Load Testing After Strengthening Of A Pre-

stressed Double Tee Beam That Had Failed In

Shear

Nestore Galati, PhD, PE(1) and Tarek Alkhrdaji, PhD, PE(1)

STRUCTURAL TECHNOLOGIES, 10150 Old Columbia Road,

Columbia, MD 21046

Introduction

▪ Fountain Square Garage is

located at the Reston Town

Center in Reston, Virginia.

▪ The roof floor framing consists of

precast, pre-stressed concrete

double tees (DT) supported by

precast concrete girders and

columns.

▪ Following a heavy snow storm, a

landscape contractor had piled up

all the snow on the roof on two DT

beams showing signs of imminent

shear failure.

Existing Conditions

FRP Strengthening Details

Strengthening procedure:

1. Restore structural integrity

of the DT beams (epoxy

injection/form and pump

delaminated concrete)

2. Strengthen damaged DT

beams with FRP U-Wrap

designed as RC member

(PT contribution considered

as zero)

FRP Strengthening Details

1 PLY U-WRAP OF FRP FULLCOVERAGE

1 PLY 12" WIDEOF FRP

ROUND CORNERS TO 12"(MIN.) RADIUS

S32 SECTION - TEE STEM FRP DETAIL

4"

(VIF

)2

2"

(VIF

)

EXISTINGSHEAR CRACK

21'-1" 28'-7"

1 PLY 12" WIDE HORIZONTALOF FRPLENGTH = 10'-7"

1 PLY U-WRAP OF FRPFULL COVERAGE OVER A

LENGTH OF 10'-0"

1 PLY 6" WIDEHORIZONTAL OF FRP

LENGTH = 2'-0"

10'-0"

1 PLY 12" WIDE HORIZONTALOF FRP

LENGTH = 10'-0"

1 PLY U-WRAP OF FRP FULLCOVERAGE OVER A LENGTH OF10'-0"

10'-0"

S32

4"

(VIF

)1'

-10

" (V

IF

)

C.4 C.8C

S31 ELEVATION - TYPICAL PRECAST DOUBLE TEE STEM FRP DETAIL

EXTEND FIRE PROTECTIONSYSTEM 6" (MIN.) BEYOND THEEND OF THE FRP WRAPS.

FIRE PROTECTION SYSTEM (CFP)NOT SHOWN FOR CLARITY

6" (MIN)VARIES 3' TO 6'

VERIFY IN FIELD

VARIES 5' TO 7'

VERIFY IN FIELD

6" (MIN.)VARIES 3' TO 6'

VERIFY IN FIELD

VARIES 5' TO 7'

VERIFY IN FIELD

EXISTINGSHEAR CRACK

▪ Load testing was performed on the

most damaged DT beam after FRP

was installed.

▪ The objective of the load test was to

confirm the shear capacity of the

repaired DT beam.

Testing Objectives

▪ The load test was designed to

verify the shear performance of

the repaired zone.

▪ All Loading was achieved using

hydraulic jacks. Hydraulic jacks

can remove the load quickly in

the event of failure.

▪ Steel spreader beams and

wood bearing pads used to

transfer load to DT stems.

Load Test Strengthening Beams

Loading on the structure

▪ Self-weight of the structure;

▪ Snow load consisting of a flat

snow load of 20 psf and snow

drift supercharge having a

triangular distribution over a

width of 10 ft and maximum

value of 41 psf;

▪ Live load of 40 psf (parking).

Failed DT Beam

Load Test Magnitude

▪ ACI 437.2 considers two sets equations when determining the load test

magnitude.

1. Statically indeterminate - Eq. (4.2.2a) through (4.2.2c)

2. Statically determined

TLM = 1.3(DW + DS) (4.2.2a)

TLM = 1.0DW + 1.1DS + 1.6L + 0.5(Lr or S or R) (4.2.2b)

TLM = 1.0DW + 1.1DS + 1.6(Lr or S or R) + 1.0L (4.2.2c)

∙

TLM = 1.2(DW + DS) (4.2.3a)

TLM = 1.0DW + 1.1DS + 1.4L + 0.4(Lr or S or R) (4.2.3b)

TLM = 1.0DW + 1.1DS + 1.4(Lr or S or R) + 0.9L (4.2.3c)

Load Test Magnitude

▪ If the deficiency in statically determinate members is not tension-

controlled, then Eq. (4.2.2a) through (4.2.2c) shall be used to determine

the TLM”.

TLM = 1.3(DW + DS) (4.2.2a)

TLM = 1.0DW + 1.1DS + 1.6L + 0.5(Lr or S or R) (4.2.2b)

TLM = 1.0DW + 1.1DS + 1.6(Lr or S or R) + 1.0L = 187 psf (4.2.2c)

∙

When testing to evaluate shear strength, the moment in the section

will influence the shear strength of the section. Therefore, it is

important to arrange the testing load that duplicate the suspected

shear deficiency.

Determining Test Load Configurations

LTM = 187 psf Eq. (4.2.2c)

PLTM =29.5 kip

A Bln = 48’-11”

T

A B

T

xt =7’-0”

wD = 75 psf

Equivalent Point Load

Critical Shear Area

Shear Distribution LTM

Concentrated

Load

Applied shear

force 8 % higher

than the one

generated by

uniformly

distributed LTM

at critical

sections

0 10 20 30 4030−

20−

10−

0

10

20

30

Distance Along the Span [ft]

Sh

ear

Fo

rce

[kip

]

Critical Shear

Area

Uniform Load

0 10 20 30 400

100

200

300

Distance Along the Span [ft]

Ben

din

g M

om

ent

[kip

-ft]

Moment Distribution LTM

Critical Shear

Area

Uniform Load

Concentrated

Load

Bending moment

applied 6.6%

higher than the

one generated

by uniformly

distributed LTM

at critical

sections

Cracking

Moment

Load Test Set Up

Reaction Beam:

2-C10x15.3 (7’ long)

Load Beam:

2-C10x15.3 (7’ long)

0.75” Diameter Rod

Anchor Plate and Bolt

Anchor Plate and Bolt

Wood Bearing Plate

Hydraulic Jack

Load Cell

6000 lbs/Stem Weight to

Prevent Uplift

Roof

Level 4

Test Setup - Shoring

Shoring tower One tower per

stem being tested.

Gap between element being

tested and shoring tower.

Shoring posts tight against

slab at level 2 and DT stem

at level 3.

Roof

Level 4

Level 3

Level 2

Shoring tower installed tight

against level 3 and DT stem

at level 4.

Shoring towers on W8

Beams spanning between

the two stems being tested.

Shoring posts below applied

counterweight

Install shoring towers on W8

Beams spanning between

the two stems being tested

6000 lbs/Stem Weight

to Prevent Uplift

Load Test Equipment

Load Cell and Jack

Data Acquisition Unit

Steel Spreader Beam with High Strength Williams Bar

Reaction Beam Hydraulic Pump

Load Test Equipment

6000 lbs./Stem Weight to Prevent Uplift

3 TO 6 kip Capacity Shoring PostsDial Gages at Underside

of DT Beams

Applied Loads

Tested DT beam was

disconnected from the

remaining structure

Deflection of the roof obtained as

the difference of the deflection

measured at level 4 and deflection

measured at level 3

Deflection Measurements

Roof

Level 4

Level 3

Deflection Measurements

What is the maximum

expected deflection? ≅ 0.2 𝑖𝑛.Dial gages with 0.001 in.

resolution are adequate

Loading Protocol

Test Load shall be applied in not less than 4

approximately equal increments

Test Load applied for 24h and final response within 24h

test load has been removed

Testing Protocol and Test Magnitudes

Load

Step

Min. Time per

Step

Load Applied at

Each Point (kips)

Total Applied

Test Load (kips)

1 - 0.3 0.6*

2 5 min 3.5 7

3 5 min 7.25 14.5

4 5 min 11 22

5 24 hours 14.75 29.5

6 - 0.3 0.6*

* Weight of equipment

Load - Time

0

5000

10000

15000

20000

25000

30000

0 200 400 600 800 1000 1200 1400 1600

Load

[lb

s]

Time [Minutes]

Acceptance Criteria

▪ The maximum and residual deflection shall satisfy

∆𝑇𝑒𝑠𝑡≤ ∆𝑀𝑎𝑥=𝑙𝑡180

= 3.3 𝑖𝑛.

∆𝑟,𝑚𝑎𝑥≤∆𝑇𝑒𝑠𝑡4

where:

𝑙𝑡= span of the member under test

∆𝑇𝑒𝑠𝑡= measured max deflection

∆𝑟,𝑚𝑎𝑥= measured residual deflection

Does Not Governs

Limit for re-testing

Test Results – Load Deflection

0

5000

10000

15000

20000

25000

30000

0 0.1 0.2

Load [

lbs]

Deflection [in.]

∆𝑇𝑒𝑠𝑡= 0.184 𝑖𝑛 ≤ ∆𝑀𝑎𝑥= 3.3 𝑖𝑛.

∆𝑇𝑒𝑠𝑡

4= 0.184 𝑖𝑛

4=0.046 in

∆𝑟,𝑚𝑎𝑥= 0.003 𝑖𝑛 ≤ 0.046 in (PASS)

∆𝑟,𝑚𝑎𝑥 ∆𝑇𝑒𝑠𝑡

Test Results – Deflection Time

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0 200 400 600 800 1000 1200 1400 1600

Deflection [

Inches]

Time [Minutes]

∆𝑇𝑒𝑠𝑡

∆𝑇𝑒𝑠𝑡

4= 0.184 𝑖𝑛

4=0.46 in

∆𝑟,𝑚𝑎𝑥

Conclusions

▪ The monotonic load testing was used to verify the shear

capacity of a double tee beam failed in shear.

▪ The load test was designed to verify the shear performance

of only the repaired zone. The remaining portion of the

beams was below cracking.

▪ The structure met the acceptance criteria of ACI 437.2 and

stayed elastic during the load test.

▪ Is a deflection criteria meaningful when testing for shear or

would a pass/fail criteria be more feasible?

Thank You

Any Questions?