For a discussion of the issues behind the expanded development of this sequence, refer to pages 16 through 25 of the Energy Design Resources Design Review Design Brief. This design brief is one in a three part series of briefs targeted at promoting energy efficiency HVAC system designs by paying attention to details, reviewing project documentation to be sure the details have been properly documented for implementation, and monitoring the project in the field to be sure that the details have been properly implemented. This series, along with many other briefs and other information describing energy efficient design practices, can be obtained by visiting the Energy Design Resources web site at.
Example:Air Handler Detailed Sequence of Operations
A discharge temperature sensor provides an input to a software based PID control loop. The output of this loop modulates the chilled water valve, economizer dampers, and hot water valve in sequence as required to control discharge air temperature. Sequencing is accomplished in software with separate outputs being provided for each component as indicated in the point list for the project. As the discharge temperature deviation signal increases, the hot water valve modulates from fully open to fully closed. A further increase in deviation signal causes the maximum outdoor and return air dampers to modulate from the full return air/minimum outdoor air position to the maximum outdoor air/no return air position. A further or continued deviation from set point causes the chilled water valve to modulate from fully closed to fully open. A decreasing deviation signal reverses the sequence.
A flexible averaging type sensor sensing the mixed air temperature in each air handling unit provides an input to a software based P only control loop. The output of this control loop is used to override the signal to the maximum outdoor air and return air dampers to prevent the mixed air temperature from falling below 40°F (adjustable). As the mixed air temperature falls below set point, the maximum outdoor air damper is modulated toward the closed position and the return air damper is modulated toward the open position. A rise in temperature above set point reverses the sequence. As long as the mixed air temperature is above the set point of this control loop, the maximum outdoor air and return air dampers are controlled by the discharge air temperature control loop.
A flexible averaging type sensor in the warm-up coil discharge provides an input to the DDC system for monitoring purposes.
The software controlling the hot water valve shall be arranged so that any time a unit is off line and the outdoor air temperature is below 40°F (adjustable), the valve shall be modulated as required to maintain the mixed air plenum at a minimum temperature of 40°F.
At outdoor air temperatures below 53°F (adjustable), the chilled water valve is forced closed regardless of the status of the air handling unit. Similarly, at outdoor temperatures above 57° (adjustable), the hot water valve is forced closed regardless of the operating status of the air handling unit, unless the system is in the warm-up mode.
A software based routine resets the discharge air temperature set point from 55°F (adjustable) to 65°F (adjustable) as the outdoor air temperature varies from 55°F to 0°F (adjustable). The discharge temperature shall never be reset above a high limit of 65°F (adjustable) or a low limit of 55°F (adjustable). The discharge air temperature reset routine shall be disabled and the discharge air temperature fixed at the low limit any time the outdoor air temperature is above 55°F (adjustable) for 15 minutes or more (adjustable) and chilled water is available.
A current switch sensing supply fan motor current provides a proof of operation input to the DDC system for use as required by the various software sequences. Additionally, hard wired and hard piped interlocks piloted by this switch perform the following functions independent of the DDC control system.
1. Closes the maximum outdoor air damper, the relief damper, the minimum outdoor air constant volume regulators, and the chilled water valve any time the fan unit is not in operation.
2. Closes the return air damper any time the fan system is not in operation because the return damper also acts as the unit return smoke isolation damper.
3. Closes the supply smoke isolation damper.
A freezestat is provided on the entering side of the hot water warm up coil. Its sensing element is strung with the mixed air temperature averaging element. The freezestat shall be a manual reset type device and shall perform the following functions if the temperature sensed by any portion of its element falls below 38°F (adjustable).
1. Trigger the DDC controller common safety trip input to cause an AHU-1 safety trip alarm message to be generated a the system alarm printer.
2. Shut down and lock out its associated supply fan regardless of the position of the drive hand-off-auto or invertor bypass selector switch.
A software based interlock overrides the discharge temperature control loop and closes the maximum outdoor air dampers and opens the return air damper fully if the outdoor air enthalpy rises above 28.5 btu/lb (adjustable) or the outdoor air temperature rises above 80°F (adjustable). The temperatures and humidities used for this interlock are global values obtained from the existing sensor system.
A constant volume regulator system shall be provided for the minimum outdoor air requirements of the air handling unit. If the air handling unit is in operation, the unit shall regulate at the scheduled minimum outdoor air flow rate. If a tenant served by the unit goes into its unoccupied cycle, this set point shall be reduced by the amount of the design minimum flow rate for the department. If the unit shuts down, the proof of operation interlock circuit shall close the regulator completely.
Seismic restraint of equipment and systems is becoming an area of increasing awareness as information gathered from recent earthquake experiences is integrated into code and construction practice. This issue is quite critical in many areas of the country and proper implementation of seismic restraint is generally not well understood in the field even though most building codes now dictate some sort of protection based on the local seismic zone. In most instances, there is little functional testing associated with the seismic restraint system. Generally, the elements are passive unless there is a seismic event. Proper design supplemented by proper installation and verification via field inspection and construction observation (i.e. verification checks) is the only assurance of proper performance when the time comes.
Failure to perform on the part of the restraint system can lead to disastrous results as can be seen from some of the illustrations included in this section. At a minimum the failures represent a waste of the capital, materials, and other resources used to provide the inadequate restraint. In addition, the failure will usually result in significant damage to the equipment the restraint was intended to protect, damage to equipment and systems in the immediate vicinity, collateral damage to the building materials and structure due to water our other fluids released when the system failed, and loss of revenue during the down time associated with the system failure, which often can be significant. In some building environments, these failures can be life threatening, either directly in terms of harm from the materials and equipment that comes loose from its mounts, or indirectly due to hazards created by failure of the systems, such as loss of life support in a health care environment or release of hazardous materials in a production environment.
In the picture on the left, the supports were selected and installed correctly, but the pad was not attached to structure. As a result, the pad jumped around and shattered in the earthquake, resulting in a failure of the system and equipment, even though the correct isolators were used. In the picture on the right, the duct and rooftop unit supports were designed for the vertical structural loads, but not the horizontal seismic loads. As a result, everything shifted during the earthquake and the system failed. Neither machine will be returned to service quickly. (Images courtesy Mason Industries) It is often the details of the design and installation of the seismic restraint and support system that will determine whether or not they succeed or fail in an actual event. This can be a little insidious since the differences between something that will work and something that will not work are often subtle and may even be hidden from view at the completion of construction. Thus the ultimate success of the restraint system can be highly dependent upon the installers and people performing the field inspections of the installation who often have the least information available to them to assist them with this task. The inspection task may or may not be delegated to the commissioning provider but should be the responsibility of someone who is actually available on site during the construction process to observe and inspect the equipment. The commissioning provider is a likely candidate for this function since site inspection and construction observation is typically a part of their work and they take a holistic view of the overall building and its systems. The commissioning provider should consider coordination of this activity in their plan and project budgeting at a minimum unless their contract asks for additional effort.
The structural designer associated with the seismic restraint system may require field tests targeted at verifying that the installed anchorage systems achieve the rated load capability anticipated by the design. While the commissioning provider may not have to write these tests, they may have to witness them in conjunction with the structural engineer in a manner similar to other more common verification checks and start-up tests like piping system pressure tests and flushing. The commissioning plan and budget should reflect these items.
From an operation and maintenance/continuous commissioning standpoint, some seismic elements require adjustment and may even require periodic inspection, testing, and other maintenance to ensure the persistence of their intended function. Examples include:
· Adjustment of the seismic snubbers on vibration isolators.
· Lubrication and adjustment of expansion and flex joints in piping and duct systems.
· Testing and verification of gas and fuel seismic shut-off valves.
Seismic restrain is one area that is not well addressed by the verification checks included in the CTPL. While a detailed discussion of this topic is beyond the scope of the FT Guide, the following information is intended to provide guidelines for commissioning providers charged with this task. Additional more detailed information can be found in the resources listed in Appendix C: Resources under this topic. The following discussions will apply in one way or another to the components discussed in all of the other chapters of the guide. Topics covered include:
· An overview of with regard to seismic restraint.
· A discussion regarding the accommodation of while still providing the necessary seismic restraint.
· used by seismic restraint systems to anchor to equipment and structure as well as between suspended duct, pipe and conduit systems and fixed equipment.
· Seismic restraint issues associated with .
· Seismic restraint issues associated with .
· Seismic restraint issues associated with .
· Seismic restraint issues associated with .
Generally, seismic requirements are set by requirements in the building codes that govern the location where the building is constructed. Since the forces generated in an earthquake can vary radically with the intensity of the quake and other factors, a trade-off must usually be made between:
· The force you are going to design to restrain against.
· The likelihood that such a force will occur (both frequency and magnitude).
· The risk and loss associated with the failure of the system to be protected if the restraint fails because it was subjected to a force greater than it had been designed to with stand.
For example, a domestic hot water recirculation pump serving an office building would, in most instances, not rate the same level of protection as the boilers serving a hospital, due to both the service provided as well as the nature of the building that the equipment is located in.
In 1999, there were three model building codes in general use in the United States:
· The BOCA National Building Code published by the Building Officials Code Administration (BOCA)
· The Standard Building Code published by the Southern Building Code Congress International (SBCCI)
· The Uniform Building Code published by the International Conference of Building Officials (UBC)
The International Building Code (IBC) represents a merger/modification of these codes. Most of the language in these codes evolved from the provisions of the National Earthquake Hazards Reduction Program (NEHRP) that is funded by FEMA (Federal Emergency Management Agency). Most of these codes go through cyclic revisions and most jurisdictions use a version of one of these codes although it may not be the most recent version. However, it is anticipated that most jurisdictions will ultimately adopt the 2000 IBC code to ensure financial backing from FEMA in the event of an earthquake.
Most codes provide a prescriptive equation that is to be used to calculate a horizontal seismic load that must be resisted by the restraint system. Typically this equation takes the following factors into consideration:
· The peak velocity that is anticipated for a quake in a given seismic zone. Generally, this factor will directly or indirectly take into account the location relative to a fault line and the soil characteristics at the site.
· The nature of the function the building performs and the importance of the equipment that is being protected relative to that function. For example, hospitals would be evaluated with more importance than offices and the life safety equipment in the hospital would be evaluated with more importance than the snow melting system. Some equations have a coefficient for each factor and some use one factor to account for both.
· The nature of the equipment’s mounting. Rigidly mounted equipment will generally not require as much restraint as resiliently mounted systems because the resilient mounts can amplify the earthquake forces due to resonance and other effects.
· The weight of the equipment.
Some codes specifically account for other factors such as the nature of the anchor used and the elevation of the equipment in the building relative to the overall height of the building into account. Others do not account for these factors directly. In addition, some codes also calculate a vertical force that must also be restrained while others only calculate the horizontal force.
As you might suspect, the different approaches will yield different results even when applied to the same item of equipment in the same location experiencing the same earthquake. This is illustrated in A Practical Guide to Seismic Restraint, published by ASHRAE., which was developed from examples in the
Seismic restraints need to be installed in accordance with the manufacturers instructions and should not place any stress on the equipment, duct, or piping systems they are installed on. It is important that the structure to which the restraint is anchored is suitable for the load that will be imposed by the restraint in a seismic event. This may mean that the contractor performing the bracing needs to submit his design and loads to the structural engineer for review prior to installation.
This table shows the results of a calculation for a boiler rigidly mounted on grade and on the roof and a pump on vibration isolators on grade and on the roof under several common building codes. Notice that all codes have similar results for the boiler mounted rigidly on grade, but show significant differences for the other conditions. Generally these differences are related to how the various codes take building height and soil conditions into consideration.
Most code requirements are aimed at keeping equipment and systems in place to prevent injury and minimize collateral damage. However, restraining a machine to prevent injury and damage may not necessary guarantee that the machine will survive the event in an operable state. There is a movement towards requiring manufacturers to submit seismic certification for their equipment. This information can then be used to develop and finalize the seismic restraint design to ensure that the equipment not only survives intact, but also is functional after the event.
On the left is a pump supported by spring isolators in a seismic housing with elastomeric grommets to limit impact loads in a seismic event. On the right is a fan that is mounted on conventional springs supplemented with snubbers that limit motion in a seismic event but allow the unit to float on the springs in normal operation. (Images courtesy Mason Industries)
In most locations, nearly all major equipment will require some sort of seismic restraint. It is also common for major equipment with rotating or reciprocating parts to require some sort of vibration isolation. Generally, these requirements are in conflict with each other. Equipment that is mounted on spring isolators must be restrained in a manner that allows the isolator to function under normal operating conditions but limits the motion during an earthquake. This can be accomplished using a spring isolator that has built-in restraints or via a combination of standard isolators supplemented by seismic snubbers to limit motion. These two approaches are illustrated in.
Bolt holes and other locations where there could be relative motion that resulted in impact loads during a seismic event should have the impact surfaces protected by an elastomer of some sort arranged to allow only limited motion before the elastomer is contacted by the hard object.
Spring isolated pipe hangers also require special considerations to allow them to remain intact through a seismic event.
(Image courtesy Mason Industries)
It is important for the connections to machinery to be designed to accommodate some motion during an earthquake. Failure to do this can place significant stress on the machinery and result in failures as can be seen from. Not all flex connectors are created equal in this regard. Some metallic connectors have been know to prevent equipment failures during an event but leak subsequent to the event due to metal fatigue.
It is also important that the connections between the seismic restraints and structure and the seismic restraint and the equipment to be restrained are properly designed and installed. The are a variety of anchoring systems in use in the construction industry and some are better than others when it comes to withstanding seismic loads. Ideally, anchors should be case in place. However, this is not possible in many situations, like a retrofit for example. In addition, a cast in place anchor can pose its own problems in terms of alignment and matching the actual equipment requirements when the equipment shows up. As a result, post-installed anchors (anchors installed subsequent to the concrete pour) are used extensively. This is one area where testing may be specified by the structural engineer.
(Image courtesy of Mason Industries)
It is also important to remember that seemingly minor differences in materials, the method of connection, or the location of a connection can make major differences in how a restraint system will perform. Aluminum and gray cast iron will probably not perform as well as ductile castings or steel housings. Field welds may affect the strength and performance of the material that is welded and may not produces as consistently predictable of a connection as a bolted connection. The location of a connection point relative to the center of gravity of the equipment or system it serves can make a significant difference in the loads it must handle.
Anchor spacing and edge distance are important parameter to consider when installing this type of anchor system. If these factors are ignored, the connection to the supporting structure can fail during a seismic event as can be seen from. Housekeeping pads are especially prone to this type of problem.
Several things need to be considered with regard to housekeeping pads in relation to seismic anchors. For the pad to be effective in holding the equipment in place it must be properly reinforced and attached to the supporting structure. Otherwise, the pad can move and/or fracture during the seismic event and the equipment will experience failure, even though the restraints and anchors were properly selected and installed. This is an obvious case where construction observation provides the only way to verify that the pad is in fact tied to the supporting structure. Edge distance between the anchor and the edge of the pad is also an important consideration.
Bracing for ductwork, piping, and conduit is generally intended to:
· Protect the duct and piping connections to the equipment it is associated with.
· Protect the connections to coils mounted in the duct system.
· Prevent swaying of the duct and piping system in a seismic event so that collateral damage to surrounding equipment and systems is prevented.
Note the use of cable style sway bracing to limit motion of vibration-isolated equipment without defeating the vibration isolation function.
(Images courtesy of Mason Industries)
Most duct, pipe, and conduit systems will require some sort of restraint although there are exceptions for smaller lines and lines located on short hangers from the supporting structure. Vertical risers required some sort of support and isolation at each floor level designed to accommodate motion while retaining the alignment of the piping. Typically, motion needs to be limited in all directions; vertically (up and down), longitudinally (back and forth in the direction of run), and transversely (side to side relative to the direction of run). The spacing of the braces usually is a function of the acceleration input from the seismic event and the type of joint used in the system. Typically, two transverse braces will be required for e longitudinal brace. For transverse braces, spacing requirements will often be in the 20 to 40 foot range. Longitudinal brace spacing will typically be in the 40 to 80 foot range. Vertical bracing is generally required at any transverse or longitudinal brace location. Regardless of the length of run, each straight run of duct, pipe or conduit will usually require two transverse braces (one at each end of the run) and one longitudinal brace. Transverse and longitudinal braces can occur at the same location, and in many cases a transverse brace located near a bend run can serve as a longitudinal brace for the run leaving the bend as long as the offset (distance between the centerline of the run and the transverse brace location) is not to great. Vertical drops will typically require bracing at or near the location of the drop. Flex connections to the equipment are generally required to protect the equipment connections from excessive loads and additional bracing of the vertical drop may be required if the change in elevation is large.
There are a variety of techniques used to provide the necessary bracing. Rigid angles or channels and flexible cables secured to the system and structure at an angle between 30° and 60° are two of the most common approaches used. The rigid angle/channel approach has the advantage of requiring fewer connections to achieve the necessary restraint. For example, at a transverse brace location, one properly sized rigid brace can limit side to side motion completely where as two cables would be required, one on each side. This is because the rigid brace can be designed to resist a tensile load as well as a compressive loads where-as a cable can only resist a tensile load. However, the need to absorb a tensile load often limits the practical length for a rigid channel or angle. In addition, when resisting the tensile load associated with a seismic event, the rigid brace will place an additional tensile load on the hanger rod (beyond the gravity load), which may exceed the hanger rod’s capacity or the capacity of its attachment to structure. Cable braces eliminate some of these problems and these advantages, when combined with the installation flexibility the offer, frequently make them the system of choice..
The pictures to the left and in the center illustrate cable and rigid channel type all direction restraints. Notice how two cables are required in each direction vs. one rigid channel in each direction since the channel can handle loads in tension and compression where-as the cables can only handle loads in tension. The picture to the right illustrates a rigid style transverse brace. Notice the different attachment points for the rigid channel to the clevis. The different attachment points can place different loads on the clevis and hanger rod and are not necessarily equivalent. Notice the clevis bolt reinforcements and hanger rod reinforcements that are visible in all of the photos. The clevis bolt reinforcements prevent the clevis bolt from buckling under compressive loads applied by the restraint. The hanger rod reinforcements perform a similar function for the hanger rods when they are subjected to compressive loads by vertical acceleration forces during the seismic event.
(Images courtesy Mason Industries)
Walls and floors can also be used as braces for duct, piping, and conduit systems assuming they are constructed of materials with sufficient capacity to resist the loads imposed. Generally, pipes, especially pipes full of water, will impose much bigger loads on a wall used as brace as compared to a duct. One notable exception to this guideline is related to runs penetrating a fire or smoke rated wall or floor because the use of the separation as a brace could compromise its other life safety functions. In addition, ducts at these locations will have a break away connection associated with the fire or smoke damper associated with the separation, making the wall or floor for use as a brace regardless of its load handling capability.
When inspecting duct bracing, it is important to remember that the point where a sway brace connects to the duct can make a significant difference in the load the brace will see. In a seismic event, the duct will tend to rotate around its center of gravity, and this rotation will place a variety of tensile and compressive loads on the sway brace. A sway brace that is suitable for restraining a duct when it is connected to the top of the duct may not be suitable for the loads the same duct would impose on the brace if it was connected to the bottom and visa-versa.
SMACNA (Sheet Metal Contractors National Association) standards provide excellent guidance with regard to the seismic bracing of ductwork and related equipment. Often, sheet metal straps are used to support ductwork. For this approach to perform in a seismic event, it is important that the duct is attached to the straps to prevent it from being thrown out of the straps due to the seismic motion. An alternative approach involves supporting and attaching the duct to trapeze style hangers and then providing sway braces to prevent motion in all directions.
The following general guidelines apply to sway bracing and supports for duct, pipe, and conduit systems.
1 Each individual trapeze may require sway bracing. Where installed, trapeze sway bracing needs to be arranged to limit motion in all directions and the ducts, pipes and conduits need to be attached to the trapeze system.
2 For insulated lines, special insulation detailing may be required to accommodate direct connection to the pipe or duct by the sway brace while preventing condensation or excessive temperatures on the sway brace due to conduction heat transfer from the brace to the duct or pipe. This is usually an issue with ducts carrying cold air, like supply ducts or outdoor air ducts and pipes carrying chilled water, domestic cold water, or steam.
The duct above used to be in the hangers prior to an earthquake. Proper sway bracing and attaching the duct to the hanger straps could have prevented this failure
(Images courtesy of Mason Industries)
3 Vertical hanger rods need to be able to absorb compressive loads as well as tensile loads. Short rods have some compressive load capability, but longer rods will require channels or angles to be clamped (not welded) to the hanger rods to accommodate the compressive loads.
4 Where beam clamps are used to secure hangers to the flanges of beams, the beam clamps need to be provided with a seismic hook arranged to engage the opposite side of the flange and prevent the clamp from being shaken loose from the beam.
5 Ductwork will often require stiffening at the brace location if it does not occur at a joint.
6 Systems with significant expansion considerations need to have the longitudinal braces designed in a manner that takes this into account. Generally, the braces will need to be coordinated with the design of the expansion anchor and control system, and often the result will be one longitudinal brace located at the midpoint of the run.
7 Ducts, pipes and conduits crossing building seismic joints may require special consideration in order to accommodate the motion at the seismic joint. This type of condition is especially common in buildings that have undergone numerous expansions over a period of years. Hospitals and other health care facilities are good examples. Differences in footing designs and structural systems can result in unusual motion at these interfaces during and earthquake.
8 Longitudinal pipe braces and braces at risers need to be connected directly to the pipe, duct or conduit. Otherwise, the pipe might slide in the support, eliminating the benefit of the brace.
9 Stacked trapezes that share hanger rods should be independently braced.
10 The fasteners used to connect the brace to structure and the brace to the trapeze, pipe, duct, or conduit should be swivel type fasteners to accommodate the rotation that will be imposed on the connection by the seismic event induced motion without over-stressing the connection.
11 Systems that are suspended on vibration eliminating type hangers should only be braced with cables to ensure that the intended level of vibration isolation is achieved. Installation of the vibration isolator is critical to the overall performance of the system in a seismic event since the isolator represents a break in the compressive load handling capability of the system. Generally, the isolators should be mounted directly to the structure and should include arrangements to limit the motion of the isolated portion of the hanger.
12 For no-up piping systems like cast iron or glass pipe, special for band clamps are available that can provide an added measure of safety with regard to joint integrity and may be desirable where lines run over critical areas like surgeries or computer rooms.
13 The connection of branches to horizontal risers needs to be designed to prevent excessive stress due to the differential motion between the riser and the connection during a seismic event.
14 Risers in open shafts should be provided with access doors at all support locations. Access flooring and catwalks in the shaft may also be desirable.
15 Flexible connectors installed in piping systems to accommodate seismic motion should be arranged so that they can be easily isolated from the system for repair or replacement. This may require additional service valves beyond what would normally; be provided.
(Image courtesy of Mason Industries)
16 Installing a stiffener on the clevis bolt for clevises at brace locations is critical for maintaining the integrity of the restrain for clevis hangers on piping systems. ( )
17 Generally, cable braces and rigid braces should not be mixed on any individual run of pipe, duct or conduit.
18 In addition to the seismic considerations, riser supports need to address fire separation issues, alignment and guidance issues, and vapor sealing issues.
Suspended equipment will typically require transverse and longitudinal restraint. Generally, the goal of the restraint system will be to limit the motion of the equipment in a seismic event. The guidelines for sway bracing this equipment are similar to those listed above for duct, pipe, and conduit systems. It is important that the braces for equipment all be connected to structure that will react in the same general manner in a seismic event. For instance, it would be best to not connect a transverse brace to a wall and the longitudinal brace to a roof or floor at the same brace location.
The unrestrained spring isolator supporting this generator experienced enough vertical displacement in the earthquake to cause the spring to be released from the housing, ultimately leading to a failure of the generator. The picture on the right is a close-up of the isolator in the picture on the left.
(Image courtesy of Mason Industries)
For floor-mounted equipment, the overall goal of the restraint is to keep the equipment in place and prevent it from overturning in a seismic event. The equipment will tend to want to roll over in a seismic event because its center of gravity is above the mounting point. Equipment with a high center of gravity will have a greater tendency towards turning over than equipment with a low center of gravity, and thus require larger restraints and anchors. Similarly, equipment with an asymmetric center of gravity will have the size of the anchors governed by the worst-case condition associated with the eccentricity. Rigidly connecting the equipment to the structure is the simplest approach to the restraint issue, but is not possible in many instances due to the need for vibration isolation, as was. This can lead to failures if not properly handled. ( ).
Regardless of the exact mounting arrangement, it is important to limit the clearance between the mounting holes provided in the equipment and the anchor. Clearances in excess of 1/8 inch can cause shear failures of the anchor bolt due to the impact load generated as the equipment shifts through the clearance during an earthquake. Gaps in excess of 1/8” should be filled with epoxy, a neoprene grommet or motion should be limited by welding a washer sized for and installed over the anchor bolt to the equipment base.
(Image courtesy of Mason Industries)
Wind loading as well as seismic loading can impact the selection of the mounting and restraint arrangements for roof-mounted equipment. Most roof-mounted equipment is mounted on a curb, typically ranging in height from 12 inches to 24 inches. To be effective in a seismic event, most curbs over 8 feet in length will require cross braces to transfer the loads from the top of the curb to the bottom at the points where it connects to the roof. The connection of these cross braces as well as the connections at the corners of the curve is critical to the success of the curb in a seismic event. This can be difficult to verify once the unit is set in place on the curb.
Many curb mounted vibration assemblies will not with stand the uplifting and overturning seismic forces generated in most seismic zones, especially if the building height is significant (). Steel systems will perform better than aluminum systems due to the strength of the steel, but additional restraints may be required in all cases to address all but the most modest seismic loads.
It is important that both the attachment of the curb to the unit as well as the attachment of the curb to the structural deck be addressed in terms of seismic loading. Due to the wide variety of roofing systems, structural support systems, curve designs, and insulation systems, the detailing required to accomplish this will be project specific. If the ducts, conduits or pipes associated with the roof top unit, cooling towers, condensing units or other roof mounted equipment are supported by the equipment, then the weight of these items (including the weight of any liquid they may contain) needs to be considered in the design and sizing of the restraint and mounting system.
Condensing units and cooling towers are typically not mounted on curbs due to the need to allow free circulation of air and to make piping connections to the basin. The seismic restraint of these systems is usually complicated by the need to provide vibration isolation due to the fans that they contain. The following general considerations apply:
1 Field erected towers deserve special consideration and should be designed and analyzed as a part of the building structure instead of something that is attached to the structure.
2 Column extensions supporting condensing units and cooling towers should be designed to transfer the seismic loads to the building structure and may require considerable cross bracing to accomplish this.
The rails and springs in these installations rolled over in the seismic event, resulting in the failure of the system
(Images courtesy of Mason Industries)
3 The vibration isolation rails typically supplied with this type of equipment may not be adequately restrained to resist the horizontal forces associated with an earthquake ( ). Additional horizontal bracing and snubbing may be required to address this issue. Steel frames with vibration isolators and snubbers may represent a desirable alternative.
4 The legs associated with air-cooled condensing units may require cross bracing to resist the horizontal seismic loads imposed upon them in a seismic event.
Many industrial sites have production processes that are quite complex compared to typical HVAC processes. Additionally, many of the materials used in these processes can be hazardous. Assumptions that would be made casually on a commercial site may be wrong or even life threatening on an industrial site. For example, on most commercial sites, you could safely assume that the puddle of liquid on the floor next to a tank is some water that leaked out of the tank, escaped from the system via a vent or drain, or was the result of condensation. On many industrial sites, the safer assumption would be that the liquid was not water and in fact could be an acid, caustic or other chemical. On the industrial site, you probably need to call the Environmental Health and Safety engineer or the lead operator and report what you have found and steer clear of the puddle. When working on an industrial site, you may want to ask for an escort familiar with the site.
Because industrial sites have higher risks, most Owners and their insurance carriers require that personnel working on the site go through training that will inform them of potential dangers and emergency response procedures. Many times, this training is site specific and quite extensive and can involve several hours or even days of classroom and practice time. It is important that commissioning providers become aware of the nature of these requirement while developing their bid or proposal. Similar training requirements exist on many sites for working in the process areas if the areas themselves are special environments. Semiconductor and pharmaceutical clean rooms are good examples of this type of location. Safety issues aside, there are often specific procedures that need to be used in these environments in order to prevent contamination of the product. Many Owners of these types of facilities will require that all contractors working in the facility go through training that familiarizes them with the general science behind their clean room environment as well as the gowning procedures and work procedures associated with maintaining that environment on their site. Again, if you as a commissioning provider are bidding or proposing work on a site with this type of facility, you should spend some time prior to finalizing your budgets and schedule to understand the training requirements and make sure you have accommodated them.
 A Practical Guide to Seismic Restraint, page 3.