metal building systems-reinforcement design

8/10/2019 Metal Building Systems-Reinforcement Design

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By Donald L. Johnson, P.E.

RE TRO FI T P ROJ EC TS CA N B E

NECESSARY FOR ANY NUMBER

OF REASONS, though changein use is one of the most com-

mon. Change of use can involveequipment loads added to roof,or the addition of sprinkler sys-tems or cable trays. In somecases, buildings need to beexpanded in length, width orheight.

Understanding the originalconstruction is a critical part ofany retrofit or reinforcementproject and Metal Building Sys-tems are no exception. MetalBuilding Systems are character-

ized by a primary framing sys-tem of rigid frames fabricatedwith bar or plate flanges andtapered webs cut from sheet orcoil. The flange-to-web weld isusually made on one side, exceptwhen the shear magnituderequires welding on both sides.

Single-span rigid frames canrange from 20 to over 200 clearspan. Modular Rigid Frames,rigid frames with interiorcolumns, can be built to almost

any width and 800 to 1,200 isnot uncommon (see figures 1 &2). Roof slope will vary with thebuilding end use, though themost common slopes are :12and :12. Slopes of 2:12 and 4:12are frequently used on recre-ational facilities and 6:12 and upare used on some church build-ings.

The secondary system usuallyconsists of Z shaped purlins orin some cases C shaped purlins

and a metal roof. The purlins areroll formed from continuous coil

Modern Steel Construction / March 1998

material in thickness from 0.04to 0.14. Depths can range from4 to 16, with 8 to 10 the mostcommon. Z sections will nor-mally have 45 degree stiffening

lips while the C sections use90-degree lips. The 45-degreelips are to permit nesting whenused in continuous systems (seefigures 3 & 4).

Spacing of the primary framesis a function of the secondaries.Simple span purlins are effectiveup to 25 while 30 and 33 baysrequire continuous purlins.Longer spans are accomplishedwith truss type members fabri-cated from cold-formed parts or

with conventional bar joists.Flange and web material in

Metal Building System rigidframes will in most cases have aminimum yield of 50 ksi. Theexception is older buildings(manufactured before 1960) thatusually used 33/36 ksi material.

Purlin and girt material can beexpected to be 50 or 55 ksi yieldmaterial unless the building wasmanufactured prior to 1958. Ifthere is doubt concerning whatmaterial was used, couponsshould be taken and tested.

Before starting a retrofit pro-ject, a designer should obtain asmuch information as possible.Considerable data can beobtained from the manufacturerif it can be identified. So called

standard buildings have notbeen produced for at least 20years; design to order has beenthe standard procedure. Thismeans that without an ordernumber, useful data will be diffi-cult to obtain. The building ordernumber and the name of themanufacturer can be obtainedfrom the owner if the originaldocuments were retained. Othersources would be the dealer whosold the building and the local

building code office, where docu-ments would have been filed forpermit purposes. In some cases,the order number can be foundon frame parts.

With the order number andthe name of the manufacturer itis often possible that the manu-facturer can supply drawingsand design calculations, whichwill greatly simplify the design-ers job. If the manufacturer datais unavailable, the only recourse

is field measurement.

REINFORCEMENT

DESIGN

FOR

METAL

BUILDINGSYSTEMS

Figure 1

Figure 4Figure 3

Figure 2


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LENGTH MODIFICATION

Extending the length of ametal building is very easy if thesite permits it. Additional unitsof frames and secondary mem-bers identical to the original canbe added to the building. The

end frames of the original build-ing must be checked. If expan-sion were contemplated in theoriginal design, the end framewould be the same design as theinterior frame, with no modifica-tions necessary. In the case of anon-expandable endwall, it canbe replaced with an intermediaterigid frame and the existing end-wall parts can be dismantled andmoved to the end of the newaddition.

Non-expandable endwalls arecommonly beam and post con-struction. If the endwall footingswere designed for this condition,they will not be suitable for anintermediate rigid frame, whichcan have a significant horizontalreaction. If this is the case, thefootings will have to be replaced.The other option would be toreinforce the existing endwall.The footing must be checked forthe increased reactions.

WIDTH MODIFICATION

Expanding a metal buildinglaterally is relatively simple withuse of a width extension (figure5). Width extensions (a lean-totype structure) are availablefrom virtually all manufacturers.

It is preferable to obtain thewidth extension from the samemanufacturer who provided theoriginal structure in order tomatch panel profiles, otherwisetransition flashing will berequired. The original wall con-

dition must be addressed. If the


sidewall is left intact with sec-ondary and paneling, the onlything that must be checked isadditional axial load in thebuilding column. If, however, thesecondaries are removed, the col-umn length increases from

approximately 7.5 (in manycases) to the full length of thecolumn. With the increasedunsupported length, the columnwill be seriously deficient with orwithout the additional load. Nor-mal reinforcement consists ofplates welded to both flanges. Ifpossible, the plates should bewider than the column toincrease the flange radius ofgyration. Staggered stitch weld-ing of the cover plates is satisfac-

tory but the width thicknessratios must be checked to pre-vent local buckling.

HEIGHT MODIFICATION

The height of a metal buildingsystem is the most difficultdimension to change. It can bedone structurally; however, theassociated problems in raisingthe roof panels and purlins canmake the project impracticalbased on cost. If simple span

purlins were used, the roof sys-tem can be dismantled a bay at atime, the roof beams raised andthe roof replaced.

The bending moment at therigid frame knee (column-to-roofbeam joint) does not change sig-nificantly when the height isincreased. The bending momentat the ridge or centerline of theframe will need to be reviewedfor possible reinforcing.

Two methods of increasing

height have been used. The firstis to fabricate a new column witha cap plate that matches thesplice plate on the roof beam.The second is to fabricate a stubcolumn with a uniform depthand cap plates on each end tomatch the existing splice plates.

LOADS

There may be the case wherean entire structure is required tobe retrofitted to a higher load;

perhaps the original specifica-tion were incorrect or insurance

rules may require a highercapacity. To significantlyincrease the overall capacity ofan entire Metal Building Sys-tem, virtually every member inthe structure would require rein-forcing. Assuming the load

increase is an overall live orsnow load increase, there are afew cost effective options. Roof system: Install addition-al purlins between the existingpurlin. The addition of continu-ous purlins is usually not practi-cal due to clearance problems,however, simple span purlins ofhigher stiffness can be installedand effectively double the roofcapacity. Care must be taken toprovide sufficient lateral bracing

to the added purlins, particularlyif they are not attached to theroof panel. The stiffness of theroof panel normally provides lat-eral support to the purlin. Insome standing seam roof sys-tems additional braces may beused. If attachment of the purlinto the roof panel is impractical,spacing of discrete bracing canbe computed using AISI SectionC3.1.2

Another method of increasing

the capacity of the roof system isto install an additional framebetween the existing rigidframes. This will reduce thepurlin span by half. The framecan be other than a rigid frame.

A post and beam frame simpli-fies the foundation require-ments, but requires intermediatecolumns. The purlins will becontinuous over the added frameand the reaction will be support-ed by a single web. This condi-

tion will need to be checked forweb crippling using AISI SectionC3.4, combined shear and bend-ing using AISI Section C3.3, andcombined bending and web crip-pling using AISI Section C3.5. Primary Framing: The use ofadded intermediate frames willdouble the capacity of the prima-ry framing as well as increasingthe capacity of the purlin sys-tem. These method doses notrequire any modification to the

existing rigid frames.If intermediate columns can

Figure 5


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be made compatible with theusage of the building, an interimcolumn (or columns) can beadded to the rigid frames. Thismethod will frequently result inless reinforcing than required fora clear span. There will be

moment reversal at the new col-umn locations and bearing stiff-eners will be required at thereactions, but the overall reduc-tion in bending moments willreduce the amount of reinforce-ment.

ANALYSIS

Before a designer can deter-mine the reinforcement detailsfor a Metal Building Systemrigid frame, an analysis must be

made for the required code load-ings plus new imposed loads.Before an analysis can be done,the frame dimensions and mate-rial sizes must be determinedusing information from the man-ufacturer or from field measure-ments. When taking field mea-surements it should beremembered that some manufac-turers use flange segments asshort as 5 and weld seams in theweb can be overlooked unless a

careful inspection is made.A number of commerciallyavailable computer programscan perform the necessary analy-sis. Some packages have directinput of tapered members, buteven the most basic program canbe used by dividing the membersinto 5 segments and inputtingthe section properties for eachsegment. With tapered mem-bers, the critical sections cannotbe determined by inspection.

The objective, when usingtapered members, is to minimizematerial usage by matching eachsection to the moment and shearat that point. Therefore all sec-tions with increased moment orshear over the original designmust be reviewed for possiblereinforcement

FRAME REINFORCEMENT

Flange reinforcement: Themain objective of flange rein-

forcement is to increase the sec-tion modulus of the section. This


can be accomplished by addingcover plates. Welding a platewider than the original insideflange permits the welder toweld in the down portionrather than overhead andincreases the radius of gyration

of the flange. In some casesreinforcement of one flange is allthat is necessary. However, theun-reinforced flange can be overstressed due to the neutral axislocation. In this case bothflanges must be reinforced.Where the inside flange normal-ly has few obstacles to installinga cover plate, the outside flangehas purlins attached at approxi-mately 5 for its entire length.One method utilizes a pair of hot

rolled angles welded to theinside of the outer flange and theweb (See Figure 6).

Web reinforcement:Additionof concentrated loads will fre-quently result in overstress ofthe web. Rigid frames support-ing roof loads are basically flex-ural members resulting in rela-tively deep members. Althoughthe use of tension field is notcommon, frames with stiffeners

to utilize tension field actionmay be encountered. The major-ity of rigid frame webs will havebeen designed per AISC SectionF4 without the use of intermedi-ate stiffeners. The use of AISCSection F4 results in relativelylow values of allowable shearstress and the allowable shearcan be doubled or even tripledwith the addition of intermediatestiffeners welded to the webs.This is an economical and quick

way to solve a web overstressproblem. If the web shear

exceeds that given by formulaF4-1 or 0.4F

y, the only solution is

more area; i.e. doubler plateswelded over the existing web. Flange to web welding:Most Metal Building Systemrigid frames will have one side

welding which works quite wellunder the low shear from a nor-mal roof load. This is not satis-factory when localized concen-trated loads are imposed on thesection. Several checks must bemade at a section supporting aconcentrated load.1. Is the flexural strength satis-

factory?2. Can the web carry the shear

without reinforcement?3. If the load is attached to the

bottom flange, can the webcarry the load in tension andthe weld must be checked forhorizontal shear and shearnormal to the weld?

4. If the load is carried by thetop flange, can the web carrythe load in bearing.?Overstress due to conditions

in 2 through 4 can be eliminatedby a combination of addedflanges to web welds and/or theaddition of a bearing stiffener at

the load point. Shear in the webmay require the addition of morethan one stiffener.

SPLICES

Metal building rigid framesuse splices of two types, the clas-sic cover plate type and the endplate moment splice. The endplate moment splice is the mostcommon.

The cover plate splice usesplates welded or bolted to the top

and bottom flanges with themoment carried by bolts or weldsin shear. ( See Figure 7). Theshear on the section is carried byshear plates bolted to the webs.In some cases, the splice isdesigned for considerably lessthan the capacity of the section,due to the location of the splicerelative to the inflection point.Reinforcement of this type ofsplice is relatively simple. Weld-ing of the plates to the flanges

can increase the capacity or thebolts can be replaced with bolts

Figure 6


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of a higher strength. If the coverplates are too small to carry thetension, supplemental plates orangles can be welded to theunder side of the flanges. Theweb splice can be reinforced byreplacing the plates and/or boltswith higher-grade bolts andthicker plates.

End plate moment connec-tions are more common in rigidframes and many companies usethis type splice exclusively (See

Figure 8). In retrofitting thistype of splice, the bolts, welds,webs and splice plates must bereviewed. A number of differentdesign procedures have beenused in the analysis of end plateconnections. The AISC Manualcontains one method; the AISCSteel Design Guide #4, ExtendedEnd-Plate Moment Connectionsis an excellent reference. Forretrofit, I have used a procedurewhere the moment of inertia and

neutral axis is computed for asection composed of the bolt

areas in tension and the beamcompression flange plus a por-tion of the web in compression.The bolt loads can then be com-puted using simple flexural theo-ry. Given the bolt loads result-ing from the new moment andaxial load on the section, thedesign checks can be made.1. Bolts - If the bolts are over-

stressed, an A490 bolt canreplace the A325 bolts whichare the standard. In extreme

cases the holes can be reamedand larger bolt diameters sub-stituted.

2. Webs The webs adjacent tothe bolts must be checked fortension stress and if the webis overstressed, a stiffener orgusset can be welded to theweb and end plate. The gus-set must be long enough totransfer the tension from thebolt into the web along thelength of the gusset.

3. Welds Web to end platewelds must be checked for thesame load that the bolts weretransferring into the web. Ifthe web is not overstressed,the welds can be increased insize or a gusset can be addedto reduce the load to the weld.

4. End Plates The end platesmust be thick enough totransfer the load from thebolts to the beam or columnwithout being overstressed in

flexure or shear. As in theprevious cases, the most eco-

nomical solution to the over-stress is to add gussets tostiffen the plate.

PURLIN REINFORCEMENT

Concentrated loads are a com-

mon addition to any building.The list of items that could behung from the roof is endless;most concentrated loads are sup-ported from a single point andthis can be a problem. Pointloads on cold-formed purlinsshould be avoided unless theyare quite small in magnitude.Before adding load to a purlin itis necessary to know its capaci-ty. Most roofs will have beensupplied with some amount of

collateral load included. A sim-ple rule of thumb to determinewhat the allowable concentratedload might be is to take theexcess uniform load capacity, ifthere is any, and find the equiva-lent concentrated load. A 25purlin with a 3 psf excess capaci-ty on 5 centers has a total loadcapacity of 5 x 25 x 3 = 375pounds or a concentrated load ofhalf of 375 or 187 pounds. Anadditional problem is the method

of hanging the load. Manyinstallers make the mistake of


Figure 7

Figure 8

Figure 9Figure 10


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using proprietary clamping orhanging devices that damage thestiffening lip and or flange.

There are devices on the marketthat allow an attachment to theweb of the purlin, which is theproper way to support the load.

If the concentrated loadexceeds the capacity of a singlepurlin, the load can be spread toseveral purlins or the purlin canbe reinforced. Spreading theload is more economical thanreinforcing and can be accom-plished by attaching a reason-ably stiff member to the bottom

flange of two or more purlins.Several methods can be used

to reinforce purlins. With simplespan purlins, two areas must bechecked, flexural stress in thespan and shear and crushing atthe support.In span reinforcing: a channelcan be attached to the web orangles can be fastened near eachflange (See Figure 9 and Figure10). Attachment of the reinforc-ing members to the purlins can

be done in several ways, welding,bolts or screws. Welding is agood structural solution, butwelding to the thin materials canbe difficult for a welder not expe-rienced in this type of welding,particularly in the positions thatwould be encountered. Bolting ismost easily done by clamping themembers in place and drilling allthe parts in place. Self-drillingscrews are the most practicalfastener, although there will be a

larger number of screws thanwould be required if bolts were

used. The member being addedshould be predrilled or punchedwith the required number of

holes so that the screw will onlybe drilling through the purlinmaterial. Spacing of the fasten-ers can be calculated using shear

values obtained from the screwmanufacturer or from AISI Sec-tion E4.3. As an example of theeffect of typical reinforcing of ageneric 9 inch purlin, 0.08 inchesthick, refer to Tables 1 and 2.Table 1 lists the properties of thebasic purlin a channel reinforc-ing member and an angle that

can be used for reinforcing, asillustrated in Figure 9 and 10.Table 2 lists the allowablemoment capacity (assumingmember is fully braced) for theun-reinforced purlin and thepurlin with the reinforcing mem-bers attached. In this case thetop and bottom angles was theminimum material solution forthe maximum moment capacity.

Support reinforcing: shearis usually not a problem but

should be checked. Web crip-pling is frequently a problem andshould be checked with SectionC3.4 of the AISI Specification. Ifthe purlin does not have a clip atthe support, one can be added. Ifthe existing clip is inadequate, astronger clip can be installed. Ahot rolled angle is usually themost expeditious solution (SeeFigures 11 and 12).

With continuous purlins, theproblem is compounded by the

purlin splice. Splice length canbe (on each side of the frame) as

little as 6 to 8 inches or as muchas 36 to 48. The review of acold-formed continuous purlinsystem can be a challenge to thedesigner that does not use the

AISI Cold Formed Specificationon a regular basis. Fortunately,several software packages areavailable that will perform theanalysis and review using thelatest AISI Specifications. A listof software packages with

description and data can beobtained from the American Ironand Steel Institute, 1101 17thSt., NW, Suite 1300, Washing-ton, DC 20036 (ph: 202/452-7100; fax: 202/463-6573).


Member Depth (Inches) Width (Inches) Lip (Inches) Thickness (Inches) Area (Sq. Ft.)

Z purlin 9.0 3.0 0.75 0.08 1.285

C Reinf. 7.0 2.5 0 0.10 1.167

Angle Reinf. 2.0 2.0 0 0.15 0.56

Table 1

Member Area Allowable Moment

Unreinf. Z 1.285 sq. in. 89.4 in. kips

Z Purlin plus C 2.45 sq. in. 135.3 in. kips

Z Purlin plus 2-Angles 2.41 sq. in. 182.87 in. kips

Table 2

Figure 11

Figure 12


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Donald Johnson is a Consult-ing Engineer from Wolfeboro,

NH, specializing in structuralsteel applications, includingcold-formed structures. Previous-ly, he was with Butler Manufac-turing Co. and was twice chair-

man of the MBMA TechnicalCommittee.

Figure 13

Figure AA

Because of the disparity

between the positive and nega-tive moments in a continuousbeam system, the positivemoment is usually only criticalin the end bay which normallywill have a heavier member;therefore the positive momentareas in the spans will frequent-ly need no reinforcement formoderate increases in load.

The critical section in a con-tinuous purlin is usually the sec-tion at the end of the lap splice

but is a function of the laplength. The solution is toincrease the lap length to a pointwhere the combined shear andmoment stresses are satisfacto-ry. If the existing splice were 2each side of the frame, a rein-forcing member that is 6 to 7long would probably move theend of the splice out of the criti-cal area. The reinforcing mem-ber could be a channel bolting tothe web of the purlin (Refer to

Figure 13). Relief holes shouldbe drilled in the reinforcingmember to clear the existingsplice bolts. It is important thatat the end of the reinforcingmember, a 2 bolt connection bemade to the existing purlin andthese 2 bolts be spaced verticallyas far apart as possible.


metal building systems-reinforcement design

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