assessment of existing structures using cyclic load testing

Concrete international / april 2010 39

By Tarek alkhrdaji, NesTore GalaTi, aNd aNToNio NaNNi

Assessment of Existing Structures Using Cyclic Load

TestingCase studies illustrate procedures recommended in aCi Committee 437 report

When a building is renovated for a change of use, the load-carrying capacity of the structural system must

be established. Load testing can be used to provide reliable verification that a given structure can safely support the calculated design loads.

Per ACI 318,1,2 the test load magnitude (TLM) is required to be reached in at least four load increments. A set of response measurements (mainly deflection) is taken after the total test load has been applied and after at least 24 hours of sustained loading. A final set of response measurements is also required 24 hours after the test load has been removed, so the total duration of the load test can exceed 72 hours. When multiple load tests are required to verify the capacities of multiple elements or configurations, significant delays and expenses can be incurred.

In the past 10 years, researchers and practitioners in the U.S. have been evaluating an alternative load test method.3 Known as the cyclic load test (CLT) method, this procedure requires the application of multiple cycles of loading and unloading (typically six). Structural adequacy is then verified by examining the linearity of the measured deflection response and magnitude of the permanent deformation after the load has been removed.

CLT investigations are typically conducted using hydraulic rams that allow the test member to be quickly unloaded at any sign of distress, improving safety and

reducing the risk of overloading or damaging the structure. Using hydraulic rams may also be more economical than using weights. Although hydraulic rams require reaction systems that can be expensive and time consuming to implement, the labor required to apply gravity loads using dead weights can also be expensive. The CLT method is discussed in greater detail in ACI 437R4 and ACI 437.1R.5

In the following case studies, the CLT method was used to verify analyses and capacities of existing structural components. The method was also used to evaluate structural behavior after members were strengthened using externally bonded fiber-reinforced polymer (FRP) reinforcement alone or in conjunction with a reinforced concrete topping slab.

CASE STUdiESFor both of the structural investigations described

herein, a building floor was evaluated for a change in use. Following industry recommendations, existing conditions were assessed by studying existing drawings, reports, and calculations; and the information was verified using on-site inspections.4,6,7 In each case, the assessments showed that strengthening would be required. Also, in each case, the CLT method was selected to minimize the duration of the testing program, as testing was required both before and after strengthening.

40 april 2010 / Concrete international

Per ACI 437.1R-075 recommendations, loading and shoring systems were designed to ensure safety, prevent collapse of the test member, and avoid damaging adjacent structural elements. Also per ACI 437.1R-07,5 acceptance criteria—including deflection repeatability, permanency, and deviation from linearity—were used to examine the performance during and after the load test (Fig. 1).

National institutes of Health Library Level B of Building 38 at the National Institutes of Health

(NIH) in Bethesda, MD, houses the National Library of Medicine. This level was being renovated to accommodate a new high-density filing system and a new carousel

shelving system to store microforms collections. The elevated slab was originally designed for 100 psf (4.7 kN/m2) live load. The filing system required that the slab be upgraded to a live load demand of 200 psf (9.6 kN/m2).

During the site investigation phase, flexural cracks were observed on the top of the slab. The discovery prompted NIH officials to request load tests to verify the capacity of the existing floor slab prior to strengthening. The load test was designed to induce loads mimicking the original design loads on certain areas of the slab.

Level B comprises a 10.5 in. (265 mm) thick concrete flat plate slab reinforced with 40 ksi (275 MPa) deformed steel bars. A typical bay is supported by 24 x 34 in. (610 x 860 mm) reinforced concrete columns on a 21 x 21 ft (6.4 x 6.4 m) grid. The CLT was conducted on a 10.5 ft (3.2 m) wide column strip located along Grid Line 12 to evaluate the current bending capacity at midspan and at the support (Fig. 2).

The TLM was determined using ACI 437.1R-075

TLM = Dw + 1.1 Ds + 1.6L Eq. (1)

where Dw is the dead load due to slab self-weight (130 psf [6.2 kPa]), Ds is the sum of the superimposed dead loads (25 psf [1.2 kPa]), and L is the specified live load (125 psf [6.0 kPa]) per the original design.

The test team also decided to perform two additional loading cycles using the load magnitude per ACI 318-05, Chapter 20.1 For this case, the TLM was

TLM = 0.85 [1.4(Dw + Ds) + 1.7L] Eq. (2)

Using Eq. (1) and (2), the TLM values were quite similar—358 psf (17.1 kN/m2) and 365 psf (17.5 kN/m2), respectively. Table 1 summarizes the moment and punching

shear capacities, fMn and fVn, and the factored bending and punching shear demands, Mu and Vu, for the column strip under investigation based on the as-designed (original) conditions. To perform the load test, concentrated loads P were applied using hydraulic rams to mimic the effect of the uniformly distributed design loads on the test slab strip. Two loading configurations were used to test the slab: Scheme 1 was used to reproduce the negative bending at Column H12; and Scheme 2 was used to reproduce the positive bending at midspan between Column G12 and H12, as shown in Fig. 2. Table 2 gives the magnitude of P for these load tests.

Index Calculation* Limit

Repeatability, IR

11

max

22max

∆−∆∆−∆

r

r 0.95 ≤ IR≤ 1.05

Permanency, IP max∆

∆ r IP ≤ 0.10

Deviation from Linearity,IDL

tantan

1−ref

i

αα

IDL< 0.25

2max∆ = maximum deflection in Cycle 2 under a load of Pmax 2r∆ = residual deflection after Cycle 2 under a load of Pmin

1max∆ = maximum deflection in Cycle 1 under a load of Pmax

1r∆ = residual deflection after Cycle 1 under a load of Pmin

iαtan = slope of secant line on load deflection envelope refαtan = slope of secant line for peak on first loading cycle

IR=

IP=

IDL =

Pmax = maximum load level achieved by Cycles 1 and 2

Pmin = minimum load level achieved at the end of Cycles 1 and 2

* Measured values used in calculations:

[ [

2

2

Fig. 1: Acceptance criteria per ACI 437.1R-075

Critical cross section72 in

J

G

H

11 12 13

J

G

H

11 12 13

Load line 2

Load line 2

Load points100 in.

Load line 1

Load line 1

72 in.

90 in.

100 in

108 in.

Load points100 in.

Critical cross section

(a) (b)

Fig. 2: Load point locations used for evaluation of Level B for the National Institutes of Health project: (a) Scheme 1, positive moment test; (b) Scheme 2, negative moment test (1 in. = 25.4 mm)


As shown in Tables 1 and 2, the existing slab has higher shear capacity than that corresponding to the new demand. Accordingly, the test load layout was intended to produce the target bending moments without necessarily achieving the shear force demand simultaneously. Additionally, for the Scheme 2 load test, it was not possible to apply the load symmetrically with respect to Column H12 due to the presence of piping at those locations. A push-

TABLe 2:ConCentrated load values P used to mimiC moment and shear effeCts of uniform TLM

Test scheme

P,kip(kN)

Mu(TLM), kip-ft

(kN-m)

Mu(P),kip-ft

(kN-m)

Vu(TLM), kip(kN)

Vu(P),kip(kN)

114.1

(62.7)93.3

(126.5)93.5

(126.8)64.2

(285.6)46.2

(205.5)

228.4

(126.3)190.6

(258.5)191.4

(259.5)64.2

(285.6)46.2

(205.5)

TABLe 1:CapaCities and demands for existing struCture

Test scheme

fMn,kip-ft

(kN-m)

Mu,kip-ft

(kN-m)

fVn,kip(kN)

Vu,kip(kN) Objectives

1120.7

(163.7)98.8

(134.0)105.6

(469.7)67.7

(301.1)

evaluate performance of column strip at positive

moment region

2212.5

(288.2)211.6

(286.9)105.6

(469.7)67.7

(301.1)

evaluate performance of column strip at negative

moment region

TABLe 3:aCCeptanCe evaluation for sCheme 2 results

Load cycles Load level

Repeatability (95 to 105%), %

Permanency (≤ 10%), %

Deviation from linearity

(≤ 25%), % Performance

1 and 2 D + Ds + L 104.3 3.9 2.8 satisfactory

3 and 4 0.75 (1.0Dw + 1.1D

s + 1.6L) 103.9 3.3 12.3 satisfactory

4 and 6 (1.0Dw + 1.1D

s + 1.6L) 101.7 9.2 24.6 satisfactory

7 and 8 0.85 (1.4[Dw + D

s] + 1.7L) 104.1 9.9 26.5 acceptable

down load test was selected (Fig. 3), using the deadweight of the floors above to resist reactions from the hydraulic rams.

Once the load test components and instruments were installed, a preliminary load of 3000 lb (13.3 kN) was applied to eliminate slack in the load system. The slab was then tested using eight loading-unloading cycles for each test configuration, including four loading levels with two cycles for each load level. Each load

cycle consisted of loading the slab in a minimum of four approximately equal loading steps, followed by at least two unloading steps. The maximum load reached in Cycles 5 and 6 corresponded to the load combination determined per Eq. (1), whereas the maximum load in Cycles 7 and 8 was per Eq. (2). Table 3 gives the load levels used in each cycle.

Results of the Scheme 1 test indicate a fairly linear behavior for positive moments. Repeatability, permanency, and deviation from linearity were within the limits prescribed by ACI 437.1R-07.5 Additionally, no new cracks were observed while performing the cyclic load test. Although existing cracks did widen during loading, they returned to their original widths at the end of the tests. Figure 4 shows the applied load cycles for the Scheme 2 test. As indicated in Table 3, the limits on deviation from linearity were not met in the last two cycles. However, because no sign of failure, such as excessive deflection or cracking, was observed, the performance of the structure was deemed acceptable.

The load test results and the preexisting top-side cracks were indications that the structure could have been subjected to loads that exceeded its original design live load of 100 psf (4.7 kN/m2). During further investigation of the building’s loading history, it was revealed that the floor was used to shore the floor above during a previous renovation. This might have overloaded the slab.


Level A

Steelpost

Loadcell Timber

block

Steel plate

Shoring

Level B

Shoringtowerbeloweachloadpoint

11121314

Hydraulicram

0

5

10

15

20

25

30

0 5000 10000 15000 20000 25000 30000 35000Tim e, seconds

Load

, kip

0

20

40

60

80

100

120

140

Load

, kN

0.85[1.4(D + Ds) + 1.7L

1.0D + 1.1Ds + 1.6L

D + Ds + L

Analytical predictions were based on a two-dimensional finite element model using commercial software (SAP 2000). The model consisted of one-dimensional “beam elements” representing existing columns and a fine mesh of “plate elements” to represent the floor slab. The concrete was assumed to be isotropic and linear elastic, and the modulus of elasticity was determined per ACI 318-08.2

Slab cracking during the test was introduced into the model by reducing the stiffness of the slab to the effective stiffness as defined in Section 9.5 of ACI 318-05.1

Figure 5 compares the analytical predictions with the experimental results for the Scheme 2 test. This figure shows that deflections measured in the first two cycles matched those predicted for an uncracked slab, whereas the measured deflections in the last two cycles are closer to deflections predicted based on a cracked slab condition. A transitional behavior can be observed on the third to sixth cycles, indicating that as the test load increased, cracks developed in the slab bringing the behavior close to that of a cracked slab at the higher load levels.

To accommodate the new design load for the Level B floor, externally bonded carbon FRP was used to increase the bending capacity of the slab. FRP strips were installed in two directions on the top and bottom sides of the slab. Design and detailing of the FRP were performed according to ACI 440.2R8 guidelines.

Commercial retail buildingTo address the needs of a potential tenant, the owner

of a commercial building in Cleveland, OH, evaluated options for upgrading the second level floor to house telecommunications equipment. The live load required for this equipment ranged from 125 to 150 psf (6.0 to 7.2 kN/m2).

The nine-story building was constructed in 1917 with a masonry skin on a concrete-encased steel frame and reinforced concrete (RC) floor system. The existing drawings provided only floor plans and geometry of the members, but no details were available for the structural steel members or steel reinforcement. The typical floor system consists of 6 in. (150 mm) wide reinforced concrete joists supporting a 3.5 in. (90 mm) concrete slab reinforced with No. 3 (No. 10) bars spaced at 18 in. (460 mm) on center. A typical joist has a total depth of 15.5 in. (40 mm) and a span of 27.6 ft (8.4 m), and the joists are 26 in. (660 mm) on center.

Dimensions of the existing joists were field verified. Condition assessment and site investigation revealed that the joists were typically reinforced with two 1 in. (25 mm) square bottom bars at midspan. About 5.5 ft (1.7 m) from each support, one bar is bent up and extends as a top bar over each support and into the adjacent span. An additional 1 in. (25 mm) straight top bar was located over the support at each end of a joist. No transverse reinforcement was

0

0.01

0.02

0.03

0.04

0.05

1 2 3 4 5 6 7 8Load Cycles

Def

lect

ions

, in.

0

0.2

0.4

0.6

0.8

1

1.2

1.4Experim ental Results

FEM Uncracked

FEM Cracked

Def

lect

ions

, mm

Fig. 5: Comparison of results for Scheme 2 test on the National Institutes of Health project

Fig. 3: CLT setup for the National Institutes of Health project. Shoring was used to distribute the reaction forces to floors above the test floor

Fig. 4: Load cycles for Scheme 2 test on the National Institutes of Health project. Load includes weight of loading apparatus


located in the joists. Based on available historical data and observed conditions, a nominal concrete strength of 4000 psi (27.6 MPa) and steel yield strength of 33 ksi (240 MPa) were used for preliminary analysis of the joists.

The proposed new loads included a superimposed dead load of 25 psf (1.2 kN/m2) for a new concrete overlay to level the slab surface and a service live load of 150 psf (7.2 kN/m2). Analyses indicated that the joists were deficient in both flexure and shear for the proposed loads, with an existing live load capacity of approximately 96 psf (4.6 kN/m2) governed by the shear strength of the existing joists. The existing shear capacity of a typical joist was estimated at 9 kips (40 kN), whereas the shear demand for the new load was approximately 11.4 kips (50.7 kN).

To eliminate the possibility of brittle shear failure, all test joists were strengthened for shear using externally bonded CFRP prior to testing. Three load tests were performed to verify the existing load-carrying capacity, controlling failure mode, and strength improvement after the strengthening systems were installed. In Tests 1 and 2, two joists were simultaneously load tested—once

Test 1andTest 2

Test 3

Full span saw cuts toisolate test joists

Load application points

Test Joist 1

Test Joist 2

Test Joist 3

Load Beam

High Strength Steel Bar

Hydraulic Ram

Load Cell

RC M icro Pile

Reaction

Beam

Spreader Beam

Timber P ads

Fig. 6: Plan of the load test area for commercial building retrofit project

before and once after they were strengthened (Tests 1 and 2, respectively). A third joist was tested after it was strengthened to examine the performance with no pre-induced damage (Test 3). The test joists were isolated by saw cutting the concrete slab to eliminate load sharing with adjacent members (Fig. 6).

Analytical modeling indicated that the maximum moment and shear forces due to the design uniform loads could be replicated using two point loads, each located 3 ft (0.9 m) from the joist midspan (Fig. 6 and 7). A pull-down-type load was used in these load tests. The load was applied using hydraulic rams that were connected to (and pulled against) a reinforced concrete micropile installed at the ground floor (one level below) to provide necessary reactions (Fig. 8). In each test, the load was applied in six cycles comprising two cycles at each of three loading levels.

Test 1 was performed on two joists isolated by saw cutting the concrete slab at mid-distance to the first adjacent joist on each side of the test joists. The two joists were then strengthened for shear with an externally bonded CFRP system. Corners on the joist stems were

Fig. 7: Loading points for two joists test for commercial building retrofit project

Fig. 8: CLT setup for Tests 1 and 2 of the commercial building retrofit project


Fig. 10: Comparison of test results commercial building retrofit project. Values were measured at P = 14,500 lb (64.5 kN)

rounded to a 0.5 in. (13 mm) radius to prevent stress concentrations, and 12 in. (305 mm) wide strips of U-wrap CFRP were installed at 16 in. (406 mm) spacing along the full span of each joist (Fig. 9). The calculated shear strength of the joists with this CFRP configuration was 13 kips (58 kN).

Deflections and crack widths were monitored in real-time during the load test. Test 1 was terminated when the midspan deflection indicated inelastic behavior. Failure of the joists was governed by yielding of reinforce-ment at the support, as evidenced by a large crack that developed on the top side of the slab. The width of this crack increased until the load test was terminated. There was no indication of failure at midspan, as midspan crack widths were stable at maximum load.

Based on the results of Test 1, it was concluded that the CFRP-strengthened joists were able to support a superimposed dead load of 25 psf (1.2 kN/m2) plus a live load of 135 psf (6.5 kN/m2). The shear performance was adequate with no shear cracks observed on the joist after the test was completed.

To resolve the observed negative bending deficiency, a bonded concrete overlay, approximately 3 in. (76.2 mm) thick and reinforced with a steel wire mesh, was installed on the same two joists after roughening the slab surface to approximately 0.25 in. (6 mm) amplitude. Test 2 was performed after the concrete overlay cured. The joists were loaded cyclically, following the same protocol as Test 1 but using a maximum test load of 85% of the factored design loads, as specified by Chapter 20 of ACI 318-05.1 This load level would not cause excessive damage to the upgraded joists, thus eliminating the need for additional repairs after the test.

As the load approached the maximum test load, a number of flexural cracks developed on the top side of the overlay at both ends of the joists (negative moment

regions). The number and distribution of the cracks indicated that sufficient bond existed between the existing slab and new overlay to transfer horizontal shear forces and produce monolithic behavior. The reinforced concrete overlay enhanced the strength and stiffness of the test joists and reduced deflection (Fig. 10). Based on the test results, the strengthened joists were rated as adequate to support their self-weight, a 36 psf (1.7 kN/m2) superimposed dead load (reinforced concrete overlay), and 150 psf (7.2 kN/m2) live load.

Test 3 was performed on a single joist that was isolated by saw cutting the slab on each side (Fig. 6). Prior to testing, the joist was strengthened for flexure using a bonded RC overlay and for shear using CFRP strips. To expedite the construction schedule and minimize construction cost, the CFRP layout for Test 3 comprised vertical strips applied only to the sides of the joist stem, thus avoiding the need to round the corners of the joists. In addition, the system provided full coverage of the side faces of the joists, as the fibers in one ply of the CFRP strip had fibers oriented in the vertical direction. The calculated shear capacity of the strengthened joist was 14 kips (62.3 kN). The purpose of Test 3 was to verify that this optimal CFRP layout would provide adequate shear performance and to examine the performance of a strengthened joist that was not previously damaged by load testing (as was the case for Test 2 joists).

In Test 3, the joist was loaded cyclically to 85% of the design factored loads. The strengthened joist had improved stiffness relative to the damaged and strengthened joists evaluated in Test 2. The strengthened joist also had almost twice the stiffness of the unstrengthened joist evaluated in Test 1. Based on the acceptance criteria parameters, the performance of the joist was considered satisfactory. As with the previous tests, no shear cracks

-0.8

-0.6

-0.4

-0.2

0

Distance From Support

Def

lect

ion,

in.

-20

-15

-10

-5

0

Test 1

Test 2

Test 3

0 0.125L 0.25L 0.375L 0.5L 0.625L 0.75L 0.875L 1.0L

Def

lect

ion,

mm

Fig. 9: CFRP shear strengthening of test joists of the commercial building retrofit project


aCi member Tarek Alkhrdaji is an engineering Manager with the strengthening division of structural Group, inc., hanover, Md. he is a member of aCi Committees 437, strength evaluation of existing Concrete structures; 440, Fiber reinforced polymer reinforcement; and 562, evaluation, repair, and rehabilitation of Concrete Buildings. he is also a member of the international Concrete repair institute.

Nestore Galati is a design engineer with the strengthening division of structural Group, inc., hanover, Md. he is a member of aCi Committees 437, strength evaluation of existing Concrete structures, and 440, Fiber reinforced polymer reinforcement.

Antonio Nanni, FaCi, holds professorships at the University of Miami and University of Naples—Federico ii. he is an active member of several aCi technical committees.

were observed in Test 3, confirming the adequacy of the alternate CFRP layout.

CHANgE iN USEIn both of the described cases, the CLT method

efficiently verified the capacities of the existing structures. For the first case, load testing was used to determine the load-carrying capacity of the existing slab, verify the cause of existing cracks, and confirm the reliability of the analytical models that were later used to determine the required level of strengthening at various locations. For the second case, the load tests provided information on the load-carrying capacity of the existing joists and their governing failure mode, confirmed the performance and composite behavior of the bonded reinforced concrete overlay upgrade solution, and allowed for optimizing the shear strengthening solution using an externally bonded CFRP system.

It should be emphasized that externally bonded CFRP reinforcement provided a cost-effective strengthening solution in both cases. Also, because load testing verified that the existing structural components had adequate capacity to carry the design service loads without the contribution of the FRP, no additional fire protection was needed for the CFRP. Only an intumescent top coat was used to provide the smoke-density and flame-spread ratings required per the governing building codes.

FUTUrE CoNSidErATioNSIncreased use of load testing can be anticipated as

more owners opt to update rather than replace existing buildings. The CLT method offers significant time and costs savings relative to the current ACI 3182 procedure, but there is a need to understand the effects of short-term creep on both old and new structures when loaded to near their capacities. It would appear that this can be achieved by using the two test methods to evaluate similar structural members and comparing the resulting deflection behaviors and residual deformations. Buildings that are to be replaced can also be used for comparative tests to failure.

references1. ACI Committee 318, “Building Code Requirements for Structural

Concrete (ACI 318-05) and Commentary (318R-05),” American

Concrete Institute, Farmington Hills, MI, 2005, 430 pp.

2. ACI Committee 318, “Building Code Requirements for Structural

Concrete (ACI 318-08) and Commentary,” American Concrete

Institute, Farmington Hills, MI, 2008, 473 pp.

3. RILEM Technical Committee 20-TBS, “General Recommendation

for Statistical Loading Test of Load-Bearing Concrete Structures In

Situ,” RILEM Technical Recommendations for the Testing and Use of

Construction Materials, E&FN Spon, London, England, 1994,

pp. 379-385.

4. ACI Committee 437, “Strength Evaluation of Existing Concrete

Buildings (ACI 437R-03),” American Concrete Institute, Farmington

Hills, MI, 2003, 28 pp.

5. ACI Committee 437, “Test Load Magnitude, Protocol and

Acceptance Criteria (ACI 437.1R-07),” American Concrete Institute,

Farmington Hills, MI, 2007, 38 pp.

6. ACI Committee 364, “Guide for Evaluation of Concrete

Structures before Rehabilitation (ACI 364.1R-07),” American

Concrete Institute, Farmington Hills, MI, 2007, 22 pp.

7. SEI-ASCE Committee 11, “Guideline for Structural Condition

Assessment of Existing Buildings,” (SEI-ASCE 11-99), American

Society of Civil Engineers, Reston, VA, 2000, 147 pp.

8. ACI Committee 440, “Guide for the Design and Construction of

Externally Bonded FRP Systems for Strengthening Concrete

Structures (ACI 440.2R-02),” American Concrete Institute, Farmington

Hills, MI, 2002, 45 pp.

Received and reviewed under Institute publication policies.

assessment of existing structures using cyclic load testing

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