To pose a technical question not listed below, please send an email to info@ viscma.com. Please consider that responses will be a consensus opinion and, therefore, may take up to 60 days to develop, depending on the complexity of the issue.

1) An air handling unit is internal spring isolated, and the fan discharge inside the AHU is flex connected to the AHU housing. The indoor AHU is floor mounted to a 6" concrete housekeeping pad in the mechanical room on the ground or second floor of a two story building. Are flex connections required for the supply and return air duct connections to the AHU housing? Is it redundant? What are the pros and cons of having the flex connection?

The answer is YES. If you want optimal reduction of noise and vibration in your facility, flex connectors on the supply and return to the AHU housing are required. Even though the fan in the AHU is internally isolated and equipped with a flex connector, it does not mitigate the casing-radiated noise that travels from the AHU into the building structure and the supply and return ductwork.

Casing-radiated noise, sometimes referred to as panel resonance, is structure-borne noise (vibration) caused by the airborne sound of the fan and other equipment in the AHU impinging on the walls of the AHU. This airborne sound energy along with any turbulence in the AHU causes the AHU housing to vibrate. Flexible connectors between the AHU and the supply and return ductwork will reduce the transmission of vibration from the AHU into the ductwork. For this same reason, we also recommend the use of external vibration isolation. The vibration isolation supplied Internal to isolate the fan only isolates vibrations from the fan. External vibration isolation will eliminate all vibration coming from the AHU housing.

If you think you have or may have a vibration problem, we suggest you contact your local Acoustical Consultant before you start your repairs. A trained acoustical Engineer can help you identify your problem and potentially save you a bundle in the end.

2) My projects are located in areas that do not normally see Earthquakes and I have not had to restrain equipment before, why am I now being asked to do so?

In reviewing seismic events on a worldwide basis, it has been found that a tremendous amount of damage has occurred as the result of only moderately sized earthquakes in low seismic areas (where little or no seismic protection was incorporated during construction). In addition, as the stresses in the earth’s crust increase with time, many areas with infrequent earthquakes have also suffered when larger events have finally occurred. This has been deemed unacceptable by FEMA (Federal Emergency Management Agency), the regulatory group that is driving the code requirements.

3) In the past, although the code specified some requirements for minor seismic restraint where my projects are located, the code officials frequently waived the requirement. Why have they suddenly become stubborn about waiving it on current and future projects?

FEMA, besides being the driving force behind the codes, is also the agency responsible for relief when an "act of god" event (such as an earthquake or hurricane) occurs. Over the last 10+ years, the relief dollars that were spent far exceeded expectations and one of the key findings was that the damage costs would have been reduced significantly, had the rules in place at the time of building construction not been waived. Insurance companies have also been pressuring localities for better enforcement as they have lost literally billions of dollars during recent earthquakes.

4) If I follow the current code standards, will my building weather an earthquake without any problems?

The codes are based on the maximum "likely" earthquake that has a 10% chance of occurrence in the next 50 years. (There is a very small chance that an earthquake can occur that is larger than this.) The codes are also "life safety" based. This means that during the above earthquake, the building and equipment will hold together well enough so that lives will not be lost. There is no requirement (except in some rare cases in hospitals, etc) that the building and equipment will retain its functionality. If continued functionality is required, a performance-based specification should be considered. (Note: A performance-based spec will typically require that all of the equipment specified for the project be shake tested to levels consistent with the design seismic forces to ensure its continued functionality.)

5) The seismic design forces have greatly increased with the advent of the current code in my area. In addition, there is a lot more information that I need to have to determine what force is appropriate. Why is this?

During the 1989 and 1994 earthquakes that occurred in California, considerable information was gathered that had not previously been available. When experts reviewed this data, it was found that factors such as soil type, proximity to a known fault, and equipment elevation in a building had a significant impact in the actual forces. The force equations were modified to take this into account.

6) Why do I not have to restrain piping or ductwork that is hung 12" or less from the ceiling?
Items that are hanging on supports react to earthquakes act as though supported on a pendulum. If the pendulum is 12" or less, its natural frequency is outside the range generated by earthquakes. Because of this, the piping and ductwork will move back and forth during an earthquake, but the motion will not be amplified with time. As long as there is nothing that won’t prevent 3-4" of lateral motion and the hanger rod is allowed to swing freely (includes some kind of swivel), pipe/duct systems have consistently survived seismic events with minimal problems. If this motion is restricted by local obstacles, damage can occur and the system would require restraint.

7) Why do I need to provide restraints on equipment that is hung 12" or less from the ceiling?

Although from a philosophic viewpoint it would seem that the same rules should apply to equipment as apply to pipe and duct, this is not the case. Equipment behaves more erratically than piping or ductwork during a seismic event because it is more local in nature. Large distributed systems like pipe and duct will tend to move laterally or even exhibit relatively minor variations in the vertical loads but will not tend to rock. Localized components like equipment will rock and as such can generate considerably higher tensile loads in the support system. In some cases this may not be an issue, but there are definitely situations where the installation geometry will develop loads that can fail the support system.

8) Why do I sometimes need to add oversized mounting plates to restraints attached to concrete?

The seismic restraint manufacturer has probably designed the steel restraint so that it is stronger than the material it is anchored to. A good example is when we are required to anchor to a thin layer of lightweight concrete. The concrete may not be strong enough on its own to hold the anchors. The addition of a larger mounting plate or embedded steel plate will distribute the forces over a larger area.

9) What is "special inspection" for anchor bolts?

The actual installation of the bolt into the concrete is a critical part of the restraint system. If the bolt is not embedded deeply enough or properly set into the concrete, it could fail in an earthquake. Special Inspection requires an independent authority to watch the installation of the bolt to ensure that it is done properly. Once this is done, for purposes of calculating the correct anchor size, the allowed mathematical value of the anchor can be increased and save the cost of many more bolts.

10) Is it possible that your building could be uninhabitable after an earthquake, even with no structural damage?

The heating and air conditioning system equipment is considered to be a non-structural component of the building. If the building has little or no structural damage but the water piping or air conditioning systems have torn loose or broken down, you still may not be able to go back to work. The resultant loss of revenue could do more damage to your business than the earthquake did. It is important to have a member of the Vibration Isolation and Seismic Control Manufacturer’s Association (VISCMA) design and/or review the anchorage of your systems.

11) You are thinking of replacing your aging air conditioning systems. Should you add seismic restraint devices with the installation?

In the event of an earthquake of a magnitude large enough to cause your air conditioning equipment and piping to break loose, you could lose the use of the building and have the risk of people being injured by falling air conditioning equipment, piping and ductwork. The current building codes are written to protect the lives of the people in your building. It is good common sense as well as a practical business decision to make sure your systems are adequately braced. Many insurance companies recognize the importance of this and often give monetary incentives to upgrade your restraint systems.

12) The mechanical equipment is all located on grade outside the building or in a central plant. Do you still have to have earthquake restraints?

The location of the equipment does not change the requirement for seismic bracing. However, the design forces that we must resist will change based upon the location of the equipment in or around the building structure.

13) Should both internal and external vibration isolation be used in a single application?

It is possible that the internal springs could resonate with the external springs. This could cause damage to the systems. If the application is near a very vibration sensitive area, the internal isolators should remain locked down, as they are when they are shipped. A good rule of thumb is that you can use one or the other, but not both. See "The Pitfalls of Combining Internal and External Vibration to Packaged Air Handling Systems", also located in the VISCMA Articles of Interest section.

14) Why should I use vibration isolators?

All mechanical and electrical equipment generates vibration. Even a small amount of vibration energy traveling through the structure can:

• Create unacceptable noise levels in commercial buildings, schools, auditoriums, etc. For example, government standards now require quieter environments for school children to enhance learning.
• Affect sensitive equipment such as in hospitals, laboratories and high tech manufacturing facilities.
• Lead to damage to the equipment or the structure.

Vibration isolation, installed during construction, will minimize the energy transmitted to the structure and provide an economical solution that leads to better and quieter environments.

Generally, most HVAC, electrical and plumbing equipment can develop disturbing noise and vibrations. Properly selecting an isolation system will minimize direct disturbances to occupants. Indirectly, vibration energy allowed to flow unchecked can travel throughout the structure and reappear unpredicted anywhere in the form of disturbing noise or vibrations. A properly written specification isolating all potentially disturbing equipment and line systems is the best insurance policy for an owner.

Additionally, sensitive equipment can be adversely affected when located near potentially disturbing equipment. Some examples are:
-Equipment within chip manufacturing facilities
-Hospital or health related equipment
-Research facility equipment

Cost to the owner is minimized when potential problems are addressed in the design stage by the dedicated professionals in the field of noise and vibration control. VISCMA was established to ensure industry standards to manufacturer specified products to meet the demands of the specifying engineer and acoustical consultant.

15) If the vibrating equipment is located outside on the ground, do I still need to use vibration isolators?

Even if the equipment is on grade on a separate slab from the building, vibration transmitting to the building structure is still a concern. Also, the addition of isolators will reduce enough vibration that it is possible to add to the effective life of the equipment.

16) When should I use inertia blocks with the vibration isolators?

There are three major reasons for including an inertia mass with the isolation. One is that it lowers the center of gravity of the system. For a tall narrow unit, this could make the installation more stable in a seismic event.

Secondly, some pieces of equipment have large unbalanced vibratory forces. An example might be large water pumps or large medical vacuum pumps. In the case of one of these applications, the large unbalanced forces could cause larger than normal amplitudes, or physical movement, of the springs under normal conditions. Large movements can cause undue stresses to flex connectors and piping connections. The inertia block adds mass to the system so that the same amount of force has to move a much greater mass, and minimize the amplitude of movement.

The third one is that some equipment manufacturers mandate a common rigid surface to minimize or eliminate flex between two components that are connected by a flex drive. 17) I have a structure with long support spans and in order to keep the vibration generated by the roof mounted equipment from causing problems, a spring with more than 2 inches of deflection is needed. When I put equipment on these springs, however, it rocks excessively in a breeze or when I push against it. What can I do to stop this?

The same "softness" in the spring coil that is necessary to reduce the transfer of vibrations into the structure also will allow the equipment to rock more easily. There is nothing that can be done to prevent this. There are things that can be done to minimize the negative effects, however. These are to ensure that the isolated equipment has a low center of gravity; the isolators are spaced as widely as possible; the system is equipped with motion limiting devices; and, if exposed to wind, is protected from direct exposure. This may require the addition of an inertia mass to lower the center of gravity or a surrounding wind barrier protection wall. The use of motion limiting devices should be restricted where possible as, when limiting the motion, they can form a path for the direct transfer of vibrations into the structure whenever the equipment is exposed to a sufficient lateral force.