Welcome to our frequently asked questions page. If you don’t find the reply you’re looking for please contact us and we would be happy to answer you directly or add your question to this page.
It may seem surprising that dampers tend to reduce the cost of structures. Achieving a given seismic performance using dampers tends to be less expensive (depending on the cost of the dampers) than if you were trying to do so by using brute force stiffening methods alone (Shear walls, stiff braces etc.).
Friction dampers tend to be less costly than other dissipation technologies because of their highly efficient rectangular hysteretic loop (less dampers required), their full integration into the lateral resisting system and their low per unit cost. It’s very common to find examples where axial and shear forces in columns are reduced by 40% to 60% of those found in other methods of seismic protection and retrofitting (Vezina, S., Proulx,P., Pall, R., and Pall, A., 1992 and Balazic, J., Guruswamy, G., Elliot, J., Pall, R., Pall, A., 2000). With this kind of reduction in the elements’ forces, elements can be better optimized and structural costs tend to be substantially reduced. It’s not uncommon to see cost reductions of up to 60% in retrofitting and to 10% in New structures.
From a lifecycle perspective, ductile technologies (yielding connectors, BRBs etc) tend to pose another challenge, in that they require replacement after an earthquake. This is of course preferable to damage of other elements, such as columns or beams, but very costly in a finished building where operations have to be disrupted to replace the ductile element. Since friction dampers don’t require replacement after an earthquake there are also significant lifecycle cost savings.
The main reason is that traditional methods tend to actually be more expensive because of their lack of energy dissipation and oversized sections. Integrating energy dissipation reduces the forces on the members allowing for more efficient structures which are ultimately less expensive. Because friction dampers do not exhibit velocity dependence, designing with them is straightforward and intuitive, making them easy to integrate.
Simply storing the earthquake energy and relying on the small amount of entropy to dissipate it is inefficient. When an earthquake that exceeds expectations occurs it’s easier to exceed the buildings capacity when there is little dissipation.
To try and store all the earthquake’s energy is ultimately impractical and leads to overly stiff and heavy structures with larger accelerations (forces). Using seismic dampers like ours, provides the structure with extra lateral resistance without causing higher accelerations. This in turn helps to achieve the same seismic performance with a more efficient (lighter) structure, bringing costs benefits to the final client. In several examples, (Vezina, S., Proulx,P., Pall, R., and Pall, A., 1992, Chandra, R., Mas and, M., Nandi, S., Tripathi, C., Pall, R., Pall, A., 2000), we see how traditional alternatives based on structural steel and reinforced concrete tend to make the structures stiffer, but at the same time, bring the problem of increasing input energy (higher mass, same ground motion). This usually carries the problem of inevitable strengthening of structural elements and foundations.
With dissipation technologies providing overall cost savings and being as easy to use as they are today, there is no reason not to integrate them.
Friction dampers are meant to be maintenance free and slip during design earthquakes or higher. Cases where the dampers would slip daily for example would be inappropriate.
Dissipating wind energy would be an example of an inappropriate use. Displacements due to wind should fall within the range of stiffness of the building before damper activation. The dampers are often used when there are strong wind loads however the slip load is recommended to be approximately 1.3times the expected wind load. The intended use of our friction damper is for design (DBE) and maximum considered (MCE) earthquakes.
Friction dampers for dissipating wind energy
Friction dampers can be designed to slip under wind forces using high wearing composite friction pads and Belleville washers (spring washers) to make up the wearing. However such a design would involve replacing or reworking the dampers periodically due this wear of the interface and fatigue of the springs. So, although feasible and possible, the periodic maintenance implication would make it costly. Viscous dampers tend to be a more appropriate technology for dissipating small amounts of energy such as wind and the maintenance can potentially be done on site with seal replacement kits and replacement silicone oil (or other working fluid). Because wind energy tends to be much smaller (sometimes orders of magnitude) than seismic energy most engineers tend to prefer not to directly dissipate wind forces and opt instead to store the energy within the stiffness of the building’s elements.
Large or small displacements can be easily accommodated. Friction dampers are the most efficient way to dissipate energy specifically because of the constant energy dissipation throughout their stroke. Moreover, Quaketek in-line friction dampers offer the largest and most reliable hysteretic loop because nearly 100% of the energy input is dissipated. Therefore, for any lateral deformation the engineer is expecting for his/her project the amount of energy to be dissipated is always proportional to the lateral deformation and the force applied. This can be particularly useful with small deformations as the rectangular hysteretic loop is the maximum dissipation available.
Quaketek friction dampers have the largest capacity for lateral deformations easily providing 130% or more of the displacement expected in the Maximum Credible Earthquake (MCE). Even with damper strokes exceeding 200mm, inline friction dampers maintain near constant slip load throughout the stroke. This constant performance is designed into the damper and validated during 100% production testing.
Stick Slip is a common phenomenon which can be observed (usually heard) in everything from hydraulic cylinders, brakes and even in fault lines! Stick-slip is responsible for the screeching of brakes or the sound produced by many musical instruments. A damper experiencing stick slip will be quite loud and screech or squeal. The effect is especially prominent in bi-metallic dampers, steel on steel or poorly manufactured interfaces.
Quaketek’s friction dampers do not exhibit stick-slip characteristics and are quiet throughout their entire travel at full load. Each damper is individually tested at load and for the full stroke to ensure that the load remains constant and that there is no sign of stick-slip or other non-desirable characteristics. The damper remains quiet throughout its entire travel and the noise generated by the damper tends to be less than 60db.
We use high-strength structural bolts in our seismic dampers which have been extensively studied and documented. Their behavior is well understood and have been used in structural connections for more than 60 years. Studies supporting AISC bolt design guidelines, Tajima (1964), Chesson and Munse (1965), and Allan and Fisher (1968), have found that bolt relaxation occurs in the highest proportion just after bolts pre-tensioning. Usually, total variation is in average 8% over an 80-90-year period. Around 80% (6.5%) of this 8% drop occurs within one week of the initial pre-tensioning. We therefore account for this bolt relaxation in the initial calibration of the dampers.
Studies have shown that changes of up to ±25% from the optimal slip load do not considerably affect the structural response. Small changes in the slip load due to relaxation will therefore have minimal affect.
Additional considerations on bolt relaxation
High wearing or soft interfaces worsens bolt relaxation in the connection, this creep could be made up for with Belleville washers. Thick Belleville washers however are themselves prone to creep and fatigue failure over time due to high stress at the edges (ASTM, Journal of Testing and Evaluation vol 42). We therefore avoid creep/wear prone elements in the friction interface altogether.
Unfortunately, bolt relaxation behavior is not well understood in the case of very long bolts (Length > 8x diameter) like those sometimes used when many plates and washers are stacked. When deciding between the use of long or short bolts AISC comments that there is not enough research background (AISC, 2002) to establish satisfactory standard pre-tensioning and installation rules.
The friction interface has been carefully designed and developed to quickly dissipate heat to the entire damper, which in turn minimizes the maximum temperature of any one point. The damper gets no hotter than a copper pipe with hot water.
Heat (energy) and dissipation
Earthquake energy has to be stored and/or dissipated. All building technologies whether standard ductile construction or using damping will store this energy and release it (usually through heat generation).
All structures will have some energy dissipation due to entropy however what is important is the rate at which the energy is dissipated. In traditional concrete buildings, the energy is stored in concrete members with very little dissipation. Once the earthquake energy exceeds the storage capacity you get ductility of the steel rebar and eventually rupture and collapse (releasing the energy). Analogous behavior occurs in structural steel structures. The more flexible a structure the more energy
In the case of yielding or ductile technologies (e.g. BRBs), the earthquake energy is used to deform the yielding core and the energy is partially stored (through elasticity and strain hardening) and partially dissipated through entropy (heats up). That heat is then transferred throughout the BRB through mostly through convection. This will occur in any material and you can see it yourself if you bend a paperclip back and forth rapidly.
A viscous damper will generate heat as all the energy is transferred into the fluid and the fluid heats up. This can in turn affect the viscosity and so the selection of the working fluid is important in order to manage these changes. The total increase in temperature will depend on the total thermal mass (mass of damper, fluid and the specific heat of the materials) and the energy input by the earthquake.
A friction damper which is essentially a coulomb damper transfers the energy directly into the surfaces which dissipate the heat through the entire damper mostly through conductive heat transfer. The amount of temperature change will ultimately depend on the thermal mass of the damper and conductivity of its elements. If the damper has insufficient thermal mass or poor conduction it will heat up excessively.
In our case we have carefully developed the friction interfaces to ensure conductive heat transfer, minimal thermal expansion and corrosion resistance. This is important because certain materials will actually become velocity dependent as they heat up. For example, friction sliding isolation bearings will sometimes use composites or PTFE which is a polymer (plastic). As plastics approach their glass transition temperatures they behave more similarly to fluids. This abrupt change in the behavior of plastics creates challenges in modelling and predictability of the elements’ performance. Further complicating the use of plastics in friction interfaces is poor heat conduction. Since friction dampers convert earthquake energy into heat it’s important that the heat be conducted away from the friction interface quickly, which is not the case with plastics and many other materials resulting in excessive heat generation especially at high velocities.
Therefore, when it comes to friction seismic dampers is better to have bigger friction surfaces and thus, bigger mass.
We have developed frictional surfaces that are protected against corrosion and that undergo and extremely small amount of wear even after many cycles. Friction dampers must be carefully designed to avoid changes in slip load over the life of the building.
Older generations of friction dampers used brass plates against steel plates to ensure a connection with low creep however these kinds of connections could suffer from galvanic corrosion in the long term. This concern lead to provisions in some building codes prohibiting bi-metallic interfaces (FEMA P-1050-1, 2015).
Using technologies originally developed for Aerospace, the Quaketek friction damper uses a low wearing hard interface which has been protected against galvanic and other forms of corrosion, while maintaining low wear and very little creep. The end result, is a friction damper that generates a stable slip force throughout the lifetime of the building.
Every Quaketek friction damper has been tested at the full slip load and full stroke. Part of our production process is a 100% test of every batch. The customer can be confident that the damper will slip at the expected load because we have tested every single one. This testing is not required by the majority of building codes (although the latest Chilean code, NCH3411 now requires it) however it would be irresponsible not to. The founders of Quaketek have been building friction dampers since 1987 and one thing is clear: no matter how consistent your manufacturing process, coupon tests or sample checks, only 100% full scale production testing will ensure accurate calibration and repeatability of performance.
In addition to our 100% production test we invite customers or their engineers to be present during our final inspection and test before shipment from our Canadian manufacturing plant. We are confident in the durability of our dampers and are happy to offer customers the option to schedule periodic tests on them at 10 year intervals.
Structural engineers selecting Quaketek dampers can be 100% confident that they are receiving a quality damper that will behave as expected when an earthquake strikes. The end result is 100% confidence in our product backed by our guarantee.
No, the performance of our friction dampers is not velocity dependent, slip load remains constant and independent of velocity. Up until the slip load is reached, the damper will behaves similarly to other elements in the structure. In fact, they are generally an integral part of it. Similar to any column or beam in the structure, only force and displacement are necessary to predict the seismic dampers’ behavior.
Friction dampers fall into the category of “displacement dependent devices” and performance is not dependent of velocity (ASCE 41-13, FEMA 1050 2015). When using our seismic dampers, the engineer can rely on the fact that the slip force will remain constant regardless of the velocity in the building. This important property drastically simplifies the integration of friction dampers into new buildings or seismic retrofits. The energy dissipated is quite simply the force multiplied by the displacement and is completely independent of velocity. This means that whether the building is oscillating at 0.5hz or 5 hz the energy dissipated is purely a function of the force applied through the distance travelled by the damper, and not the velocity.
Velocity dependent elements
In contrast, when using other velocity dependent dissipation technologies, the engineer must spend considerable effort calibrating the structure against the expected ground motions building velocities and may not be able to reduce the input energy as efficiently as with our seismic dampers.
Contrary to popular belief though, a friction damper is not necessarily velocity independent. A friction damper using certain composites or Teflon (PTFE) in the interface for example can exhibit velocity dependence. As temperatures at the interface rise and approach easily reachable glass transition temperatures, plastics can start to behave more similarly to fluids than solids, causing velocity dependence.
Another common misbelief it that the forces in ductile elements such as yielding braces, BRBs or yielding connectors, will not exhibit velocity dependence. The rate at which a material yields with respect to time is called the strain rate, is well understood and has been extensively studied for more than 50 years (ASTM Journal of Materials Vol 1, no 1, 1966). Discrepancies in yields force as high as 20% due to differences in strain rate are not uncommon.
This will depend on the performance targeted by the engineer. Seismic friction dampers make achieving this performance objective easier, allowing for buildings that re-center even after large earthquakes. It is a common misconception that the building will re-center by the effect of the damping device alone. It is actually the elasticity of the structure’s elements that provide the re-centering forces to return the building to its original position. The main cause of permanent deformations in buildings are damage due to yielding or failure of structural elements. Using damping devices, such as friction dampers, engineers can dissipate the earthquake energy instead of allowing that energy to damage structural elements.
The amount of damage tolerated in a structure will depend on the performance criterion. For operational buildings, structural elements should be kept elastic in order to ensure minimal to no damage to the building. In cases where the performance objective is rather lifetime safety, some permanent deformation is tolerated and some of the earthquake energy will be absorbed by plastification of structural elements. The re-centering capability of a building is therefore dependent on a function of how much of an earthquake’s energy can be stored elastically within its structure and how quickly the energy can be dissipated.
The objective of any damping device is to dissipate energy as efficiently as possible and thereby protect the structure. Friction dampers are most efficiently used within elastic structures which provide the energy storage and some hysteretic damping while the damper provides only energy dissipation.
Self centering dampers
Friction dampers can also be designed to provide re-centering capabilities (energy storage). The problem is that performing the task of re-centering within the damper sacrifices energy dissipation capacity (storing the energy instead). Therefore, designing with them is not as efficient since their hysteretic loops are much smaller than pure energy dissipation type dampers.
The other question the engineer must ask him/herself is: What is the building I’m trying to re-center? This is a consideration that is usually overlooked, especially during retrofits. If the building is going to have considerable plastic deformation after the design earthquake, what is the purpose of re-centering? The other structural elements might be too damaged to be worth re-centering. Is the engineer sure that it will withstand the re-centering forces? Will it collapse in the attempt of re-centering?
For these reasons, it is very important that, from the beginning of the design, the engineer decides on the performance criterion for the building and that this is clear to the client. This is equally important when performance criterion are set by default in code usually being between Life Safety (LF) and Collapse prevention (CP), as the client may not have the same level of understanding as the design engineer of the code.
This understanding of performance objectives help the engineer better understand what task the dampers are trying to accomplish for the building. Usually, when this reflection is done, the engineer realizes that the most cost-effective way to re-center a building after an earthquake is by giving a portion of the earthquake force to the seismic friction dampers and the remaining to the rest of the structure.
The optimal slip force in the dampers produces the highest energy dissipation while transferring the least amount of energy (moments) to the frame. Parametric studies have shown that this force is produced when the dampers take less than 50% of the seismic shear in a given storey. Although you can assign the dampers as low or as high a force as you deem appropriate (especially when using large reduction factors), the closer you are to this optimum slip load the more efficient your design will be. In structural seismic retrofitting for instance, this usually prevents the need to strengthen structural elements and foundations. In the case of new structures, it allows engineers to design more efficient and economical structures.
For quick calculations, use a value of approximately one third of the seismic shear of a given story or less if using small reduction factors. This force divided by the cosine of the angle formed by the brace and the floor will be your Optimum Slip Load.
When computing the shear force in the building you must do it initially without dampers. Then proceed to locate them in the model and apply the parameters in the design section.
Please don’t hesitate to contact our design team to receive support on how to find the slip Load in your project and to how integrate the dampers.
Sometimes the seismic design force has been so significantly reduced by reduction factors (ductility) that it ends up being of smaller magnitude than the wind load in some combinations. When this occurs, the easiest solution is to increase the Slip Load slightly, so that it is at least 30% higher than the wind governing case. By doing this, you avoid over stiffening the building, thus bringing less acceleration and therefore forces in the seismic case.
With this small change, the structure has received a very efficient solution because it has received the exact increase in stiffness it needed, no more. This is because dampers work as common braces below the slip load. Therefore, the building will continue limiting seismic forces at a force only slightly higher than it was previously, bringing the building to an even higher level of performance against the MCE.