THE EVER-EVOLVING SAGA OF 432 PARK AVENUE

Once celebrated as the flagship of Manhattan’s supertall condominium towers, 432 Park Avenue has become mired in controversy and legal battles. Standing at nearly 1,400 feet, it was the tallest residential building in the world at the time of its completion in 2015, and touted as a marvel of Engineering. But behind its “façade” (more on that later) was a building plagued by complaints of operational, cosmetic, and structural defects.

In 2021, the building’s condominium board filed a lawsuit against the developers/sponsors, citing, among other things, negligence, construction and design defects, and most recently, allegations of fraud. The developers, in turn, dragged in more parties, who then dragged in more parties, and so on. The suit now includes over thirty defendants, multiple proposed amendments to the original complaint, the filing of more than twenty-seven motions, and at least two appeals. For the first few years of litigation, little specific information was revealed about the details of the alleged defects. However, as the case continued, with various motions and responses and supporting documentation added to the case file, more detailed information became available through the published court documents. The case also includes its fair share of drama, including the recent allegations of fraudulent misrepresentations to unsuspecting buyers and the NYC Department of Buildings against a number of parties, including the Architect and Engineer of Record.

The primary load-resisting system of 432 Park is its exposed exoskeleton of perimeter columns and spandrel beams, which acts like a structural tube with perforations. That’s right, the average passerby is looking at the primary structural element of one of the tallest buildings in New York City. The continued reference to this critical structural element of the building as a “façade” is misleading, intentionally or not, as that term typically applies to non-structural exterior cladding, windows, and finishes, and not the primary load-resisting system of a building, much less the second tallest residential buildings in the world (Sorry Steinway Tower, your lattice apex doesn’t count in my book).

The conditions of the building as outlined in the lawsuit, visible from online images and videos, and reported in various news articles indicate that the structure suffers from a stiffness deficiency, which is resulting in excessive movement, cracking, and associated effects, during even modest wind events. These conditions will likely only worsen with time if the source(s) of the underlying issues are not adequately addressed, and as the building experiences windstorms of increasing intensity. While the court documents mention superficial repairs and protection of existing cracks, they do not say if or how they intend to address the underlying cause of existing and future cracking or the future performance of the building. The images also show extensive cracking and even spalling (sections breaking off) in the exposed perimeter structural frame. Reports of new cracks forming years after construction completion and the re-emergence of cracks that were previously repaired is an indication that they are not entirely associated with concrete material deficiencies at the time of construction, as alleged in the court documents. Such construction material-based deficiencies would not be expected to cause new or re-emerging cracks so long after installation, except when subjected to a subsequent applied bending or tensile stress.

If a picture is worth a thousand words, then a video is worth a million. The first realization that something was amiss with this building occurred in 2016, when I found links to YouTube videos of one of the Tuned Mass Dampers (TMDs) in action on October 28, 2015. The videos were taken during a field trip from the annual conference of the Council on Tall Buildings and Urban Habitat (CTBUH). There was a surprising magnitude of lateral sway of the TMD, which was visually estimated to be roughly 3-feet in each direction, predominantly in the north-south direction. Equally concerning was the movement in the fluid viscous dampers (VDDs), which were stroking what appeared to be as much as 1.5-feet. A review of the weather data for that time indicated that the movements were occurring at a time when the maximum sustained surface wind speeds reported at surrounding airport weather stations were less than 25 mph, and wind gusts were less than 40 mph. Those are relatively modest winds compared to the service level wind speeds that the building should have been designed to easily accommodate, and substantially less than the design level hurricane force winds. The winds were predominantly from the east (ESE), indicating predominantly across-wind motion of the TMDs (i.e. north-south swaying). Such movements perpendicular to the direction of wind are consistent with vortex shedding phenomena (more on that later). Interestingly, none of the attendees appeared to be experiencing any visible discomfort, indicating that the TMD was serving its intended function of limiting human perception to acceleration, even while undergoing significant lateral sway and stroking. VDDs are hydraulic devices that incorporate precision-fabricated seals. If they were stroking that hard during such a modest wind event, then they are likely stroking to a significant degree on a nearly constant basis. That may explain why the VDDs have been experiencing premature seal failures, as indicated in the court documents. So, why was the TMD system experiencing so much movement during a relatively routine wind event? To begin to answer that question, one needs to understand the special relationship between the structural properties of tall slender buildings and the wind forces they are subjected to.

Most people likely think of wind load in simple terms as the pressure exerted against an object that is oriented perpendicular to the direction of the wind (the “along-wind” direction). If you hold a flat object perpendicular to the wind, you can feel the force (F) of the wind pressure (P) acting over the surface area (A) of the object. If the surface area of the object is increased, the felt force increases (F = P x A). One would also observe that as the wind speed increases, so does the pressure/force. In fact, wind pressure (P) increases exponentially with increased wind speed (V). Thus, P α  V2 . However, this easily understood phenomenon is only one of many components of the wind response of tall slender buildings, and actually the least significant. Of far greater significance is the dynamic, or resonant, component of response, which occurs when fluctuating pressures around the building interact with the structure’s dynamic properties (it’s mass, stiffness, and damping) to produce inertial forces. This dominant component of the wind response produces an effect that is similar to earthquake loading on building structures, which has no external applied force, but rather internally generated (invisible) inertial forces when ground movements are transmitted up through the structure, causing accelerations (A) of the structure’s mass (M). Recall from high school physics and Sir Isaac Newton that F = M x A.

As wind flows around a tall building, it generates a wake, forming swirling air patterns at the back of the building. These swirls, called vortices, break away (shed) from the building faces in an alternating, repeating pattern in a phenomenon called vortex shedding. If these cyclical vortices become organized along the building height and occur at a consistent and repeated rate, or “frequency”, the associated alternating vortex-induced pressures can cause the building to sway side to side in the direction perpendicular to the direction of wind flow. That is why vortex shedding is referred to as an “across-wind” (or cross-wind) phenomenon. Vortex shedding is caused primarily by steady wind flow, not turbulent and fluctuating winds and gusts, because it takes time for the vortex-induced pressures to build up and become organized. The frequency (rhythm) of these alternating vortex-induced pressures is a function of the wind velocity. The higher the velocity, the higher the frequency of alternating vortex pressures, but only up to a point. Since wind turbulence around the building can have a disruptive effect on the formation and organization of vortices, and wind turbulence also generally increases with wind speed, vortex shedding occurs at regularly occurring wind speeds that are typically much lower than storm-level wind speeds. At higher wind speeds, the flow becomes too turbulent for the vortices to become organized. Thus, vortex shedding occurs within a predictable range of wind speeds (flow velocities).

When the vortex shedding frequency is at or near the building's natural frequency of oscillation, a condition called resonance can occur, and the across-wind forces can become dramatically amplified. The wind speed (velocity) at which the vortex shedding frequency matches the building structure’s natural frequency of oscillation is called the critical velocity. That is the wind speed at which maximum across-wind resonant loading is anticipated. The most common analogy for resonance is that of pushing a child on a swing. We inherently know to time each push of the swing to correctly match the period of oscillation (period is the inverse of frequency) of the swinging child. Doing otherwise would slow or even stop the continued movement. Since vortex shedding occurs at relatively moderate and frequent wind speeds, if resonant response is not mitigated, the building may undergo a great number of stress cycles even in its early life. Such regular repetitive stress cycles make the structure susceptible to fatigue-like failures.

When the amplitude of the building’s lateral displacement (sway) is large enough that the building's motion can influence the flow and wake dynamics around it, an additional phenomenon called vortex lock-in can develop. The vortex shedding frequency can become coupled to, or “locked into”, the building's natural frequency of oscillation, even when the wind speed changes. This can allow the resonant response to occur at higher velocities, and thus higher pressures, than it would otherwise. This phenomenon has an analogy in real life when driving behind a semi-trailer. Under certain conditions, the back ends of trailers can be seen swaying from side to side in a continual rhythmic manner. The frequency (rhythm) of the sway can be observed to remain relatively constant even with variations in speed.

Dealing with cross-wind response to vortex shedding can become a delicate balancing act because the shedding frequency is based on the wind environment and shape of the building, which are controlled by Mother Nature and the Architect, respectively; while the natural frequency of the building is based on the building’s dynamic properties, which is controlled by the Structural Engineer. So, how does a design team deal with these issues?

It stands to reason that the most effective way to mitigate the effects of vortex shedding would be to somehow prevent or disrupt the organization of the vortices in the first place, essentially eliminating the problem altogether. For most tall buildings, this can be accomplished by orienting or shaping the building or adding appendages (fins/spoilers, etc.). Altering the building shape in plan (asymmetry, softening/chamfering corners, etc.) can disrupt the wind flow to limit the formation of vortices, while altering the building shape in elevation (tapering, twisting, setbacks) can prevent vortices from becoming organized in a consistent repetitive manner. A review of the world’s tallest buildings reveals that designers go to great lengths to shape their buildings to mitigate these issues. The designers of the current tallest building in the world, the Burj Khalifa, have described the shape of the building as being mostly determined by the wind tunnel testing. This approach relies entirely on the ability of wind tunnel testing, and sometimes analytical computational fluid modeling, to accurately reflect the flow characteristics around full-scale buildings. This requires replicating the actual wind environment, using significantly scaled-down, simplified physical models tested in a laboratory by blowing air from large fans at them.

Another method that attempts to achieve a similar goal to shaping is the introduction of porosity (openings/slots/holes) to allow wind to pass through the building to disrupt the organization of the vortices along the height of the building. This method relies even more so on the ability to simulate the flow behavior, because the disruptive behavior of these mitigating geometric features is modeled at an even smaller scale (i.e., 1500-ft aerodynamically shaped building compared to say the two-story tall openings in the case of 432 Park). Since the wind flow through such openings is what is relied upon to disturb the vortices, anything obstructing or limiting the countering wind can reduce their mitigating effect.

As an alternative to the above-described external geometric and flow-disruptive techniques, mitigation of across-wind response can also be achieved by modifying the structure's dynamic properties (mass, stiffness, and damping). Although these measures are typically employed in conjunction with the above-described external mitigation techniques, they can be employed alone in some cases. However, when employed alone, they are a somewhat risky and audacious “brute-force” approach in that they allow the vortex shedding to occur and rely entirely on the Engineer's ability and confidence to predict the structure's behavior to counter it accurately.

Since the wind response of tall slender buildings is dependent on the structure’s dynamic properties, it is essential to identify the contribution of those properties. The mass, or weight, of the building is relatively predictable and consistent, so it is primarily the stiffness and damping that are variables that drive the structural behavior under wind excitation.

Damping is the dissipation of vibrational energy to reduce the amplitude and duration of oscillations. All structures possess some level of inherent damping due to friction and straining of the materials. Referring back to the analogy of a child on a swing. If one stops pushing the swing, and the child does not force any additional movements themselves, the swing will eventually stop oscillating, which may take several cycles to stop. The number of cycles it takes for the swing to stop oscillating is a measure of damping. If there were no damping, meaning theoretically no friction on the swing hinges or chains, no air resistance, etc., the child would swing forever with no additional input (you wish, Dad). If, instead, the swing had rusty hinges with a lot of rubbing, then the swing may stop oscillating after just a few cycles. Such conditions would also make it harder for the pusher to maintain the amplitude and duration of oscillations. Damping is measured as a ratio, or percentage, of “critical damping”, which is the damping level that would result in no oscillation. For reference, the inherent damping level of building structures under wind excitation varies between roughly 1%-3% of critical. Increasing the level of damping, or energy dissipation, always has a mitigating effect on resonant response. But what if the inherent damping of a building structure alone is insufficient to control resonant response to an acceptable levels?

When the inherent damping of a building structure is insufficient to control resonant response to an acceptable level, additional “supplemental” damping can be added. There are various ways to achieve supplemental damping in building structures, but the most efficient way is with the introduction of auxiliary oscillators. These subsystems are essentially secondary structures possessing mass, stiffness (at least an effective stiffness) and damping of varying degrees. Moreover, in the same way that the primary structure (the building) has a natural frequency of oscillation, so does the auxiliary oscillator. The goal is to “tune” the auxiliary oscillator to have the same frequency as the building, but oscillate completely out of phase (opposite direction) with the building’s oscillation. One such auxiliary oscillator system used commonly to add supplemental damping is the so-called tuned mass damper (TMD), which is essentially a simple mass pendulum (swinging weight). Like the building’s inherent damping, the inherent damping of a TMD is also relatively small, consisting mostly of energy dissipation through friction against moving parts. That is why TMDs are typically provided with some form of viscous damping, most commonly fluid viscous damping devices (VDD). These act like shock absorbers that you may find on vehicles and bicycles. Fluid viscous dampers consist of cylinders filled with a viscous fluid (usually silicone oil).         When the structure oscillates, a piston connecting the building and the TMD moves (strokes) through the fluid and forces the fluid through small orifices or valves, creating resistance by converting kinetic energy (motion) into thermal energy. As you can imagine, these devices are highly engineered with precision fabricated components, especially the various seals, valves and orifices. Recent advances in damping technology have introduced less sensitive alternative systems. One such system now used with TMDs incorporates extension pieces (paddles) connected to the mass that move through fluid-filled containers (think of a pool of molasses). The properties of the fluid (i.e., density, viscosity, etc.) are tailored to provide the desired resistance to the motion of the paddles.

While increasing damping always decreases the resonant inertial forces, the same cannot be said for the other important dynamic property of the structure, stiffness. Structural stiffness is the resistance of a structural element or assembly of structural elements to deformation or displacement under load. Plucking a tight string (high stiffness) results in oscillation at a high frequency (many cycles per unit time), whereas a soft, loose string (lower stiffness) oscillates at a lower frequency. The natural frequency of a building structure is essentially the measure of how many cycles of oscillation per unit time (e.g. cycles per second) the building would experience if one were to displace (pluck) the top of the building laterally and then let it go. Although the natural frequency of building structures is based upon their construction materials and components, they tend to fall within a band of expected values based on their height. This is primarily because building codes and standards of practice impose certain limitations on building displacements, which are proportional to the frequency. Wind pressure fluctuations in the along-wind direction also have an expected and recognizable frequency. Fortunately, for most buildings, the fluctuation frequencies of the wind are much lower than those of the building. However, for very tall buildings, as the natural frequencies decrease with greater height, the risk of resonance between the two becomes greater. Therefore, increasing the building’s natural frequency (increasing the stiffness) almost always decreases wind response in the along-wind direction. But what about the across-wind response?

For across-wind response, decreasing the natural frequency of the building (i.e., making the building more flexible) forces the critical velocity for vortex shedding to decrease to a lower speed that may be associated with more regularly occurring wind events. But the level of excitation associated with those events would be less than that associated with more severe, less frequent, higher wind speeds, all other things being equal. Increasing the building frequency ( i.e., stiffening the building) forces the critical velocity to increase to a higher speed that, although may occur less frequently, could have a higher response due to the stronger level of excitation. Remember, faster wind speed means higher pressure. But the critical velocity can only be increased so much because, as explained previously, there will be a wind speed at which the flow becomes too turbulent for the vortices to continue. So, should tall slender buildings be stiff or flexible? The answer depends on what aspect of the design criteria is of most interest.

Different design criteria (goals) are based on different recurrence intervals of wind. Serviceability design focuses on issues like deflection, vibration, durability, performance of MEP systems, etc.,  ensuring the building remains usable for its intended purpose and function under normal conditions. Strength (ultimate) design focuses on its ability to withstand extreme demands without collapse. In a tall, slender, high-end condominium building, serviceability issues like the human perception of acceleration (motion sickness) may be governed by regularly occurring wind speeds expected to occur on average, say once every month, while damage to finishes and non-structural components may be governed by less frequent, but still reasonably expected, wind speeds occurring, say once every 20 years.  The structural capacity of the building against collapse may be based on wind speeds expected to occur once every 500-1000 years, etc. The design of the building must be comprehensive, addressing all of the goals, and some are obviously more critical than others. While the owner of the highest penthouse in the world may reasonably accept some minor inconveniences in service and a little bit of discomfort during a windstorm likely to occur every say 5-10 years, experiencing the unsettling feeling of motion sickness in your 100-million dollar apartment, or having sloshing water in the toilets, or reduced elevator speed on a monthly basis would be something entirely different. Moreover, most people would agree that having fire doors and elevators that function in any event is more important than controlling human perception of motion, which is somewhat subjective anyway. However, If controlling human perception of acceleration under frequently occurring wind events is the governing design criteria, as is often the case for tall slender residential towers, one may reasonably conclude that making the building more flexible can be an effective way to mitigate vortex-induced across-wind response for more frequent wind events. But, that approach carries a big disclaimer. Such an approach assumes that everything else associated with that flexibility has been addressed adequately for very frequent lower amplitude events, less frequent service level events, and extreme design level events.

While a designer may be tempted to intentionally limit the stiffness of the structure in an effort to avoid resonance at the occupancy comfort levels of excitation, there can be significant consequences for other sources of wind response. For example, while the across-wind response in tall slender buildings is typically larger than the along-wind response at the wind speeds associated with vortex shedding, tall flexible buildings having very low oscillation frequencies can become susceptible to large, low-frequency gusts in the along-wind direction. Such large fluctuations occurring at much higher wind speeds can cause an amplified dynamic force in a more impulsive manner than the oscillatory/cyclic type of resonant response associated with vortex shedding. Under such conditions, increased stiffness becomes an important mitigating property. Additionally, the larger sway amplitudes associated with flexible buildings render them more susceptible to vortex lock-in, which as described before can expose the building to vortex shedding at higher levels of wind excitation than anticipated. Large displacements can also result in geometric nonlinearities (stability issues) as the gravity load of the building acting over a large displacement generates additional (secondary) overturning forces, further increasing the displacement, generating yet additional overturning forces, and so on. Then there are the obvious issues with too much flexibility, such as the potential for basic serviceability issues like cracks, noise, leaks, etc.

As described previously, the conditions of the building, as outlined in the lawsuit, visible from online images and videos, and reported in various news articles, are consistent with a stiffness deficiency in the lateral force-resisting structure. Stiffness deficiencies are generally the result of design deficiencies (structural design and wind engineering), material deficiencies, construction defects (installation and protection of materials and methods of construction), or some combination. However, it is important to distinguish between deficiencies associated with materials themselves, as opposed to improper handling, storage, transportation, placement, protection, curing, connection, etc. of those materials. It is also important to distinguish between the actual behavior of the materials and the theoretical behavior assumed in the design.

For concrete buildings, the lateral stiffness of the structure is inversely proportional to the degree of cracking in the lateral force-resisting elements. That means that with greater cracking comes lower stiffness (lower frequency) and usually a higher deign wind load. Concrete cracks due to a variety of reasons, not the least of which is when applied tensile stresses exceed the tensile strength of the concrete. However, there are many other reasons why concrete cracks, even when there is no apparent tensile force present. During the curing process, concrete undergoes volumetric changes, thermal and moisture variations, formwork displacement, etc. Therefore, material and construction deficiencies and defects can also play a role in the degree of cracking in the concrete. Fractures (cracks) in concrete are not like fractures in bones. Fractures in bones can heal over time and essentially fuse back together, but cracks in concrete never heal themselves; they only worsen and propagate under increasing and repetitive cyclic stress in a fatigue-like manner, potentially causing irreparable harm to the structural integrity of the building. Fortunately, crack widths are limited by the degree of restraint provided by the reinforcing within the elements, assuming, of course, that the reinforcement is installed properly and can provide adequate tension-stiffening to the concrete.

The 432 Park Avenue structure utilized 14,000-psi high modulus, self-consolidating, architecturally exposed concrete. While concrete mixes with such a high strength had been used on a few building projects in New York City by the time the 432 Park project started, achieving such high strength requires careful proportioning of specialized admixtures, aggregates, and cementitious materials. The difference with 432 Park was that, as an architecturally exposed structure, the concrete utilized white cement, which had not been used in the existing 14,000-psi concrete mixes. An Article in ENR magazine from 2015, now referenced in the court documents, described the challenges the design team faced in developing a mix that could meet the structural design criteria, while being buildable. That required using specialty admixtures, the effects of which were tested using at least twelve full-scale mock-ups.

The perimeter frame elements (columns, beams, etc.) of the building are relatively large, which can introduce additional issues related to the placement and curing of so-called “mass-concrete”. Mass-concrete is generally defined as any volume of concrete placed in which a combination of the dimensions of the member, the boundary conditions, the characteristics of the concrete mixture, and the ambient conditions can lead to undesirable thermal stresses. These thermal stresses can cause cracking, damaging chemical reactions, and a reduction in the long-term strength, structural integrity, and monolithic action. Recent testimony in the ongoing case available in the court records indicated that the mock-ups exhibited cracks that may have been an advanced warning of such mass-concrete effects. But even when the concrete material itself is not deficient, the manner in which it is placed and cured can be. Additionally, the formwork system, joints, reinforcing, and items embedded within the concrete can all affect the final condition of the hardened concrete.

The 2011 structural peer review report for the building, available through the DOB website and now included in the published court documents, revealed that the main governing factor on the design of the lateral force-resisting system was the acceleration performance of the building during very frequently anticipated, almost monthly (0.1-year return period), wind events. Yet, there is little information in the court documents or media about occupants suffering from motion sickness (seasickness). Additionally, there were no apparent indications of concern among the attendees of the October 28, 2015 tour of the TMD’s posted on YouTube, indicating that the design goal of controlling human perception of motion has apparently been achieved thus far. However, the court documents and news articles describe conditions within the building that are more consistent with excessive differential inter-story movement between the structure and attached components and finishes, not human discomfort due to acceleration. The report also states that at least at that time, there was no recognized standard for acceleration limits for occupant comfort for such frequent windstorms, and, in the author’s experience, other supertalls have not explicitly considered such frequent windstorms. So perhaps the design team focused too much of their efforts on controlling the occupant comfort levels for cross-wind response accelerations for wind speeds expected on an almost monthly basis, and not enough attention was given to plain old-fashioned structural displacement and flexibility.

The peer review report indicated that the structural design of the building anticipated an overall top displacement under a 50-year wind load of 0.4% of the building height (i.e., H/250), with a period of vibration of approximately 14 seconds. For future reference, frequency (f) is the inverse of period (T). The report does not mention if the 14-second period applied to the frequently occurring windstorms that were reportedly the primary focus of concern or if it is associated with more extreme design-level windstorms. In either case, the magnitude of those two parameters alone should have raised some eyebrows, as they are well beyond what design engineers should expect for such a significant building (It was, at one time, the tallest residential building in the world). To put this into perspective, according to presentations and papers available online, the tallest building in the world, Burj Khalifa, which stands approximately 2700 feet tall (almost twice that of 432 Park), has a period of roughly 11 seconds. Other supertall buildings over 2000 feet tall, such as Kingdom Tower Jeddah, Ping’an Finance Center, and Shanghai Tower, for example, have reported periods of less than 12 seconds. Other supertall buildings within the height range of 432 Park, such as Shanghai World Financial Center, Wuhan Greenland Center, and Taipei 101, have reported periods of less than 9 seconds. A 50-year wind displacement of H/250 is roughly one-half of the H/500- H/400 limit that most engineers would recognize as the maximum top displacement limit for a tall building to avoid problems. The H/400 limit, although not required by US codes or based on any specific calculation, has served as a reliable limit and industry standard for at least half a century. Older experienced engineers would immediately recognize it as kind of an industry standard that is grilled into the minds of young engineers in their training as tall building designers, with the obvious message being that if the top of your building is designed to displace more that L/400, you are asking for trouble, similar to the problems reported by the building’s occupants throughout the court documents. Recently, many Engineers have begun to ignore this limit, with the justification that it is not necessarily applicable to buildings that exhibit a predominantly flexural mode of deformation (I.e., minimal expected inter-story differential movement). However, such an assumption can easily be validated by observing the actual behavior of the building. Since the conditions and complaints described in the lawsuit and media are consistent with excessive inter-story differential movement, it would be reasonable to question if such a lax overall displacement limit was justified. Recall, as described earlier, at least one unit owner hired a team to eliminate the “differential movement” between the building and interior finishes.

Although building displacement limitations are typically intended to address serviceability conditions, there are many cases where serviceability conditions can have associated safety-related effects, such as operation of life-safety systems, operation of egress doors, detached components falling from the building, etc.

The design of any tall slender building is heavily dependent on wind tunnel testing, which is done at significantly reduced scale (typically 1:500). Although modern wind tunnel testing is a time-tested and reliable method of determining overall wind-induced forces on building structures, assessing highly localized effects is more challenging because they typically involve much reduced scale. For these reasons, wind tunnels sometimes create separate larger-scale models to verify accurate representation of localized effects and associated parametric studies.

The approach of the design team and wind tunnel to mitigate the wind-induced response of the building was to disrupt the vortex shedding effect with the addition of two-story high openings through the building spaced approximately every fifteen stories,  in addition to providing supplemental damping. The introduction of so-called “blow-through” floors was intended to allow wind flow through openings in the exterior envelope to disrupt the organization of wind vortices. Since wind flow through such openings is what is relied upon to disturb the vortices, anything obstructing or limiting the countering wind flow can reduce the mitigating effect. As such, one would reasonably expect that the mitigating effect of the wind flow around and through the blow-through floors was based upon much larger-scale wind tunnel testing using detailed representations of the various components present at those locations. It is not clear from the publicly available information if such large-scale localized wind tunnel testing was implemented for the two-story tall blow-through floors. Notwithstanding, it is conceivable that in reality the blow-through openings are just not as “open” as was assumed in the wind engineering assessment process. The grid of wide columns and deep beams, the size and texture of the metal drum enclosures, and even the hand railings, all cause some degree of disturbance to air flow through these “open” floors. Interestingly, the court records indicate that the railings on at least some of the blow-through floors have been jostling loose from fatigue, an indication that they are likely "catching” a lot of wind.

The design of a TMD supplemental damping system is only as valid as the predicted long-term behavior of the building to which it is attached. If the TMD mass sways more than anticipated, that can cause unexpected demands on attachments and elements of the structure, as well as components of the TMD system itself. The court documents suggest that continual malfunction of the fluid viscous dampers (VDDs) may be a source or contributing cause of the reported problems with the building. Still, it is far more likely that their malfunctions are a symptom of a problem, one that is causing unanticipated excessive stroking of the VDDs on a regular basis. The seals and components on VDDs are intended to have a long service life, but can become overworked and fatigued. The YouTube videos of the VDDs stroking substantially during a modest level wind event suggest that the TMD may be swinging beyond its expected displacement level. Visual cues in the videos indicate that the TMD was swaying as much as 3-feet in each direction, and the VDDs were stroking as much as 1.5-feet, which seems excessive for the wind speeds at that time.

Unlike most supertall buildings throughout the world, the vast majority of which were shaped specifically to mitigate wind response, the shape of 432 Park was preordained by the architect, and the Engineers were forced to deal with wind response in a more head-on, brute force manner; one that required a delicate balancing act between making the building flexible enough to avoid occupant discomfort to acceleration while stiff enough to avoid issues associated with excessive sway. Although openings (blow-through floors) were created through the building in an attempt to disrupt vortex shedding, their mitigating impact was clearly limited even under the best circumstances, given the fact that the building still required the largest TMD system in the world to control the acceleration. To put this into perspective, the tallest building in the world, Burj Khalifa, has no supplemental damping system.  By not shaping 432 Park Avenue as the primary means of mitigating wind response, the design team was forced to rely on an aggressive and ambitious approach that was sensitive to variations in analytical assumptions and actual in-place conditions, with reduced resiliency and robustness to deal with unknown or unanticipated issues, not the least of which would be material and construction defects. It is very risky to design a building of this magnitude and significance with such sensitivity to stiffness. It would be like walking a tightrope while wearing pointe shoes. Many Engineers would likely agree that if they found themselves in a position where they were making the tallest residential building in the world intentionally more flexible, they should carefully reconsider their approach.

As described previously, decreasing the building’s natural frequency (increasing the stiffness) always results in a decrease in wind response in the along-wind direction and usually also in the across-wind direction, especially under design-level winds. It stands to reason that the reverse is also true, and buildings with stiffness deficiencies experience higher wind loads. If wind response is inversely proportional to the lateral stiffness of the building, and the lateral stiffness is inversely proportional to the degree of cracking in the concrete, then with greater cracking comes higher wind load. Additionally, a reduction in stiffness results a reduction in the building’s frequency, which can further alter the dynamic interaction between the wind and the building.  If this all sounds very complicated, that is because it is. It is no wonder that most supertall buildings rely on shaping as the primary source of wind response mitigation  

Since the cracking in the exposed structural frame of 432 Park Avenue was evident even while the building was in the early stages of construction, the design-level wind loading may now be underestimated. Additionally, the building may now also be more susceptible to vortex lock-in. While much of the cracking may not have initiated from applied stress, subsequent repetitive cyclic stress from wind loading since the time the project was constructed has likely exploited the already ruptured concrete and propagated existing cracks while also causing additional cracks. The court documents indicate cracks that were previously repaired have reappeared, and new cracks have and continue to develop. A common assumption by many Engineers is that concrete maintains its full section properties until applied tensile stresses exceed the rupture stress of the material. While this is typically a valid assumption, even when there are minor thermal and shrinkage cracks, in the case of 432 Park, the cracking is so pervasive that even with stress levels below the materials rupture stress, the stiffness of the structural frame is significantly reduced, resulting in a decreased overall lateral stiffness. Furthermore, the natural frequency of a concrete building is actually not constant, as it varies based upon on the state of stress and cracking, and reinforcing within the elements.  If the building experiences windstorms of increasing magnitude in the future, the associated higher stresses in the concrete can produce more cracking, further reducing stiffness, which increases wind loads, causing higher stresses in the concrete, resulting in more cracking and higher wind loads, and so on and so on. It becomes a cycle of progressive failure, as additional repetitive stress-induced cracking becomes insidious. When the wind loads on the building don’t vary significantly over time, as has been the case thus far in the history of 432 Park Avenue, this cycle of progression can reach a state of convergence, and the problem becomes dormant and the conditions consistent; that is until there is a much greater magnitude wind event.

Merely sealing the cracks in the exposed structural frame against water/moisture exposure is like covering a deep wound with a Band-Aid to keep dirt out, but not treating the underlying internal wound itself. If such cracks are also unprotected from environmental exposure, the concrete and embedded reinforcing can deteriorate from corrosion. While elastomeric surface coatings and crack sealing can protect against continued deterioration from environmental exposure, they do not reverse the underlying cracks and associated loss of stiffness. While adhesives can be injected into cracks under pressure, the depth of penetration is limited in cases where the crack widths are narrow. This is especially true for horizontal cracks in vertical elements (columns and walls) that support gravity load. Cracks in such elements are difficult to inject reliably to any significant depth because when the elements are in a state of compression, the crack widths are relatively narrow. Additionally, if and when those elements experience a net tension force, the cracks will experience additional widening anyway. Additionally, suppose wind-related structural deficiencies are addressed based on the conditions existing while in a state of dormancy. In that case, those measures can quickly be reversed once the building experiences a much higher wind event. 

Anyone who drives on New York’s highways would be very familiar with the detrimental effects of corrosion of embedded steel elements such as reinforcing bars. Cracked and spalled concrete on abutments, piers, and walls is an all too common condition found concrete highway structures. The heavily corroded (rusted) and deteriorated steel exposed when the concrete falls off (spalls) can be alarming, as sometimes the steel is completely disintegrated. Now, imagine if similar conditions were occurring on exposed reinforced concrete elements 1000-feet above the ground. Although most people may think of concrete as a solid impermeable material, it actually acts more like a sponge when exposed to water and moisture, and that does not even consider the effect of wind-driven rain, which at the height of 432 Park Avenue has the effect of being hit with a fire hose. So, the idea that the design team initially allowed the primary lateral force resisting structural element for what was at the time the tallest residential building in the world to be left unprotected and directly exposed to the elements is difficult to comprehend. Notwithstanding any desire the developers and architects may have had to retain the natural look of the exposed concrete, the design professionals involved should have anticipated that significant corrosion issues could eventually develop and should have insisted that protective details be incorporated into the design from the outset.

The conditions shown and described in the publicly available court documents, visible from online images and videos, and reported in various news articles and media are consistent with a structure that suffers from a stiffness deficiency. If not addressed properly, the conditions can result in the structure entering a cycle of progressive failure, even while it may currently be in a period of dormancy until it experiences higher wind events. The building still has not been tested by a hurricane, and has barely experienced tropical storm-force winds. Although there are solutions that can be implemented to stop the cycle of progressive deterioration, as with anything, you cannot solve a problem unless you first fully understand it. At this point, it is not clear if any of the involved parties have such an understanding. The more cynical view and concern is that in a rush to placate the immediate complaints of the building's occupants and reach an expeditious settlement of the lawsuit, the identification of the actual causes of the issues with 432 Park is not being given the necessary attention. This may include making temporary and eventually ineffective repairs, more Band-Aids.

After years of litigation, its second set of counsel/attorneys, and two prior amended complaints, the plaintiffs recently filed a motion to amend their complaint yet again, this time alleging fraud against multiple defendants, as well as aiding and abetting fraud against the project's Structural Engineer of Record. The fraud claims are based on an alleged cover-up and conspiracy to defraud the plaintiffs and prospective owners, in addition to making deceptive representations to the New York City Department of Buildings (DOB) about the severity and significance of cracks in the exposed structure. Really? Has anyone notified the DOB about this??

The plaintiffs argue that their new allegations were based largely on recent discovery and deposition testimony and that they only recently learned the critical facts necessary to uncover and plead the underlying cause and origin of the structural defects, most notably the results of multiple pre-construction mock-ups to evaluate the performance of the concrete. The Plaintiffs initiated the lawsuit in September of 2021 and have presumably been diligently investigating the alleged defects. The plaintiff’s expert team is in a superior position with regard to the investigation into the cause and origin of the defects because they have had unfettered access to the building to evaluate, measure, test, and monitor the various issues alleged in their suit. With regard to the challenging concrete mix design and pre-construction mock-ups, at least two articles about the mix design and mock-ups were published in Engineering News Record (ENR) as early as January 28, 2015. Furthermore, structural and architectural drawings as well as the structural peer review report have been available online since at least 2019, before the lawsuit was even filed. All one had to do was perform a thorough Google search to find that information. So it is difficult to comprehend that after more than three years of litigation and presumably diligent investigation, the plaintiff’s experts are just now figuring out the cause and origin, or at least what they believe to be the cause and origin, of the structural defects. After all, their original complaint, filed more than three years ago, alleged “structural life-safety” issues. Those serious allegations were presumably substantiated with a thorough Professional Engineering assessment. It is reasonable to question why the condominium board has allowed this building to remain occupied and units to be bought and sold with the knowledge of so many alleged structural life-safety issues. And now that the plaintiffs experts, at least some of whom are presumably Licensed Professional Engineers, have such knowledge of the cause and origin of the alleged “life-safety” issues with the building, as well as the alleged misrepresentations by the developer and design team to deceive the New York city Department of Buildings (DOB); have they exercised their professional obligation to notify the DOB of these allegations?

With such an enormous collective asset value in the building, there is a point at which the alleged defects could become too big for anyone involved on both sides of the case to tolerate. A 1% liability exposure on, say, a 100-million-dollar claim could result in a 1-million-dollar judgment, which is a tolerable, survivable amount. Companies and individuals can survive that kind of judgment. However, if this turns into something like a 1-billion-dollar claim, that would be an entirely different story. Even a token 1% liability exposure on such an amount could result in a 10-million-dollar judgment. That would easily exhaust most insurance policies, corporate or personal assets, at least for a more minor third-party defendant. Simply put, there is only so much money available in the pot. For the Plaintiff’s part, having a problem that is too big to fix within a reasonable period and for a recoverable monetary cost would leave them holding the bag, one full of the shattered remains of their once valuable assets. So there may be an incentive among all parties in the litigation, even those with minimal exposure, to minimize the full extent of the problems with this building. That may explain why they keep referring to the primary lateral force-resisting element of the building as a “façade”. But what about the public? Don’t we have a right to know if there is a serious problem with one of the tallest buildings in New York City?

The public cannot rely upon the litigants in a high-stakes lawsuit involving parties with multiple competing interests to provide them with assurance that chunks of concrete will not be spalling off this building and raining down to the ground in the future. Perhaps it is time for the New York City Department of Buildings and the City government to take a more proactive approach in ensuring that the safety and interests of the public are not taking a backseat to the interests of the litigants in a lawsuit. At a minimum, they should investigate whether they have in fact been intentionally misled as to the severity of the structural defects in the primary lateral force-resisting element of one of the tallest residential buildings in the world, not a “façade” as they may currently understand it. Perhaps it is time for the DOB to conduct an independent investigation and provide oversight and structural health monitoring in the interests of public safety. Such an involvement would not be without precedent. One only needs to study the history of the Millennium Tower (aka the Leaning Tower of San Francisco ) to understand how any self-interests of the litigants can quickly become overshadowed by the interests of the public, by way of their governmental representatives.

It will be interesting to see how this saga plays out and if we ever learn the truth about this building and its alleged defects. Only time will tell.