Bursting and Spalling in Pretensioned U-Beams - Ferguson ...

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tested at transfer and under shear loads in the near future under Research Project5831, with these test results determining the adequacy of such details.5. Measured stresses in bottom-flange lateral bars never exceeded the AASHTO limit(20 ksi), despite the fact that the reinforcement provided in this zone was 30% of thatrequired by AASHTO LRFD. As the recent code change (2008) requiring “splitting”reinforcement in this zone seems to have been based on Washington State DOTstandard practice rather than the results of a specific research program, the coderequirement may be overly conservative. Similar results from other beam types (e.g.box beams, as planned in later portions of Research Project 5831) would be necessaryto substantiate these findings, however.6. No trend in the measured bursting and spalling stresses could be linked to the endblockdesign alternative (large triangular or small skewed end block) chosen for theskewed end; neither end block can be recommended as providing superiorperformance at prestress transfer. Should the finding of structural equivalence bereplicated in shear testing and further bursting/spalling testing, the small skewed endblock may be recommended for temperature-related reasons.7. Bursting reinforcement should be provided in all locations at which strands first bondto concrete, be they near the beam end as in the usual case, or further from the beamend in the case of debonded strands or dapped ends. With limited experimental workthus far conducted examining the effects of different levels of bursting reinforcement,it is recommended that bursting reinforcement should be designed similarly to the“splitting” reinforcement in AASHTO LRFD, to resist 4% of the applied prestressingforce (or increment of applied prestressing force newly bonded at that section) withan allowable design stress of 20 ksi for the transverse reinforcement.190
5.3 RECOMMENDED FUTURE RESEARCHContinued development and testing of new transverse-reinforcement details forU-beams and box beams will be the next steps for Project 5831. Limited study of burstingand spalling stresses in beams with debonded strands has also begun under the auspicesof the project; however, it seems that the subject may be deserving of a more intensivetreatment. One reason often given (PCI, 1985; Itani & Galbraith, 1986; Tadros et al.,2007) for why bursting and spalling stresses are not necessary to consider for beams inservice is the beneficial clamping effect of the beam reaction in the region close to thesupport. For beams with debonded strands, a significant portion of the prestressing forcemay be transferred in locations far from the end support.Work with bursting and spalling database has made it clear that members ofdifferent cross sections can behave differently at transfer. Increasing the database size byconducting more tests on pretensioned beams with extensively-instrumented end regionswould be invaluable to the understanding of bursting and spalling behavior. As noted inthe previous section, particularly lacking are tests of pretensioned beams with varyinglevels of bursting reinforcement. As bulb tees members have proved particularlyvulnerable to cracking problems, their use in such a study would be natural.Continued study of lateral bursting stresses will also be interesting. As notedpreviously, requirements for bottom-flange lateral reinforcement in wide pretensionedconcrete members were recently added to AASHTO LRFD (2008). Should U-beams andother members with much less than the LRFD-required lateral reinforcement continue toshow serviceable lateral bursting zones, there may be cause to readdress this provision.An extreme case for lateral bursting, in which the lateral-bar stress in a beam withdebonded strands would be measured between widely-spaced, bonded strand groups,would be valuable to investigate for the purposes of code calibration.191
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CopyrightbyDavid Andrew Dunkman2009
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Bursting and Spalling in Pretension
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Bursting and Spalling in Pretension
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2.4.2.3 Itani & Galbraith, 1986 ...
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4.2.2.4 Beam 1: Lateral-Bar Burstin
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List of TablesTable 2.1 Range of st
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Figure 2.25 Mechanical gage points
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Figure 3.30 Thermocouple placement
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CHAPTER 1Introduction1.1 IMPETUS FO
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At present, current end-region deta
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Figure 2.1 Severe cracking in a pre
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through use of tensioned, high-stre
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As splitting stress is bond-related
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Figure 2.6 Effect of bursting & spa
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magnitude of the end-region transve
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FEA can readily output color maps o
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2.3.1.3 Plastic Analysis of Beam En
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force can increase by 400% (Yettram
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2.4 EXPERIMENTAL STUDIES OF BURSTIN
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The trial specimens included one 40
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mechanical gage (2 in. gage length)
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Bursting stress(normalized by prest
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proprietary post-tensioning anchors
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Though Marshall and Mattock placed
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ectangular beam” (p. 21). At high
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Most beams observed to crack did so
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The behavior of these beams was dom
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2.4.2 Studies of End-Region Transve
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Figure 2.28 I-beams studied by Mars
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transverse forces compared to those
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(a) Actual spalling stress variatio
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stirrups in Gergely, Sozen and Sies
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50% more reinforcement than require
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strength concrete, increasingly hig
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the reinforcement located within th
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Typical crack patterns for the beam
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“Very common”crack locationPred
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Figure 2.38 Typical gage locations
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2.4.2.7 Smith et al., 2008Most rece
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the provided transverse reinforceme
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Itani and Galbraith (1986) reported
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Figure 2.45 Confining reinforcement
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cracks located along lines of prest
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odies relevant to pretensioned conc
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than that in another shape. For thi
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Figure 2.47 Required splitting rein
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No attempt was made in the Tentativ
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No citation is provided in the PCI
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Section Propertiesy b22.4 in.A 1120
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minimum for greater skews. For beam
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treatment (counting the transverse
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Pretensioned/post-tensioned beams h
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Figure 2.57 Harped strands in webs
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transversereinforcementwithin voidp
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2.8 SUMMARYIn this chapter, literat
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3.2.1.1 Linear and Nonlinear Stress
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Empirical expressions can be used t
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where: 0.021 = spalling force,
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3.3 CONCRETE PROPERTIES3.3.1 Concre
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It was reasoned that the dimensiona
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Splitting tensile-strength testing
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For temperature-match curing, therm
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3.4.1 Reinforcing-Bar Mechanical Te
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Grade 60 rebar yielded at 65 ksi an
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Equation 3.10where:ε1,2 = gage str
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Figure 3.17 Typical strand failure
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SouthNorth North120NorthSouthFigure
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Beam 1 also included an additional
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Figure 3.22 Strand strain-gage inst
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Figure 3.25 Interface board used to
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eam has a “hot spot” (measureme
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3.6 BEAM FABRICATION3.6.1 Beam Cast
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Figure 3.33 Installing the interior
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In order to use a pretensioning bed
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standardside-formprofilespecialtape
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Figure 3.41 Strands installed, tens
Page 156 and 157: Correspondence between the values o
Page 158 and 159: Figures 3.45 to 3.50 show the fabri
Page 160 and 161: Figure 3.49 Casting end block (and
Page 162 and 163: Figure 3.52 Detensioning by gradual
Page 164 and 165: CHAPTER 4Results & Discussion4.1 OV
Page 166 and 167: The strain time history for each ga
Page 168 and 169: 1DNorthSouth1E18 ksi1C1C,E1D1521B1B
Page 170 and 171: Figure 4.4 Beam 1 prestress losses
Page 172 and 173: crack adjacent to this location was
Page 174 and 175: The temperatures in the end blocks
Page 176 and 177: Table 4.4 Beam 2 applied prestressi
Page 178 and 179: Figure 4.10 Beam 2 prestress losses
Page 180 and 181: terms of force distribution seems t
Page 182 and 183: limits were only exceeded at locati
Page 184 and 185: spalling stresses were measured in
Page 186 and 187: transverse reinforcement contained
Page 188 and 189: Based on Figure 4.18, the 4% Pi des
Page 190 and 191: Uniform and linearly-decreasing tra
Page 192 and 193: 4.3.5 Contributing Factors to High
Page 194 and 195: stress ratio multiplied by a typica
Page 196 and 197: Figure 4.25 Relation between total
Page 198 and 199: 4.4.1 Recommended Texas U-Beam Rein
Page 200 and 201: Figure 4.28 Proposed end-region det
Page 202 and 203: At present, AASHTO LRFD (2009) lack
Page 204 and 205: CHAPTER 5Conclusions5.1 SUMMARY OF
Page 208 and 209: APPENDIX ADatabase of Transverse-Ba
Page 210 and 211: 194Figure A.1 U-beam with skewed en
Page 212 and 213: 196Figure A.3 Shallow Texas bulb te
Page 214 and 215: 198Figure A.5 Lightweight-concrete
Page 216 and 217: 200Figure A.7 Washington bulb tee (
Page 218 and 219: 202Figure A.9 Nebraska bulb tees an
Page 220 and 221: 204Figure A.11 Nebraska bulb tees a
Page 222 and 223: 206Figure A.13 Shallow I-beams
Page 224 and 225: where:= lower-bound concrete tensil
Page 226 and 227: B.3.4 Transmission Length (Clause 6
Page 228 and 229: Equation B.11where:= bursting force
Page 230 and 231: 50High Web gage4030Max Moment heigh
Page 232 and 233: C.2 CONCRETE RELEASE STRENGTHA bott
Page 234 and 235: Table C.1 Specimen Design for Beams
Page 236 and 237: Table D.1 Beam 1 match-cured cylind
Page 238 and 239: D.1.2 Beam 28Release-Point Cylinder
Page 240 and 241: Cylinder Strength (ksi)131197Ambien
Page 242 and 243: D.2 REINFORCING-BAR MODULUS & STREN
Page 244 and 245: APPENDIX EEnd-Block Temperature Pro
Page 246 and 247: E.2 BEAM 2T max = 142°F∆T = 30°
Page 248 and 249: REFERENCESREFERENCED STANDARDSAmeri
Page 250 and 251: Fédération Internationale du Bét
Page 252 and 253: Davis, R.T.; Buckner, C.D. & Ozyild
Page 254 and 255: Lin, T.Y. & Burns, N.H. (1981), Des
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Stone, W.C. & Breen, J.E. (1984),
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VITADavid Andrew Dunkman was born 2
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