HomeMy WebLinkAboutT-MOBILE WTF, 1108 W. VINE DR. - PDP - PDP110013 - REPORTS - CORRESPONDENCE-HEARINGs ((
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STRUCTURES CONSULTING, PC
Based on the above discussed design criteria and failure modes, it is our opinion the fall
radius for this structure is predicted to not exceed 20 ft.
If you have any questions regarding the above information, please contact me at (503) 587-0101.
Sincere Regards,
Adrian McJunkin, PE
President
PO Box 7293 • Salem, OR 973b3.503.587.0101 • jar 503.316.1864 L
uwiv.wsrengmem.ng.com �^ 1 C VON C
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STRUCTURES CONSULTING, PC
The loads generated by this wind speed, along with the weight of the tower sections and any ice
loading, if considered, are used to design the structural system. To fail the tower structure, the
wind must exceed all of the estimates for magnitude and duration, as well as overcome the factor
of safety determined from the design, and also the factor of safety inherent to the applicable
material design codes.
The design criteria used for this monopole are as follows: SNt-
Design Wind: 90 mph (3-second gust) per ANSI/TIA 222-G-2
90 mph (3-second gust) per 2009 IBC
Ice: 0" escalating ice @ 50 mph (3-second gust) wind for Larimer Co., CO
Failure Modes
The failure modes for monopole structures are dependent on many factors, but due to their
simplistic design and limited number of components, the "weakest link" is typically not difficult
to determine.
Single section monopoles would typically fail near the base. Considering a "catastrophic
failure", the fall radius would be nearly equal to the height of the monopole. With multiple
section monopoles, failure would tend to occur at a splice or near the base, and likewise, the
worst case fall radius would be equal to the height of the monopole. The multiple section
monopole could also produce fall radii equal to the distance from the top of the monopole to any
splice or point where the pole shaft strength changes abruptly.
Note that rarely does a "structural failure" of a monopole include any part of the structure
actually striking, or falling, to the ground. As is the case with most structural systems, the onset
of material deformation (i.e. bending, warping, twisting, etc.) relieves the localized overstress
condition by transferring load to adjacent, or nearby, structural elements. Unless the duration of
the "overstress" (or conditions causing it) becomes nearly continuous, a "catastrophic failure"
(i.e. the monopole actually falling over) would never occur.
The (preliminary) design of this structure indicates that the "weakest link" is the antenna mast
section (stressed in bending), with a stress ratio of 82%. It is highly unlikely that a "structural
failure" would include anything more than a visible bending, or tilting, of the upper twenty foot
section of the monopole. This tilting would be an obvious indication that further investigation
was required.
The predicted failure mode for this, or any structure, is applicable only to a specific final design.
If any single item of the design process is altered in any way, it would be necessary to investigate
and determine a new failure mode.
PO Box 7293 • Salem, OR 97303 • 503.587.0101 • fax 503.316.1864
www.wscengineering.com
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STRUCTURES CONSULTING, PC
August 9, 2011
T-Mobile
7901 E. Lowry Blvd.
Suite 800
Denver, CO 80230
Subject: Failure Mode for Proposed 70' — 0" Slim -Line Monopole
Site: DN03637F Animal House
Dear T-Mobile,
The following is our design criteria and failure modes for communications structures, along with
the predicted failure for the above proposed monopole:
Design Criteria
Communications tower structures designed by Wireless Structures Consulting, PC (WSC) are
typically sized in accordance with the latest revision of the ANSUTIA 222 Standard entitled
"Structural Standards for Steel Antenna Towers and Antenna Supporting Structures." The 2009
International Building Code (IBC), in section 3108, Radio and Television Towers, specifically
requires that "Towers shall be designed and constructed in accordance with TIA-222." This
standard has been approved by ANSI, who has generated the standard ANSI/ASCE 7, titled
"Minimum Design Loads for Buildings and Other Structures."
The ASCE 7 standard, which is the basis for design loading for practically every building code
and standard in the country, has dealt with the design of antenna support structures for over 40
years. The TIA-222 standard, based on provisions of this nationally known specification, has a
long history of reliability. At its core philosophy is it's first and foremost priority to safeguard
and maintain the health and welfare of the public.
The TIA-222 standard dictates minimum wind loading (the predominate loading on a tower
structure) for each county in the United States. It is WSC's policy to use the wind loading listed
in the latest TIA-222 standard as a minimum loading, unless the customer specifies a larger
value. Statistically, the wind speed listed in the TIA-222 standard has been determined to be that
wind which has a 50 year mean recurrence. This wind is also a "3-second gust" wind, which by
definition, is the highest sustained gust over a 3 second period, at an elevation of 33 feet above
ground level. This "3-second gust" wind is then modified with factors based on the structure
height, and terrain, or exposure, conditions at the actual project site. Note that these factors, in
all cases, increase the design load applied to the structure.
PO Box 7293 • Salem, OR 97303 • 503.587.0101 • fax 503.316.1864
www. wscengineering. cam