1、68 2008 ASHRAE ABSTRACTModern building codes have brought a revolution to seis-mic restraining of equipment and systems. With this revolutioncomes confusion and installations that are substandard cost-ing the construction industry time and dollars. The anchoringof equipment is well understood and re
2、latively simple. Andthere is a new paradigm to qualify the equipment to operateafter an earthquake for emergency type buildings. But that isa different story. This paper addresses the installation, design,and code issues associated with seismic restraints on duct-work, piping, and conduits. The seis
3、mic restraining of thesesystems is not relatively simple and requires anchorage toceilings/roofs, lateral restraints in the form of cables or struts/angles, and some sort of attachment to the system. For thispaper, the lateral restraints will be referred to as supports andthe vertical bracing of the
4、 system is a hanger. This languageis comparable to the language used in the building codeswhich include both the International Building Code (IBC) andASCE 7-05.INTRODUCTIONThere are two purposes for this paper. The first purpose isto provide some basic design information to size seismicsupports for
5、ductwork, piping and conduits. Basic designinformation can be used to determine the capacity of thesupport. The capacity should be in terms of tension andcompression loads in the lateral support. For example the seis-mic demand will be transferred to the building structurethrough a lateral support c
6、onsisting of a cable or strut that is ata 45 degree angle (plus or minus 5 degrees). All parts includingthe duct attachment, cable or strut, structural connectionbrackets, and anchorage should have capacity to properlytransfer the load (demand) from the duct to the building struc-ture based on the l
7、oad in the lateral support.The second purpose of this paper is to identify the issuesin the current International Building Code and providereasonable changes that will be submitted for adoption. Thispaper could be used to explain the relevant issues andproposed changes to a building code official wh
8、ich may beused as current enforcement. Adoption of these suggestedchanges to be applied in current building projects by theAuthority Having Jurisdiction. Obviously there is someimplied responsibility until these changes either are adoptedor if these changes become standard practice by the engineer-i
9、ng community as the practical way to implement a seismicrestraint design.INTERNATIONAL BUILDING CODE REQUIREMENTSThere are several terms and factors used in throughout thispaper that originate in the current code requirements of ASCE7-05. These terms basically come from the equation thatequates a dy
10、namic force developed by earthquakes and uses aprescriptive method to determine an equivalent static force tobe applied at the center of gravity of any component or systemcomponent. The basic equation provided in ASCE 7-05 is:FP= 0.4 aP/RP SDS IP (1 + z/h) WP/1.4 (1)FP= the equivalent static force t
11、o be applied at the center of gravityaP = component amplification factor; this factor accounts for the natural frequency of the systemSDS= design spectral acceleration; this is the design seismic value at the site specific location in the USPiping, Ductwork, and Conduit Seismic RestraintsJames A. Ca
12、rlson, PEMember AHSRAEJames A. Carlson is Senior Nuclear Design Engineer for Omaha Public Power District, Fort Calhoun Station, Fort Calhoun, NENY-08-0112008, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 11
13、4, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.ASHRAE Transactions 69WP= total weight of component and attachmentsRP= component response modification factor; this is a
14、damping value or a value of the ability of the component to absorb dynamic loads based on engineering judgmentIP= importance factor; his factor is to provide for risk associated with critical facilities like hospitals, emergency response facilities, or hazardous installationsz = height of component
15、attachmenth = average roof height of structureThe factor of 1.4 provided at the end of Equation 1 is a conver-sion from an ultimate strength design to an allowable stressdesign depending on how the system is attached to the primarybuilding structure.Equation 1 is the basic equation to calculate the
16、equiva-lent static force which is directly proportional to the SDS. Asthe SDSvalues go up, the seismic demand increases linearly.There are many areas in the United States with very low valuesof SDS. Applying Equation 1 to high seismic areas where thesystems are located to the roof results in high va
17、lues of FP.Therefore it is important to check the static load calculationwith this basic equation and the other equations that limit thevalue of FPto a minimum and maximum value which is alsoprovided in ASCE 7-05.For applications to ductwork and piping, the basic equa-tion and the equations for the
18、minimum and maximum valuescan be converted into FP/WP. This is a unit less factor thatdefines the relative acceleration as shown in Equation 2. ForFP/WPless than 0.15, the systems may be exempt by excep-tions in the building codes. This paper provides design infor-mation for values of FP/WPgreater t
19、han 0.15.FP/WP= 0.4 aP/RP SDS IP (1 + Z/h)/1.4 (2)DUCTWORKDuctwork, in a structural sense, is a very rigid long hollowsheet metal beam. Different from piping, the horizontaldisplacement of the ductwork in the lateral direction in-between the seismic supports are relatively small and there-fore the n
20、atural frequency is never evaluated. However, theproblems associated with the seismic supports in the construc-tion industry are monumental caused by the ambiguity in thebuilding code requirements.Code Issues for DuctworkThe most current building code (International BuildingCode, IBC 2006) has been
21、revised many times over the past 10years with major updates each time its published. Most of therequirements for seismic supports were moved into a refer-ence document ASCE 7 in 2002. Designing seismic supportsis complex enough without constantly changing the locationof the information and rearrangi
22、ng or renumbering the refer-ence material. With all of the changes, there are still manyrequirements for ductwork that are ambiguous and allowcontractors, manufacturers and design engineers to makeinterpretations which results in applications that do not meetthe codes intentions. Requirements for du
23、ctwork seismic supports originatefrom a SMACNA guideline that was first printed in 1976. Thiswas the first guideline that really addressed seismic supportsfor ducts and pipes. It became accepted by everyone in theconstruction industry. This guideline contained two excep-tions for ductwork; 1) if a d
24、uct is less than or equal to six squarefeet, or 2) if the hangers were equal to or less than 12 inches,than the duct does not require seismic supports.Code requirements for seismically restraining ductworkadopted these same two exemptions used in the SMACNAguideline. These exceptions have become to
25、be known as the12 inch rule and the six square feet rule. The 12 inch rule isclearly defined in the building code for piping such that the 12inches is measured from the top of the pipe to the buildingstructure. The attachment location is not specifically stated forductwork in the building codes. Als
26、o, the requirement for allhangers in the duct run must be 12 inches or less to apply thisexemption as discussed in the code for piping. There is a prob-lem with the 6 square feet rule as well. The six square feet rule exemption is not being applied thesame throughout the county. The six square feet
27、exemptiononly applies to facilities with an importance factor of 1.0 asintended by the building code. There is no exemption for facil-ities with an importance factor of 1.5. However, industry hasdecided that this six square feet rule exemption should applyto facilities with an importance factor of 1
28、.5. This contradic-tion to the building code is what has created many problems.The argument is if there are no exemptions, then exemption forfacilities with an importance factor 1.0 applies to both. In real-ity, by not having an exemption for facilities with an impor-tance factor of 1.5 is really to
29、o conservative and wasteful formany applications.Requiring all the ductwork to be restrained in facilitieswith an importance faction of 1.5, even those small six inchround ducts used in restrooms, is not the answer. A reasonableexemption of 4 square feet for important facilities would reallyreduce t
30、he ambiguity of the codes and reduce the problemswith bidding or installing supports on small ductwork. Theprecedence for an exemption is based on experienced data inthat small ductwork has not failed in past earthquakes. Thebuilding code has another exemption for in-line equipmentmounted in the duc
31、twork less than 75 pounds. The failure ofducts less than 4 square feet, which weights less than 75pounds, will probably cause less damage than the failure of a75 pound device. This exemption should only be allowed if thefailure of the duct does not cause the space used for emergen-cies to be unusabl
32、e.In a perfect world all of the ductwork in an importantfacility would be restrained in order to be operable per thebuilding codes. The California Codes and building officialsallow the exemption of six square feet to be applied in all facil-70 ASHRAE Transactionsities. And these systems have perform
33、ed very well with a fewexceptions. So it makes sense that important facilities shouldhave some exemption. Since the six square feet rule is condi-tionally acceptable, then four square feet should have a slightbetter performance for the risk associated with facilities thathave an importance factor of
34、 1.5.The exemption for the six square feet rule is based oncommercial applications that use light weight sheet metal.There are some installations that use heavier or thicker sheetmetal (i.e., grease ducts). In these cases, a smaller duct mayweigh twice the six square feet duct made with 24 gauge she
35、etmetal, which is not the intent of the building code. A codechange should be adopted to modify the six square feet rulewhich includes criteria to limit the weight per foot.Additional changes to the building codes should alsoconsider the spacing of seismic supports. This information isalso provided
36、in the SMACNA guideline. Since the code usesthe SMACNA guideline for the six square feet rule, why notuse the information in the SMACNA guideline for the spacingof seismic supports. This requires some discussion of the useof the SMACNA guideline. For many years prior to themodern codes (IBC 2000), t
37、he SMACNA guideline hasprovided an application of seismic support systems for duct-work, piping and conduit to service the construction industry.With the modern codes also came cost effective changes to thesupport systems. Today, most of the support systems usecables in typical commercial applicatio
38、ns and do not use thedetails in the SMACNA guideline. The SMACNA guides orstrut systems are still applicable to industrial applicationswhich may require frequent modifications to the pipingsystems. Cables do not facilitate these types of changeswhereas trapeze rigid brackets do allow for piping or c
39、onduitreplacement.The difference between the application of cables and strutsystems or systems used in the SMACNA guide is that cablesystems use a 45 degree angle which are attached at the cornersof the duct shown in Figure 1. The angle of the cable relativeto the ceiling can be 30 to 60 degrees. Th
40、e configuration doesnot separate the longitudinal from the transverse supports asidentified in the SMACNA guide. This basically separates theduct into smaller subsections. There are really no transverse orlongitudinal supports. Each duct section is seismicallysupported as if it is a long piece of eq
41、uipment. This is the leastexpensive layout for attachments to the duct and attachmentsto the primary building structure.The SMACNA guide spacing requirements can be easilymatched up to the Seismic Design Categories. For all intentsand purposes, the SMACNA Seismic Hazard Level (SHL) A(FP/WP= 0.48) is
42、 equivalent to the Seismic Design CategoryC and the SHL AA (FP/WP= 1.0) is equivalent to the SeismicDesign Category D, E, and F. There are a few cases where theSeismic Design Category C is applied to an equivalent SHL B(FP/WP= 0.30). For ductwork, piping, and conduit; the spac-ing is the same for bo
43、th SHL A and SHL AA up to FP/WP =2.0. So for ductwork, the SMACNA can be used as a referenceto set the spacing 30 feet on center. Typically, the contractorwill construct the duct in 4 feet sections. A more practicalspacing for seismic supports should be slightly increased to 32feet on center. Unlike
44、 piping which has a different spacingbased on the size of the pipe, the ductwork spacing suggestedin the SMACNA guideline does not change with duct size.This is because, as stated earlier in the paper, ductwork is rigidand indifferent to natural frequencies. There is a requirementto reduce the spaci
45、ng by 2 if the FP/WP 1.0.The last exemption added to the building codes shouldaddress flexible ductwork. All flexible ductwork less than 16inches in diameter or less should be exempt or if the flexibleconnection is less than 12 inches long regardless of size for fanconnections.The next revision of A
46、SCE 7-05 will hopefully beupdated to reflect the changes previously discussed.Designing Supports for Ductwork SystemsNow that the code requirements are all cleared up, thedesign of these systems is simple. Table 1 can be used to iden-tify the vertical hanger rod size and Table 2 can be used tocalcul
47、ate the total weight of the duct based on the FP/WP. Therod hanger size is an important piece of information to thesupplier of seismic restraint devices. The appropriate attach-ment to the duct relies on the correct size of the hanger rod size.Twenty-two gauge sheet metal weights about 1.7 poundsper
48、 square feet. This includes 10% as recommended bySMACNA to account for hangers and insulation. The weightof a 24 36 in. duct made from 22 gauge sheet metal is about17 pounds per foot. The weight of a 24 24 in. duct made from22 gauge sheet metal about 13.6 pounds per foot. A 33%reduction in area only
49、 reduces the weight of the duct by 20%.Many duct systems can be constructed from 24 gauge sheetmetal which results in a slight reduction of weight. Mostdesigners will use the six square feet rule to determine if a ductrequires seismic supports.It is a simple calculation to determine the seismic load onthe seismic support from the weight per foot, support spacingand the angle of the support cables. This can be determinedusing equations in the chapter 54, Seismic and Wind RestraintDesign, 2007 ASHRAE HandbookHVAC Applications.There is a simple method. The designers will dete
