banging bolts

are often described as soundinglike a high-powered rifle shot.Typically this raises concernamongst the occupants as to thesafety of the building. Thispaper will explore the cause ofrandomly occurring bangingbolts and explain why occupantsshould not be concerned abouttheir safety. The accompanyingpaper describes one investigationand develops a hypothesis for thecause; this paper examines other

aspects of the banging bolt syn-drome.

BACKGROUND

In general, the banging boltphenomenon is attributed to thesudden slip of tightened highstrength bolts (HSB) in a boltedconnection. The bang is a sud-den release of strain energy inthe form of noise. Researchersinvolved in testing HSBs havefor years observed the slip asnoted in various publications(see references in preceedingarticle) and reported in figure5.14 of “Guide to Design Criteriafor Bolted and Riveted Joints” byKulak, G.L., J.W. Fisher, andJ.H.A. Struik, 2nd Edition, 1987,John Wiley & Sons, New York,New York.

In this test example, a slip of0.06 in. or approximately 1/16”, isindicated for a 10 row lap spliceconnection. This amount ofmovement is the differencebetween the nominal bolt diame-ter and the bolt hole. In fieldconnections with similar boltrows, the maximum slip, if itcould occur, would be substan-tially smaller because fabricationand erection tolerances wouldplace some of the bolts dimen-sionally much closer to a bearingcondition, therefore giving thebolts less distance to move whenthe slip occurs.

BANGING BOLT PHENOMENON

The banging bolt phenomenonrandomly occurs in buildings,when bolted connections sudden-ly slip as the applied load over-comes the clamping forceenduced friction in the connec-tion. The author has investigat-ed incidence of this behavior in

Modern Steel Construction / November 1999

By R.H.R. Tide, P.E., S.E.,D.Sc.

ONE OF THE PERFORMANCEPHENOMENA OF OCCUPIEDSTEEL BUILDINGS THAT

RAISE OCCUPANT’S CONCERN is thepossibility of banging bolts.Shortly after a building is occu-pied and for the next year or two,the occupants may hear loud andsharp noises that originate fromwithin the structure. The noises

BANGING BOLTS—ANOTHER PERSPECTIVE

A look at the what and how of this noisy phenomenon

Figure 1: Typical exposure of fire protected moment connections.

building connections that can bedivided into fundamentally threecategories. The first category isa multi-bolt shear connectionsimilar to the one described byRobert Schwein in the previousarticle where the beams supporta composite metal floor system.The second category is the multi-bolt shear connection in industri-al buildings without a floor sys-tem. The third category is amoment connection with multi-bolt web shear connection andwelded beam flanges.

In several projects where thebanging noise was reported,investigations were performed tofind actual connection move-ment. It was observed that thesprayed on fire protection corrod-ed the surface of the beam weband shear tab as shown in Fig. 1.Removal of the fire protectionand corroded mill scale obliterat-ed any evidence of joint move-ment having a magnitude of onlya couple of hundredths of aninch. Subsequent wire brushingas shown in Fig. 2 further hin-dered detection of any jointmovement but allowed examina-tion of the joint to verify thatweld fractures had not occurred.

The randomness of this phe-nomenon occurs because no sud-den slip with the resultant“bang” will occur as long as anyof the bolts in the connectiongroup are already in bearing inthe direction of the force. Insteel framed buildings, the gravi-ty weight typically pulls the sup-ported members down so thatone or more bolts will be in bear-ing. As a result, most buildings,each with hundreds of connec-tions, will not experience thebanging bolt phenomenon.However, whenever the erectionprocedure results in bolts thatare nearly centered in the holeswhen tightened, such as employ-ing drift pins, or other erectionconditions the banging bolt phe-nomenon may occur. Ironically,as snug-tight bolts, slotted holesand shear tab usage becamemore prevalent, the incidence ofbanging bolts appears to haveincreased. The absence of paint


on steel is also considered a vari-able that has increased theoccurrence of banging bolts. Asreported by P.C. Birkemoe, paintallows the connection to gradual-ly slip into bearing without theresulting noise (“High StrengthBolting: Recent Research andDesign Practice.” Proceedings ofW.H. Munse Symposium onBehavior of Steel Structures—Research to Practice, 1983, W.J.Hall and M.P. Gaus, Editors,ASCE, New York, New York.)

CONNECTION FORCES

The connection geometry thatresults in bolts located centrallyin their respective bolt holes sothat slip can occur is one of thecontributing variables to bang-ing bolts. The other critical vari-able is the force that causes thebolts to slip. Actually, a forceexists in nearly every connection.The preceeding article bySchwein has identified concreteslab shrinkage as one of the pos-sible causes of forces that induceslip in the connections of steelframed structures.

Another possibility is the tim-ing of steel beam erection, themetal deck installation andpouring of the concrete slab. Ateach step the steel beam and theconnection at each end will be atsome base temperature.Subsequently, as the fire protec-

tion is applied and the buildingis enclosed, whether the buildingis heated or cooled to a stabletemperature of approximately 70degrees F may result in a tem-perature differential. The tem-perature differential magnitudefrom the time of bolt installationand tightening to enclosed build-ing temperature is then a ran-domly occurring variable. Atemperature differential of 20degrees F or slightly higher isnot unreasonable. For weldedmoment connections with boltedwebs, flange weld shrinkage isanother possible driving forcesource. Finally, as the buildingis occupied, the live load increas-es within the first few monthsand may become the triggeringcondition that causes a few con-nections to bang.

The force due to the tempera-ture differential can be calculat-ed from the basic thermal forcecondition if it is assumed thatthe columns at each end of abeam and the floors above andbelow provide a relatively highdegree of restraint, yet anotherrandom variable. The axialstress in a beam for a tempera-ture change when the framing ateach end is assumed to fullyrestrained is given by:

fa = Eα ∆Twhere:

Figure 2: Complete removal of fire pretection at edge of bolted connection.

fa = axial stress, psiE = modulus of elasticity, 29 x

(10)6, psiα = coefficient of thermal

expansion, 6.5 x (10)-6,in/in/°F

∆T = temperature differential, °F

The appropriateness in con-sidering thermal shrinkage as acause of the bangs can be demon-strated by a simple example cal-culation. A W24x55 beam isframed into shear tabs that arewelded to columns at each end.The beam web is attached to theshear tabs with five ASTM A3257/8”-diameter bolts. The beamarea is 16.2 sq. in. Assume a 20degree F drop in temperaturefrom the time of bolt installationto occupancy.

The axial stress is computedas 3.77 ksi corresponding to abeam axial force of 61.1 kip.Assume that the bolts have beenpre-tensioned to the specifiedforce of 39 kip and that the fay-ing surfaces having a coefficientof friction of 0.3. Therefore, forthe 5 bolts, the frictional force is58.5 kip which is approximatelyequal to the thermally inducedforce. Based on the statedassumptions slip is possible ifthere are no bolts in bearing.The effect of the live load can beaccommodated in the example byslightly changing one of the vari-ables.

In moment connections, trans-verse shrinkage across the weldcan be estimated using the AWSequation (AWS, “WeldingHandbook”, Volume 1, 8thEdition, 1991, American WeldingSociety, Miami):

S = 0.2Aw/t + 0.05Rwhere:S = transverse weld shrinkage, in.Aw = cross sectional area of weld, in2.t = thickness of flange plate, in.R = root opening, in.

Appropriate assumptions forthe variables in this equationand for axial displacement(∆=PL/AE) will show that weldshrinkage can also result in aforce that exceeds the frictionforce developed by the HSB

clamping force.Schwein also raises some pro-

ject specific issues that should beconsidered in the context of otherinvestigations. The beam com-pressive force is attributed to dif-ferential shrinkage in the con-crete slab due to variations inwater/cement ratios. Duringother investigations, crackinghas been observed in slabs alongthe girder lines and hairlinecracking has also been observedat other locations, both of whichcould relieve beam compressiveforces.

Similarly, in only one direc-tion the metal deck could resistthe concrete shrinkage forces.Petrographic examinations of theconcrete in the suspect and adja-cent slabs could predict thewater/cement ratio and the relat-ed differential shrinkage poten-tial accurately enough to isolatethe floor banging locations.Obtaining accurate temperatureand cloud cover records floor-by-floor throughout the constructionsequence would allow one to esti-mate the temperature differen-tial conditions. It is unlikelythat the actual floor-by-floorerection fit-up conditions wouldbe documented. Therefore know-ing the temperature differentialbetween the time of erection andthe time of building occupancyonly allows a qualitative evalua-tion of the induced thermalforces. Knowledge of slab crack-ing patterns, water/cementratios and temperature rangesetc. would facilitate a more in-depth evaluation.

The bolt installation proce-dure is another contributing fac-tor. However, contrary to C.J.Carter (“What are ‘banging bolts’and how do they affect structuralsteel framing?”, SteelInterchange, Modern SteelConstruction, Vol. 39, No. 7,July, 1999)), whether the boltsare installed to a snug-tight con-dition or a prescribed pre-tensionfor a slip-critical connection isnot significant in the occurrenceof the banging bolt phenomenon.For ¾”- and 7/8”-diameter HSBproperly installed by the cali-

brated wrench, turn-of-nut, ten-sion control (TC) or load indica-tor method, the snug-tight condi-tion typically results in bolttensions close enough to the pre-scribed pre-tension for slip-criti-cal connections. As a result, slipunder thermal conditions andservice loads will be a randomoccurrence independent of theinstallation technique.

Loosening connection bolts toinduce banging bolts was anexcellent concept that allowedSchwein to confirm the overallbanging bolt hypothesis.

SUMMARY & CONCLUSION

The reported occurrence ofbanging bolts has become morefrequent in recent years. Thebang occurs when high strengthbolts (HSB) slip into bearingwith the release of strain energyin the form of noise. Because itoccurs so randomly in relativelyfew steel framed buildings con-structed each year and for only asmall percentage of the connec-tions in those buildings, specificdata is not readily available.The randomness can be attrib-uted to fabrication/erection toler-ances that for the most partresult in HSB bearing conditionswhich prevents HSB slippage.The increase in banging boltsincidents also coincides with theincrease in use of single plateshear tabs and slotted holes, aconnection configuration that islikely to have all bolts in a non-bearing condition in the horizon-tal direction. The decrease inthe use of painted steel membersalso contributes to the increasein sudden slippage.

Driving forces causing thebanging bolt phenomenon can beattributed to concrete floor slabshrinkage, differential tempera-ture conditions between the timeof bolt tightening and buildingenclosure, and weld shrinkage inmoment connections. Erectionconditions that require forcedalignment of steel members canalso act as a driving force. Thesubsequent live load that resultsfrom building occupancy withinthe first few months to a year or


so is the triggering force thatcauses the slip and resultingbang. It is probable that the dif-ferential temperature conditionis the primary driving forcebecause banging bolts have beenheard in buildings without con-crete slabs. In one instance, anindustrial building with weldedconnections and without a con-crete slab also experienced thebanging bolt phenomenon.

In conclusion, as the accompa-nying article also indicates, thebanging bolt phenomenon is anindication that a steel framedbuilding is reaching its finalequilibrium position but thebanging is not of any concernregarding design or safetyissues.

R.H.R. Tide, P.E., S.E., D.Sc.,is a Senior Consultant, Wiss,Janney, Elstner Associates, Inc.,Northbrook, IL and also a mem-ber and chair of subcommitteeson high strength bolts for bothAISC and RCSC Committees.


banging bolts

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