Assessment of Existing Structures Using Cyclic Load Testing

Transcription

1 Assessment of Existing Stuctues Using Cyclic Load Testing Case studies illustate pocedues ecommended in ACI Committee 437 epot By Taek Alkhdaji Nestoe Galati and antonio nanni When a building is enovated fo a change of use the load-caying capacity of the stuctual system must be established. Load testing can be used to povide eliable veification that a given stuctue can safely suppot the calculated design loads. Pe ACI 38 the test load magnitude (TLM) is equied to be eached in at least fou load incements. A set of esponse measuements (mainly deflection) is taken afte the total test load has been applied and afte at least 4 hous of sustained loading. A final set of esponse measuements is also equied 4 hous afte the test load has been emoved so the total duation of the load test can exceed 7 hous. When multiple load tests ae equied to veify the capacities of multiple elements o configuations significant delays and expenses can be incued. In the past yeas eseaches and pactitiones in the U.S. have been evaluating an altenative load test method. 3 Known as the cyclic load test (CLT) method this pocedue equies the application of multiple cycles of loading and unloading (typically six). Stuctual adequacy is then veified by examining the lineaity of the measued deflection esponse and magnitude of the pemanent defomation afte the load has been emoved. CLT investigations ae typically conducted using hydaulic ams that allow the test membe to be quickly unloaded at any sign of distess impoving safety and educing the isk of oveloading o damaging the stuctue. Using hydaulic ams may also be moe economical than using weights. Although hydaulic ams equie eaction systems that can be expensive and time consuming to implement the labo equied to apply gavity loads using dead weights can also be expensive. The CLT method is discussed in geate detail in ACI 437R 4 and ACI 437.R. 5 In the following case studies the CLT method was used to veify analyses and capacities of existing stuctual components. The method was also used to evaluate stuctual behavio afte membes wee stengthened using extenally bonded fibe-einfoced polyme (FRP) einfocement alone o in conjunction with a einfoced concete topping slab. Case studies Fo both of the stuctual investigations descibed heein a building floo was evaluated fo a change in use. Following industy ecommendations existing conditions wee assessed by studying existing dawings epots and calculations; and the infomation was veified using on-site inspections. 467 In each case the assessments showed that stengthening would be equied. Also in each case the CLT method was selected to minimize the duation of the testing pogam as testing was equied both befoe and afte stengthening. Concete intenational / apil 39

2 Pe ACI 437.R-7 5 ecommendations loading and shoing systems wee designed to ensue safety pevent collapse of the test membe and avoid damaging adjacent stuctual elements. Also pe ACI 437.R-7 5 acceptance citeia including deflection epeatability pemanency and deviation fom lineaity wee used to examine the pefomance duing and afte the load test (Fig. ). National Institutes of Health Libay Level B of Building 38 at the National Institutes of Health (NIH) in Bethesda MD houses the National Libay of Medicine. This level was being enovated to accommodate a new high-density filing system and a new caousel Index Calculation* Limit Repeatability I I R R = Pemanency I P Deviation fom Lineaity I DL.95 I R.5 I P = I P. ef I DL <.5 = imum deflection in Cycle unde a load of P = esidual deflection afte Cycle unde a load of P min = imum deflection in Cycle unde a load of P tan α i = slope of secant line on load deflection envelope tan = slope of secant line fo peak on fist loading cycle α ef tan α i I DL = tan α * Measued values used in calculations: = esidual deflection afte Cycle unde a load of P min P = imum load level achieved by Cycles and P min = minimum load level achieved at the end of Cycles and Fig. : Acceptance citeia pe ACI 437.R in. (a) in. Load points [ J H Load line Citical coss section Load line G [ 3 9 in. 8 in. in. shelving system to stoe micofoms collections. The elevated slab was oiginally designed fo psf (4.7 kn/m ) live load. The filing system equied that the slab be upgaded to a live load demand of psf (9.6 kn/m ). Duing the site investigation phase flexual cacks wee obseved on the top of the slab. The discovey pompted NIH officials to equest load tests to veify the capacity of the existing floo slab pio to stengthening. The load test was designed to induce loads mimicking the oiginal design loads on cetain aeas of the slab. Level B compises a.5 in. (65 mm) thick concete flat plate slab einfoced with 4 ksi (75 MPa) defomed steel bas. A typical bay is suppoted by 4 x 34 in. (6 x 86 mm) einfoced concete columns on a x ft (6.4 x 6.4 m) gid. The CLT was conducted on a.5 ft (3. m) wide column stip located along Gid Line to evaluate the cuent bending capacity at midspan and at the suppot (Fig. ). The TLM was detemined using ACI 437.R-7 5 TLM = D w +. D s +.6L Eq. () whee D w is the dead load due to slab self-weight (3 psf [6. kpa]) D s is the sum of the supeimposed dead loads (5 psf [. kpa]) and L is the specified live load (5 psf [6. kpa]) pe the oiginal design. The test team also decided to pefom two additional loading cycles using the load magnitude pe ACI 38-5 Chapte. Fo this case the TLM was TLM =.85 [.4(D w + D s ) +.7L] Eq. () Using Eq. () and () the TLM values wee quite simila 358 psf (7. kn/m ) and 365 psf (7.5 kn/m ) espectively. Table summaizes the moment and punching shea capacities fm n and fv n and Load points J Load line Citical coss section H Load line Fig. : Load point locations used fo evaluation of Level B fo the National Institutes of Health poject: (a) Scheme positive moment test; (b) Scheme negative moment test ( in. = 5.4 mm) (b) G the factoed bending and punching shea demands M u and V u fo the column stip unde investigation based on the as-designed (oiginal) conditions. To pefom the load test concentated loads P wee applied using hydaulic ams to mimic the effect of the unifomly distibuted design loads on the test slab stip. Two loading configuations wee used to test the slab: Scheme was used to epoduce the negative bending at Column H; and Scheme was used to epoduce the positive bending at midspan between Column G and H as shown in Fig.. Table gives the magnitude of P fo these load tests. 4 apil / Concete intenational

3 As shown in Tables and the existing slab has highe shea capacity than that coesponding to the new demand. Accodingly the test load layout was intended to poduce the taget bending moments without necessaily achieving the shea foce demand simultaneously. Additionally fo the Scheme load test it was not possible to apply the load symmetically with espect to Column H due to the pesence of piping at those locations. A push- Table : Capacities and demands fo existing stuctue Test scheme Table : Concentated load values P used to mimic moment and shea effects of unifom TLM Test scheme fm n -ft.7 (63.7).5 (88.) P 4. (6.7) 8.4 (6.3) M u -ft 98.8 (34.).6 (86.9) M u (TLM) -ft 93.3 (6.5) 9.6 (58.5) fv n 5.6 (469.7) 5.6 (469.7) down load test was selected (Fig. 3) using the deadweight of the floos above to esist eactions fom the hydaulic ams. Once the load test components and instuments wee installed a peliminay load of 3 lb (3.3 kn) was applied to eliminate slack in the load system. The slab was then tested using eight loading-unloading cycles fo each test configuation including fou loading levels with two cycles fo each load level. Each load M u (P) -ft 93.5 (6.8) 9.4 (59.5) V u 67.7 (3.) 67.7 (3.) V u (TLM) 64. (85.6) 64. (85.6) Objectives Evaluate pefomance of column stip at positive moment egion Evaluate pefomance of column stip at negative moment egion V u (P) 46. (5.5) 46. (5.5) cycle consisted of loading the slab in a minimum of fou appoximately equal loading steps followed by at least two unloading steps. The imum load eached in Cycles 5 and 6 coesponded to the load combination detemined pe Eq. () wheeas the imum load in Cycles 7 and 8 was pe Eq. (). Table 3 gives the load levels used in each cycle. Results of the Scheme test indicate a faily linea behavio fo positive moments. Repeatability pemanency and deviation fom lineaity wee within the limits pescibed by ACI 437.R-7. 5 Additionally no new cacks wee obseved while pefoming the cyclic load test. Although existing cacks did widen duing loading they etuned to thei oiginal widths at the end of the tests. Figue 4 shows the applied load cycles fo the Scheme test. As indicated in Table 3 the limits on deviation fom lineaity wee not met in the last two cycles. Howeve because no sign of failue such as excessive deflection o cacking was obseved the pefomance of the stuctue was deemed acceptable. The load test esults and the peexisting top-side cacks wee indications that the stuctue could have been subjected to loads that exceeded its oiginal design live load of psf (4.7 kn/m ). Duing futhe investigation of the building s loading histoy it was evealed that the floo was used to shoe the floo above duing a pevious enovation. This might have oveloaded the slab. Table 3: Acceptance evaluation fo Scheme esults Load cycles Load level Repeatability (95 to 5%) % Pemanency ( %) % Deviation fom lineaity ( 5%) % Pefomance and D + D s + L Satisfactoy 3 and 4.75 (.D w +.D s +.6L) Satisfactoy 4 and 6 (.D w +.D s +.6L) Satisfactoy 7 and 8.85 (.4[D w + D s ] +.7L) Acceptable Concete intenational / apil 4

4 4 Level B Level A 3 Shoing Steel plate Hydaulic am Steel post Shoing towe below each load point Load cell Timbe block Fig. 3: CLT setup fo the National Institutes of Health poject. Shoing was used to distibute the eaction foces to floos above the test floo Load [.4(D + D s ) +.7L.D +.D s +.6L D + D s + L Tim e seconds Fig. 4: Load cycles fo Scheme test on the National Institutes of Health poject. Load includes weight of loading appaatus D eflections in Expeimental Results FEM Uncacked FEM Cacked Load Cycles Fig. 5: Compaison of esults fo Scheme test on the National Institutes of Health poject Load kn D eflections m m Analytical pedictions wee based on a two-dimensional finite element model using commecial softwae (SAP ). The model consisted of one-dimensional beam elements epesenting existing columns and a fine mesh of plate elements to epesent the floo slab. The concete was assumed to be isotopic and linea elastic and the modulus of elasticity was detemined pe ACI Slab cacking duing the test was intoduced into the model by educing the stiffness of the slab to the effective stiffness as defined in Section 9.5 of ACI Figue 5 compaes the analytical pedictions with the expeimental esults fo the Scheme test. This figue shows that deflections measued in the fist two cycles matched those pedicted fo an uncacked slab wheeas the measued deflections in the last two cycles ae close to deflections pedicted based on a cacked slab condition. A tansitional behavio can be obseved on the thid to sixth cycles indicating that as the test load inceased cacks developed in the slab binging the behavio close to that of a cacked slab at the highe load levels. To accommodate the new design load fo the Level B floo extenally bonded cabon FRP was used to incease the bending capacity of the slab. FRP stips wee installed in two diections on the top and bottom sides of the slab. Design and detailing of the FRP wee pefomed accoding to ACI 44.R 8 guidelines. Commecial etail building To addess the needs of a potential tenant the owne of a commecial building in Cleveland OH evaluated options fo upgading the second level floo to house telecommunications equipment. The live load equied fo this equipment anged fom 5 to 5 psf (6. to 7. kn/m ). The nine-stoy building was constucted in 97 with a masony skin on a concete-encased steel fame and einfoced concete (RC) floo system. The existing dawings povided only floo plans and geomety of the membes but no details wee available fo the stuctual steel membes o steel einfocement. The typical floo system consists of 6 in. (5 mm) wide einfoced concete joists suppoting a 3.5 in. (9 mm) concete slab einfoced with No. 3 (No. ) bas spaced at 8 in. (46 mm) on cente. A typical joist has a total depth of 5.5 in. (4 mm) and a span of 7.6 ft (8.4 m) and the joists ae 6 in. (66 mm) on cente. Dimensions of the existing joists wee field veified. Condition assessment and site investigation evealed that the joists wee typically einfoced with two in. (5 mm) squae bottom bas at midspan. About 5.5 ft (.7 m) fom each suppot one ba is bent up and extends as a top ba ove each suppot and into the adjacent span. An additional in. (5 mm) staight top ba was located ove the suppot at each end of a joist. No tansvese einfocement was 4 apil / Concete intenational

5 Full span saw cuts to isolate test joists Load Beam Test Joist Test Joist Load application points Test and Test Timbe P ads Speade Beam Test Joist 3 Test 3 Fig. 6: Plan of the load test aea fo commecial building etofit poject Load C ell H ydaulic Ram H igh Stength Steel Ba Reaction Beam R C M ico Pile Fig. 7: Loading points fo two joists test fo commecial building etofit poject Fig. 8: CLT setup fo Tests and of the commecial building etofit poject located in the joists. Based on available histoical data and obseved conditions a nominal concete stength of 4 psi (7.6 MPa) and steel yield stength of 33 ksi (4 MPa) wee used fo peliminay analysis of the joists. The poposed new loads included a supeimposed dead load of 5 psf (. kn/m ) fo a new concete ovelay to level the slab suface and a sevice live load of 5 psf (7. kn/m ). Analyses indicated that the joists wee deficient in both flexue and shea fo the poposed loads with an existing live load capacity of appoximately 96 psf (4.6 kn/m ) govened by the shea stength of the existing joists. The existing shea capacity of a typical joist was estimated at 9 s (4 kn) wheeas the shea demand fo the new load was appoximately.4 s (5.7 kn). To eliminate the possibility of bittle shea failue all test joists wee stengthened fo shea using extenally bonded CFRP pio to testing. Thee load tests wee pefomed to veify the existing load-caying capacity contolling failue mode and stength impovement afte the stengthening systems wee installed. In Tests and two joists wee simultaneously load tested once befoe and once afte they wee stengthened (Tests and espectively). A thid joist was tested afte it was stengthened to examine the pefomance with no pe-induced damage (Test 3). The test joists wee isolated by saw cutting the concete slab to eliminate load shaing with adjacent membes (Fig. 6). Analytical modeling indicated that the imum moment and shea foces due to the design unifom loads could be eplicated using two point loads each located 3 ft (.9 m) fom the joist midspan (Fig. 6 and 7). A pull-down-type load was used in these load tests. The load was applied using hydaulic ams that wee connected to (and pulled against) a einfoced concete micopile installed at the gound floo (one level below) to povide necessay eactions (Fig. 8). In each test the load was applied in six cycles compising two cycles at each of thee loading levels. Test was pefomed on two joists isolated by saw cutting the concete slab at mid-distance to the fist adjacent joist on each side of the test joists. The two joists wee then stengthened fo shea with an extenally bonded CFRP system. Cones on the joist stems wee Concete intenational / apil 43

6 Deflection in Test Test -.8 Test 3 -.5L.5L.375L.5L.65L.75L.875L.L Distance Fom Suppot Deflection mm Fig. 9: CFRP shea stengthening of test joists of the commecial building etofit poject Fig. : Compaison of test esults commecial building etofit poject. Values wee measued at P = 45 lb (64.5 kn) ounded to a.5 in. (3 mm) adius to pevent stess concentations and in. (35 mm) wide stips of U-wap CFRP wee installed at 6 in. (46 mm) spacing along the full span of each joist (Fig. 9). The calculated shea stength of the joists with this CFRP configuation was 3 s (58 kn). Deflections and cack widths wee monitoed in eal-time duing the load test. Test was teminated when the midspan deflection indicated inelastic behavio. Failue of the joists was govened by yielding of einfocement at the suppot as evidenced by a lage cack that developed on the top side of the slab. The width of this cack inceased until the load test was teminated. Thee was no indication of failue at midspan as midspan cack widths wee stable at imum load. Based on the esults of Test it was concluded that the CFRP-stengthened joists wee able to suppot a supeimposed dead load of 5 psf (. kn/m ) plus a live load of 35 psf (6.5 kn/m ). The shea pefomance was adequate with no shea cacks obseved on the joist afte the test was completed. To esolve the obseved negative bending deficiency a bonded concete ovelay appoximately 3 in. (76. mm) thick and einfoced with a steel wie mesh was installed on the same two joists afte oughening the slab suface to appoximately.5 in. (6 mm) amplitude. Test was pefomed afte the concete ovelay cued. The joists wee loaded cyclically following the same potocol as Test but using a imum test load of 85% of the factoed design loads as specified by Chapte of ACI This load level would not cause excessive damage to the upgaded joists thus eliminating the need fo additional epais afte the test. As the load appoached the imum test load a numbe of flexual cacks developed on the top side of the ovelay at both ends of the joists (negative moment egions). The numbe and distibution of the cacks indicated that sufficient bond existed between the existing slab and new ovelay to tansfe hoizontal shea foces and poduce monolithic behavio. The einfoced concete ovelay enhanced the stength and stiffness of the test joists and educed deflection (Fig. ). Based on the test esults the stengthened joists wee ated as adequate to suppot thei self-weight a 36 psf (.7 kn/m ) supeimposed dead load (einfoced concete ovelay) and 5 psf (7. kn/m ) live load. Test 3 was pefomed on a single joist that was isolated by saw cutting the slab on each side (Fig. 6). Pio to testing the joist was stengthened fo flexue using a bonded RC ovelay and fo shea using CFRP stips. To expedite the constuction schedule and minimize constuction cost the CFRP layout fo Test 3 compised vetical stips applied only to the sides of the joist stem thus avoiding the need to ound the cones of the joists. In addition the system povided full coveage of the side faces of the joists as the fibes in one ply of the CFRP stip had fibes oiented in the vetical diection. The calculated shea capacity of the stengthened joist was 4 s (6.3 kn). The pupose of Test 3 was to veify that this optimal CFRP layout would povide adequate shea pefomance and to examine the pefomance of a stengthened joist that was not peviously damaged by load testing (as was the case fo Test joists). In Test 3 the joist was loaded cyclically to 85% of the design factoed loads. The stengthened joist had impoved stiffness elative to the damaged and stengthened joists evaluated in Test. The stengthened joist also had almost twice the stiffness of the unstengthened joist evaluated in Test. Based on the acceptance citeia paametes the pefomance of the joist was consideed satisfactoy. As with the pevious tests no shea cacks 44 apil / Concete intenational

7 wee obseved in Test 3 confiming the adequacy of the altenate CFRP layout. Change in use In both of the descibed cases the CLT method efficiently veified the capacities of the existing stuctues. Fo the fist case load testing was used to detemine the load-caying capacity of the existing slab veify the cause of existing cacks and confim the eliability of the analytical models that wee late used to detemine the equied level of stengthening at vaious locations. Fo the second case the load tests povided infomation on the load-caying capacity of the existing joists and thei govening failue mode confimed the pefomance and composite behavio of the bonded einfoced concete ovelay upgade solution and allowed fo optimizing the shea stengthening solution using an extenally bonded CFRP system. It should be emphasized that extenally bonded CFRP einfocement povided a cost-effective stengthening solution in both cases. Also because load testing veified that the existing stuctual components had adequate capacity to cay the design sevice loads without the contibution of the FRP no additional fie potection was needed fo the CFRP. Only an intumescent top coat was used to povide the smoke-density and flame-spead atings equied pe the govening building codes. Futue consideations Inceased use of load testing can be anticipated as moe ownes opt to update athe than eplace existing buildings. The CLT method offes significant time and costs savings elative to the cuent ACI 38 pocedue but thee is a need to undestand the effects of shot-tem ceep on both old and new stuctues when loaded to nea thei capacities. It would appea that this can be achieved by using the two test methods to evaluate simila stuctual membes and compaing the esulting deflection behavios and esidual defomations. Buildings that ae to be eplaced can also be used fo compaative tests to failue. Refeences. ACI Committee 38 Building Code Requiements fo Stuctual Concete (ACI 38-5) and Commentay (38R-5) Ameican Concete Institute Famington Hills MI 5 43 pp.. ACI Committee 38 Building Code Requiements fo Stuctual Concete (ACI 38-8) and Commentay Ameican Concete Institute Famington Hills MI pp. 3. RILEM Technical Committee -TBS Geneal Recommendation fo Statistical Loading Test of Load-Beaing Concete Stuctues In Situ RILEM Technical Recommendations fo the Testing and Use of Constuction Mateials E&FN Spon London England 994 pp ACI Committee 437 Stength Evaluation of Existing Concete Buildings (ACI 437R-3) Ameican Concete Institute Famington Hills MI 3 8 pp. 5. ACI Committee 437 Test Load Magnitude Potocol and Acceptance Citeia (ACI 437.R-7) Ameican Concete Institute Famington Hills MI 7 38 pp. 6. ACI Committee 364 Guide fo Evaluation of Concete Stuctues befoe Rehabilitation (ACI 364.R-7) Ameican Concete Institute Famington Hills MI 7 pp. 7. SEI-ASCE Committee Guideline fo Stuctual Condition Assessment of Existing Buildings (SEI-ASCE -99) Ameican Society of Civil Enginees Reston VA 47 pp. 8. ACI Committee 44 Guide fo the Design and Constuction of Extenally Bonded FRP Systems fo Stengthening Concete Stuctues (ACI 44.R-) Ameican Concete Institute Famington Hills MI 45 pp. Received and eviewed unde Institute publication policies. ACI membe Taek Alkhdaji is an Engineeing Manage with the Stengthening Division of Stuctual Goup Inc. Hanove MD. He is a membe of ACI Committees 437 Stength Evaluation of Existing Concete Stuctues; 44 Fibe Reinfoced Polyme Reinfocement; and 56 Evaluation Repai and Rehabilitation of Concete Buildings. He is also a membe of the Intenational Concete Repai Institute. Nestoe Galati is a Design Enginee with the Stengthening Division of Stuctual Goup Inc. Hanove MD. He is a membe of ACI Committees 437 Stength Evaluation of Existing Concete Stuctues and 44 Fibe Reinfoced Polyme Reinfocement. Antonio Nanni FACI holds pofessoships at the Univesity of Miami and Univesity of Naples Fedeico II. He is an active membe of seveal ACI technical committees. Concete intenational / apil 45