Satisfying occupant comfort criteria is a common challenge in the design of slender high-rise buildings. When wind-induced
accelerations exceed the acceptable limits for human comfort, structural engineers typically try to increase building mass or
stiffen the building. Nevertheless, this approach may not be a cost effective or feasible option, especially when the
structural system has already been determined and the option of introducing additional structural components is limited.
When a project is initiated, structural engineers estimate wind loads in compliance with building codes and standards and
adjust them, if necessary, based on their past experience and knowledge of the area. In New York City, it is not uncommon to
find wind loads for a nearby building of similar scale. However, when it comes to wind loads, often the information from
neighboring buildings is not applicable to the building under design due to the complex nature of wind-structure interaction.
In addition, even though design standards are available to structural engineers to compute wind loads, these standards are
not sufficient to predict all possible issues discovered after wind tunnel testing is completed.
Two slender towers, Building A, (the real name of Building A is not identified, per the request of the building owner), and W
New York Downtown Hotel and Residence (referred to as the W Downtown Hotel in this article), recently built in New York City,
exemplify the aforementioned cases.
Structural System
Building A is a 60-story, 650-foot tall residential building. The building footprint at the typical floors is 150 feet by 65
feet, and the building slenderness ratio is 10:1. A dual system, combining moment frames using a typical 10-inch thick flat
plate slab with columns and shear walls, is used as a lateral load resisting system to resist wind and seismic loads. In
order to maximize the efficiency of each structural component, shear walls extend the full 65-foot width in the north-south
direction at the base of the building and a thicker 12-inch thick flat plate slab is used at the 35th floor and above, where
their frame action is more effective. This enabled engineers to increase the stiffness and mass of the building to a certain
extent without elongating the building periods, which generally increase the resonance portion of wind induced responses.
The W Downtown Hotel is a 57-story, 627-foot tall mixed-use building (Figures 1 and 2) with a slenderness ratio of 11:1,
located one block south from the World Trade Center site. The footprint of the typical floors is 124 feet by 57 feet. The top
24 floors are high-end condominium units and the bottom 30 floors are occupied by amenities, mostly hotel rooms and furnished
residences. Two mechanical floors are strategically used for lodging full height belt walls (full height reinforced concrete
spandrel beams connecting exterior columns) at the 31st floor and 100-inch deep belt beams at the 57th floor. These belt
walls and beams supplement the framed tube action initially provided by the exterior columns and flat plates but reduced by
transfers at the 6th floor.
Wind Responses Obtained from Wind Tunnel Testing
Wind responses under two configurations of the surroundings were studied to predict the most critical wind loads for
designing the structure and evaluating the serviceability performance. In both cases, the future neighboring buildings
provided a shielding effect, and benefits from the future adjacent buildings turned out to be more significant in the east-
west direction. Despite these benefits, both towers were designed for wind loads under the present configuration, since it
resulted in the most critical wind responses.
For the initial study, stiffness of the structures under two different return periods (50 year and 10 year) were estimated
based on the modified code specified wind loads (knowing that code specified wind loads do not include cross wind responses
and torsional responses). After the initially estimated wind loads were provided by a wind tunnel lab, a more precise
analysis was preformed to estimate cracked sections. Then the corresponding structural dynamic properties under these
estimated wind loads were sent back to the wind tunnel testing lab. The final wind loads for the design of structural members
and evaluation of serviceability performance were established after several iterations, in order to reach compatible results
between the wind responses and the stiffness of the structures corresponding to estimated cracked conditions.
The W Downtown Hotel was initially designed as a 55-story building, and the peak acceleration of this structure was estimated
to be 17.4 mg (milli-g: 1/1000th of gravity acceleration) at the 53rd floor. By the end of the design development phase, the
owner decided to add two more floors to the building. Assuming a damping ratio of 2% of the critical damping for the selected
structural system of the tower, which are comprised of flat plates and shear walls coupled with shallow link beams at every
level, the peak acceleration of the taller building excluding influence of hurricanes was estimated to be 19.4 mg at the top
occupied residential floor (55th floor). For Building A, the peak acceleration of the 650-foot tall structure was estimated
to be 22.4 mg at the topmost occupied residential floor.
Acceptable Limit in Accelerations
For both Building A and the W Downtown Hotel, the wind studies indicated that accelerations were excessive at the floors
where long-term occupants will reside. The commonly acceptable range for 10 year peak accelerations is 15 mg to 18 mg for
residential towers and 18 mg to 20 mg for office towers. This acceptable range can be varied depending on the natural
frequencies of the structure, as occupant’s sensitivity to motion decreases when the natural frequencies of buildings are
lower. Therefore, buildings with longer periods can generally allow larger accelerations in terms of perception to motion.
As the initial studies indicated that the accelerations of both buildings were excessive, the structural modifications, such
as increasing the stiffness and increasing the general mass, were investigated. According to the engineer’s study in
cooperation with the wind tunnel testing lab, adding massive shear walls at the base of the building for Building A and
improving frame tube action by enlarging exterior columns and reducing exterior spans for the W Downtown Hotel would have
produced the targeted accelerations. However, these modifications would have required architectural compromises and reduction
of valuable space. As an alternative option, introducing supplementary damping systems to improve the performance of both
structures under the 10 year return period wind loads was explored.
Supplementary Damping Systems
Various types of supplementary damping systems (SDS) were considered: a tuned mass damper (TMD), a deep tuned sloshing damper
(DTSD), a shallow tuned sloshing damper (STSD) and a tuned liquid column damper (TLCD). For both towers, a TLCD was not a
feasible option because of space limitations. A tuned mass damper was also excluded due to the higher cost and maintenance
requirement in comparison with a deep tuned sloshing damper (DTSD) which was eventually selected. Two levels of target
performance were investigated during an initial study for the W Downtown Hotel. Level 1 performance aimed for 2.0 mg of
reduction in the peak acceleration and Level 2 performance aimed for 3.0 mg to 4.0 mg of reduction. In the end, Level 2
performance was achieved.
Tuned Sloshing Damper (TSD)
A tuned sloshing damper utilizes liquid waves to absorb energy from vibrating structures through wave travel and viscous
action in a partially filled tank of liquid. The tank is designed so that the liquid surface wave has a frequency "tuned" to
be near the fundamental frequency of the building for the optimal performance of a tuned sloshing damper. The frequency of
the liquid is determined by the density, length, width and depth of the liquid.
During the initial wind study, it was found that the majority of the excessive peak acceleration of Building A was in the
north-south direction. The W Downtown Hotel was somewhat different from Building A. The acceleration in the north-south
direction was also the primary contributor to the large acceleration but, due to mainly across wind responses, the
acceleration in the east-west direction was not negligible. Therefore an effort was made to reduce accelerations in both
directions. Considering the contribution of accelerations in each direction, a one-directional-tuned-deep-sloshing damper (18
feet x 45 feet x 11.6 feet high) (Figure 3) and a bi-directional-tuned-deep-sloshing damper (27.25 feet x 23.16 feet x 8 feet
high) (Figure 4) were evaluated to be the most cost-effective and space-optimal option for Building A and the W Downtown
Hotel respectively.
Construction of Dampers
A tuned sloshing damper consists of a damper tank, liquid and screens or vertical hangers generating turbulence of water in
motion. When a wind event begins, the liquid resonates out of phase with the structure and energy is dissipated from the
liquid by flowing through these devices. For Building A, three slat screens parallel to the short direction of the damper
tank (Figure 5) were installed in the tank. Similarly, for the W Downtown Hotel, nine paddles were hung from the ceiling of
the concrete tank (Figure 6). Each paddle consists of 1-inch thick by 10-inch wide galvanized steel plates welded to two 1-
inch thick by 4?-inch wide plates to a create cross shape section. These paddles work in both directions to provide
additional damping and ultimately result in reduced peak accelerations. The total construction cost of a damper for the W
Downtown Hotel was estimated to be less than $200,000.
Frequency Measurement and Tuning
Construction of the damper tanks, made of cast-in-place concrete, needed to proceed with the rest of the concrete
construction. Hence it was important to confirm the predetermined dimensions of the damper tanks prior to their construction
based on the measured building frequencies. These measured building frequencies were compared with the estimated building
frequencies using FEM (Finite Element Method) analysis. Considerations for the in-situ conditions at the time of measurement
had to be taken into account. Non-cracked sections were assumed under the ambient wind loads. A reduced building mass, which
excluded the weight of the missing mechanical equipment, was used. Lastly, the higher strength of the tested and in-place
concrete in some vertical members was incorporated in the structural model.
For both towers, the measured building frequencies were in a range of 10% of their estimated building frequencies (Table ).
The concrete damper tank for each damper was built per the original design without any modifications.
For the W Downtown Hotel equipped with the bi-directional-tuned-sloshing damper, the water depth was primarily tuned for the
natural frequency of the building in the north-south direction. The dimension of the damper tank in the east-west direction
was left flexible for future adjustments, which would consist of constructing additional layers of concrete masonry unit
walls at the north wall or at the south wall of the damper tank. However, interim monitoring results indicated that
adjustments in the tank dimensions of the damper basin were not necessary.
After construction of the W Downtown Hotel was completed, the final monitoring was performed to ensure the performance of the
TSD that was filled with 36 inches of water, predetermined from the initial study. This final monitoring indicated that the
measured frequency of the damper was slightly different from the measured frequencies of the completed structure. As a result
of this measurement, the water level of the damper was adjusted to 27 inches.
Summary
Within the last five years, more buildings have been equipped with supplementary damping systems. Since buildings are getting
taller and more slender, conventional methods to improve their performance in terms of motion perception may no longer be
cost-effective. These traditional methods such as increasing stiffness or generalized mass without negatively affecting
building frequencies, result in increased construction costs and loss of valuable space. From two buildings recently designed
and built in New York City, engineers have learned that a tuned sloshing damper can be a competitive alternative to those
traditional means. Also, this system can be fitted to the buildings not only to decrease accelerations but also to reduce
wind loads, as long as the supplementary damping system is properly tuned for the building frequencies under the considered
wind loads.?
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