   |
|
|
 |
Cutting-Edge Design Yields Efficiency and
Accuracy |
| The design of an
overall structural system is only as reliable and accurate as the quality of
the information being input. This is especially true with complex, large
tensioned fabric structures. By the nature of their material properties, these
unique structures allow a virtually endless array of shapes and sizes to
produce numerous design options for the architect and/or owner. The effect of
this range of alternatives is that many of the unique structures that have been
built do not fall into any standard loading profile. This is especially true in
the case of wind load. Snow load being predictable does not pose such a
challenge for the engineer to predict the loading profile utilizing published
data and guidelines. Wind, however, has different loading characteristics for
complex shapes, different wind directions and different topographical
conditions. For this reason, Span Systems performs boundary layer wind tunnel
testing for highly complex membrane shapes that we design and build. The
process for one structure that Span Systems has designed is explored here in
general terms. The subject project is the new Arrival canopy at Dallas/Fort
Worth International Airport terminal D. This produces accurate wind pressure
patterns across all areas of the structures and for all wind directions and
environmental factors. Typically we perform the following: |
- Building the model:
This critical step
requires the correct choice of materials applied with the correct scale, along
with the incorporation of any locally influential buildings or topography.
Shown below is a typical rendering and model for the referenced
project.
The position of the fabric roof in its
surroundings |
Overall Model of the Structure with Membrane Roof
in Roadway |
- Applying the pressure taps and
instrumentation:
Choosing the location of the pressure reading devices
is important to ensuring that the results provide the most germane set of
output information. This also determines the profile of the output data for use
in loading structural models for computer analysis purposes.
Location of
pressure taps |
- Running the test:
Depending on the level
of symmetry of the model and its surrounding structures, wind directions are
chosen to insure that all wind possibilities are accounted for and critical
values are generated. Flow visualization experiments were performed by oil mist
and laser sheet. The airflow was illuminated by laser sheet parallel to the
ground below and over the roof and perpendicular to the ground and roof. The
flow field was recorded by digital video recorder. The wind directions and
positions of the illuminating laser sheet are included in the typical project
report. In order to realize a relatively high time resolution, the wind
velocity during the visualization was 2m/s at reference height 10 m.
Wind Directions |
|
- Compiling the data and generating
output:
The critical information for the purposes of analysis is the
pressure distribution generally described with a graphic and numerical
compilation of the pressure coefficients throughout the membrane in question
for various wind directions. Also of interest are loading delays that cause
possible harmonic behavior, as the load may not be instantaneous at all
locations. This is especially important in the case of numerous localized
obstructions in close proximity to the subject structure.
| PRESSURE
DISTRIBUTION AND COEFFICIENTS |
Northern fly,
1,2*cprms, 0 degree cp(us-)-(ls+) |
1,2*cprms, 225degree
cp(us-)-(ls+) |
|
|
|
