)
The various terms in the equation above are explained below.
Term
Value
Explanation
P
To be
computed
Kz
0.87
Height and Exposure factor (Art. 3.8.4, Table 3-5). Value applies to
structures less than 16.4 feet high.
G
1.14
Gust effect factor to account for dynamic interaction of the structure
with gusting (varying speeds with time) winds.
V
150 mph
Ir
0.54
This is the wind pressure to be used for design of structures.
Basic Wind Speed
Wind Importance Factor (Art. 3.6.3, Table 3-3). This recognizes the
(importance) need for a structure to withstand high winds – a hospital
is given higher importance than a roadside sign for example. There
are two values given here, and value leading to the higher overall
wind pressure is to be used. For roadside sign structures, which is
what the GFRC pillars support, the value is a function of the
Recommended Minimum Design Life of 10 years (Table 3-3). The
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Wind Capacity of GFRC Pillars
January, 2006
US Sign and Fabrication Corp.
_______________________________________________________________________________________________
two values of Ir are (1) 0.71 to be used with 100-mph winds, and (2)
0.54 to be used with the design wind speed. The lower value actually
leads to a higher wind pressure.
Cd
1.17 (sign)
Drag coefficient (Art. 3.8.6, Table 3-6). The value is obtained by
interpolation for a sign panel with aspect ratio of 1.6 (for the largest
sign panel that is 8 feet wide and 5 feet high).
Cd
2.0 (pillars)
Drag coefficient (Art. 3.8.6, Table 3-6). The value applies to square
shaped members (as seen in plan), that have sharp corners as is the
case with the GFRC pillars.
The wind pressure for the panel and the pillar is computed using the equation given
above, as follows:
Pressure on the sign panel
P = 0.00256 (0.87) (1.14) (0.54x1502) (1.17) = 36 pounds per square foot (psf)
Pressure on the pillar
P = 0.00256 (0.87) (1.14) (0.54x1502) (1.20) = 62 pounds per square foot (psf)
The total pressure on the assembly is computed as the sum of the pressures on one
sign panel and two pillars. The wind is taken as flowing directly at the face of the sign
(direction 1) for maximum wind area. To account for winds incident at an angle to the
panel, 20% of the force generated by wind perpendicular to the panel is applied in a
direction parallel to the plane of the sign panel (direction 2).
Wind force on the sign panel
Ws = 8 x 5 x 36 = 1440 lbs
Wind force on the pillars (average width is taken as 21 inches over the full height of
6 feet)
Wp = 2 x 21x 6 x 62/12 = 1302 lbs
The total wind force is therefore,
W = Ws + Wp = 1440 + 1302 = 2742 lbs
The wind load on each pillar is 1440/2 + 1302/2 = 720 + 651 = 1371 lbs
The wind forces computed above are applied 3.0 feet above the base for the pillars
and 3.5 feet above the base for the sign panel. The forces experienced at the base are as
follows:
Base Shear = 1371 lbs in direction 1
Bending moment = 720 x 3.5 + 651 x 3.0 = 4473 ft-lbs in direction 1
Base Shear = 0.2 x 1371 = 274 lbs in direction 2 in direction 2
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Wind Capacity of GFRC Pillars
January, 2006
US Sign and Fabrication Corp.
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Bending moment = 0.2 x 4473 = 895 ft-lbs in direction 2
In the next section, the components of base anchorage are checked for the forces
computed above.
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Wind Capacity of GFRC Pillars
January, 2006
US Sign and Fabrication Corp.
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Section 2: STRUCTURAL ANALYSIS OF GFRC PILLAR FOUNDATION
This study examines the adequacy of the following components of the assembly:
1. 0.25 inch diameter stainless steel studs that connect the panel to the pillar.
2. 3/8 inch diameter J-shaped anchor bolts in the footing.
3. 12 inch diameter concrete footing in the ground.
1) 0.25 inch diameter stainless steel studs
The tensile (T) and shear (V) forces on each bolt, located at each corner of the
panel, are computed as:
T = 1440/4 = 360 lbs
V = 0.2 x 360 = 72 lbs
The corresponding stresses (the cross-sectional area of a ¼ inch bolt is 0.05 sq.
inch) are:
σt = 360 / 0.05 = 7,200 pounds per square inch
σv = 72 / 0.05 = 1,440 pounds per square inch
Both values are well within allowable limits, compared to the yield strength of
stainless steel, which is at least 35,000 pounds per square inch.
2) 3/8 inch diameter J-shaped anchor bolts in the footing
There are two frames that can be used for the GFRC pillars, a large frame
measuring 15” by 17.5” and a small frame measuring 11.25” by 6.5”. However, both
provide a similar level of resistance to overturning. This is due to the fact that the
arrangement of the anchor bolts is very similar in both frames, and it is these four bolts
that provide the necessary resistance. The bolts are laid out on a 6” x 7.5” rectangle on
the large frame, and on a 5.5” x 6.5” rectangle on the small frame. Since it is not always
possible to predict how the frame inside a pillar will be aligned, as a conservative
measure the worst position is assumed to exist for this study.
The tensile (T) and shear (V) forces on one bolt, located at the critical corner of the
panel, are computed as:
T = 4473/(2 x 5.5) + 895/(2 x 6.5) = 475 lbs
V = (1371 + 274)/4 = 411 lbs
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Wind Capacity of GFRC Pillars
January, 2006
US Sign and Fabrication Corp.
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The corresponding stresses (the cross-sectional area of a 3/8 inch bolt is 0.11 sq.
inch) are:
σt = 475 / 0.11 = 4,318 pounds per square inch
σv = 411 / 0.11 = 3,736 pounds per square inch
Both values are well within allowable limits, compared to the yield strength of
ASTM A615 (used for rebars) steel which is at least 60,000 pounds per square inch, or
ASTM A307 (used for threaded rods) steel which has a yield strength of least 33,000
pounds per square inch. The J-shape ends ensure that the bolts will not be pulled out of
the concrete. Note that both the forces and stresses will be slightly lower for the larger
frame.
3) 12 inch diameter concrete footing in the ground
The concrete footing can fail in one of the following manners.
a) Pulling out of the J-bolts from the top of the footing: This is not likely to
happen due to two reasons – one, usually only two bolts have any significant
tension, and two, the J-bolts have more than adequate strength to be pulled
out under the forces computed above.
b) Failure of the concrete footing at due to shearing or fracture: The shearing
resistance of the concrete footing is computed as:
Vc = 2(√f’c)(A) where f’c = 3,000 psi for commonly used concrete
A = cross-sectional area of footing = 113 sq.in
Vc = 2(54.8)113 = 12, 348 lbs
This is more than adequate to resist the applied force of 1371 lbs, hence this
mechanism is unlikely to happen. Similarly, fracture of the footing is
unlikely due to the small magnitude of the force compared to the strength of
the 12 inch diameter concrete section.
c) Sliding of the footing in the horizontal direction due to wind flowing towards
the face of the sign or its front face: This requires the shearing failure of a
wedge of soil in the back of the footing, and is not likely to happen due to the
large forces (soil generates considerable passive pressures) required to push
the soil. Incidentally, the force applied to the sign is the same regardless of
which direction the wind is flowing in, towards the face of the sign or
towards its back.
d) Overturning of the footing about a point located at the top of the footing on
its back side: This requires the slip failure, along a curved surface, of a soil
wedge in front of the footing. The slip failure is resisted by the shearing
strength of the soil along the surface of the wedge, and by the weight of the
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Wind Capacity of GFRC Pillars
January, 2006
US Sign and Fabrication Corp.
_______________________________________________________________________________________________
wedge itself which must be lifted up in order for the failure to occur. This
mechanism is investigated in detail below.
The following assumptions are made regarding the nature and engineering
characteristics of the soil that the footing is built in:
1. Unit weight of saturated soil = 120 lbs per cubic foot
2. Angle of shear in soil = 30 degrees
3. Cohesion of soil = 0. This is the value commonly used for sandy soils.
This is based upon the supposition that the sign assemblies are likely to be
installed in improved locations, where the native organic soil usually
comprising a mix of sand and clay has been replaced with sandy backfill
during grading and leveling operations. This is a conservative assumption,
since the presence of clayey material in the soil will lead to an increase in
the shearing resistance obtained in the soil.
4. Multiplier for obtaining total surface area = 3.0. This accounts for the
increase in resistance due to the mobilization of soil adjacent to the wedge
being considered for analysis.
The overturning moment at the pivot point (top of footing) is the vector
sum of 100% moment in the main direction and 20% moment in the
perpendicular direction, as given below:
Mapplied = √(44732 + 8952) = 4562 ft-lbs
In order to simplify calculations, a plane sided wedge was used in the
computations instead of a curved surface; this represents a slight measure of
over-conservatism that is introduced in the analysis.
The restoring moment, or MR, is generated by three different forces:
• Weight of the concrete footing acting 6” away from the point
• Weight of the soil wedge acting at the centroid of the triangle
• Shearing resistance along the bottom face of the soil wedge
The factor of safety (FS) against overturning was selected as 1.25.
Normally, a minimum factor of safety of 2.0 is required for all structures such
as footings and retaining walls used in buildings and highways. Since the sign
structures in this study do not qualify as critical or essential structures, it was
judged prudent to lower the factor. In addition, all design codes allow a 1/3rd
increase in allowable stresses under wind loads, hence the actual factor of
safety is (1.25)(1.33) = 1.66, as compared to the normal factor of 2.0
Analyses were conducted for three different depths of the concrete
footings by computing the MR provided by each for the design wind speed of
150 mph. The 2’-0” and 3’-0” deep footing did not provide a FS greater than
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Wind Capacity of GFRC Pillars
January, 2006
US Sign and Fabrication Corp.
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1.25, hence the allowable wind speeds were reduced to acceptable values by
back-calculation. The results are as follows:
Depth of Concrete
Footing
2’-0”
FS against
overturning
0.53
3’-0”
1.02
3’-6”
4’-0”
1.53
2.22
Remarks
Not acceptable for 150 mph wind speeds.
Can be used safely in areas with
maximum wind speeds of 98 mph.
Not acceptable for 150 mph wind speeds.
Can be used safely in areas with
maximum wind speeds of 135 mph.
Acceptable in all areas
Acceptable in all areas
SUMMARY
It will be useful and necessary to refer to the map showing contours for wind
speed given in either of the references listed earlier, in order to determine the applicable
wind speed for any location in the US. Attention must be paid if the signs are to be
located within the areas designated as Special Wind Zones which are identified on the
map; these zones can experience very high peak gust speeds, an example being Mt.
Washington in New Hampshire. Local knowledge of the wind speed history is required
in order to ascertain applicable speeds.
•
The 2.0 feet deep footing should not be used in areas with peak 3-second wind
gust speeds greater than 100 mph. This excludes all coastal areas for the Gulf and
the Atlantic states in the US. They can be used in most inland locations, with the
exception of special wind zones.
•
The 3.0 feet deep footing can be used in most locations with gust wind speeds
listed as being less than 135 mph on the map. Care must be taken if the location
is in the special wind zones which are identified on the map.
•
The 3.5 feet deep footing can be used everywhere, and with care in special wind
zones.
•
The 4.0 feet deep footing can be used anywhere.
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