Post tensioning system for prefabricated building panel

Abstract

An improved post tensioning system for prefabricated building panels in which a concentric uniform radial compression is maintained in the slab. A post tensioning cable is placed in the concrete slab to form a perimeter loop starting from one corner of the slab to a point where the cable entered the slab and at a point turning 90.degree. to follow that portion of the cable in the periphery to a point midway up the Y axis and then turning 90.degree. across the X axis to bisect the slab and cross the opposite parallel portion of the cable to exit out the adjacent side of the slab. This creates a cable pattern with no less than 2.5 parallel cables in any direction.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to prefabricated concrete building panels and, more particularly, to improvements in post tensioned concrete slabs in which there is a better distribution and an increase in compression forces in slabs which are relatively small and thin in section, but which have high strength.

2. Background Description

In U.S. Pat. No. 4,432,175 issued to Rodney I. Smith discloses a post tensioned concrete slab in which one or more continuous reinforcement cables are positioned in a mold, around and near its outer periphery. These cables are lubricated and sheathed to prevent adherence to the concrete. The concrete is poured and cured, and each cable is post tensioned and anchored. The tensile force of each cable is therefore exerted toward the center of the slab as well as from side to opposite side. This results in a slab that can be relatively small and lightweight but has high strength, resistance to cracking and deterioration, and is relatively impermeable to liquids and gases.

While the prior design was, and continues to be, an excellent building product, the distribution of compressive forces tends to be strongly biased along one of the major axes of the panel. This is due to the rigid mold and straight line cable requirement of pretensioning and the single post tensioned perimeter loop design which exits strands at a single corner which attempts to provide uniform compression force in both principle axes of a small rectangular slab. This, in turn, meant that the product just met the criteria for post tensioning under the American Concrete Institute (ACI) guidelines. As a result, an improved design for the post tensioned building panel is highly desirable.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an improved post tensioning system for prefabricated concrete building panels of relatively small size where linear post tensioning would not work due to short cable lengths.

It is another object of the invention to provide a prefabricated concrete building panel in which the variation in compressive forces is reduced while the average compressive force is increased.

According to the invention, there is provided a prefabricated concrete construction panel which overcomes the limits of post tensioning as disclosed in U.S. Pat. No. 4,432,175. Specifically, by maintaining a uniform concentric radial compression in the slab, the benefits of limiting latent cracks to a width allows autogeneous heading of the latent crack to take place, through the tendency of the concrete to reunite and form a new crystalline seal from the radial compression forces and continued presence of sufficient humidity in the concrete. In the practice of the invention, a post tensioning cable is placed in the concrete slab to form a perimeter loop starting from one comer of the slab to a point where the cable entered the slab and at a point turning 90.degree. to follow that portion of the cable in the periphery to a point midway up the Y axis and then turning 90.degree. across the X axis to bisect the slab and cross the opposite parallel portion of the cable to exit out the adjacent side of the slab. This creates a cable pattern with no less than 2.5 parallel cables in any direction and reduces concentrated forces in one corner of the slab.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which:

FIG. 1 is a plan view of a concrete building panel according to the invention;

FIGS. 2 and 3 are enlarged views of a portions of FIG. 1 showing the tensioning of the cable;

FIG. 4 is a model of a 12'.times.10'.times.4" panel composed of 480 six inch squares using the new loop configuration according to the invention;

FIG. 5 is a stress contour map of a building panel made in accordance with the invention showing compression in the Y axis;

FIG. 6 is a stress contour map of a the same building panel as in FIG. 4 showing compression in the X axis;

FIG. 7 is a model of a 12'.times.10'.times.4" panel composed of 480 six inch squares using the old loop configuration;

FIG. 8 is a stress contour map of a building panel modeled in FIG. 7 showing compression in the Y axis; and

FIG. 9 is a stress contour map of a the same building panel as in FIG. 7 showing compression in the X axis.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1, there is shown a post tensioned concrete building panel indicated generally at 1 provided with a post tensioned cable 3 toward its outer periphery. The opposite ends of the cable 3 are secured by wedges 5 and 7.

The cable 3 forms a perimeter loop starting from one comer of the slab to a point where the cable entered the slab and at a point turning 90.degree. to follow the existing cable to a point midway up the Y axis and then turning 90.degree. across the X axis to bisect the slab and cross the opposite parallel cable to exit out the adjacent side of the slab. This creates a cable pattern which no less than 2 and in some places 3 (or more) parallel cables in any direction. The cable 3 is preferably a hard drawn steel cable, but any flexible high-tensile strength material can be similarly employed.

The cable 3 is within a plastic sheath 15 which forms a channel extending around the periphery of the slab 1 and across the central portion of the slab 1. Cable 3 will be lubricated to facilitate its movement within sheath 15.

The post tensioning process is illustrated in FIGS. 2 and 3. When the concrete, which forms building panel 1 is poured, the sheath 15 and cable 3 are positioned so that the opposite ends of cable 3 extend outwardly from one comer (FIG. 2) and a side opposite that corner (FIG. 3). The one corner is provided with an anchor 17, such as the anchor component of the VSL S5N anchorage system. The anchor 17 has an aperture 19 in registry with a pocket 16 formed in the edge of the slab for access to the end of cable 3 and anchorage system 17 and 5. Likewise, as shown in FIG. 3, a second anchor 21 having an aperture 23 in registry with a pocket 22 formed in the edge of the slab for access to the opposite end of cable 3 and anchorage system 21 and 7 is provided in the side of the panel.

After the concrete has become relatively hardened (approximately twenty-four hours after it has been poured), there is an initial tensioning of the cable 3. This initial tensioning adds stripping strength to the slab that allows the mold to be removed more easily and with minimal surface deterioration. To accomplish this initial tensioning, the mold is removed and wedges 5 (FIG. 2) are applied to one end of the cable 3. The wedges are composed of two frustro-conically shaped halves which are partially inserted into an aperture and frictionally clamp the cable by means of teeth formed in the interior surfaces of the wedges.

With the wedges 5 gripping the one end of the cable 3, the other end of the cable is connected to a tensioning device 25, such as a hydraulic jack or similar device, for pulling the cable 3 in the direction indicated by the arrow in FIG. 3. A tensioning force is exerted on the cable 3 until an optimum stress in the cable is approximately equal to 0.7 F.sub.u (F.sub.u =270 kips per square inch (ksi)) is attained. This tensioning of the cable draws the wedges 5 into the aperture 19, thereby anchoring that end of the cable 3. While the cable is tensioned, the wedges 7 (FIG. 3) are applied to the end of the cable which extends beyond aperture 23. Tensioning device 25 is then released, and the tensile force of the cable 3 draws the wedges 7 into aperture 23, resulting in a final stress on the cable of 0.7 F.sub.u. The cable is trimmed and the pockets 16 and 22 are filled.

The tensioning process results in a post tensioned building panel which, due to the improved configuration of the cable 3, results in a better distribution of compression forces in small slabs less than twenty feet across. For such slabs, a finite analysis demonstrates the improved results of the present invention. The analysis of the new building panel configuration of the post-tensioned cable was performed on a finite-analysis program developed by DAST Consulting. The program input utilized the known stress/strain relationship and thermal coefficient of expansion and contraction of steel and a temperature delta to induce a given stress in the linear beam element presenting the post-tension cable. The concrete slab was modeled as a 12'.times.10'.times.4" thick plate composed of 480 six inch squares, as generally shown in FIG. 4. The model parameters were consistent with the limitations and requirements of ACI (American Concrete Institute) Standard 319-95, Chapter 18, "Prestress Concrete". Stresses were applied to both ends of the cable and friction loss due to bending of the post-tensioned cable was configured according to ACI guidelines. The computer generated stress contour maps for the building panel modeled as shown in FIG. 4 showing compression in the Y and X axes are shown in FIGS. 5 and 6 respectively, and the tabularized stress as shown in Table 1. The concrete slab of the cable configuration of U.S. Pat. No. 4,432,175 was also modeled as a 12'.times.10'.times.4" thick plate composed of 480 six inch squares, as generally shown in FIG. 7. The computer generated stress contour maps for the building panel modeled as shown in FIG. 7 showing compression in the Y and X axes are shown in FIGS. 8 and 9 respectively, and the tabularized stress as shown in Table 2.

TABLE 1______________________________________STRESS VALUES (BY ELEMENT) - NEW LOOP CONFIGURATION Sx-x Sy-y______________________________________1 -106 -472 -670 -22183 -671 -22034 -86 -675 -77 -56 -116 -237 112 -138 14 219 -2 2710 -70 -1311 -68 -3912 -7 -1313 -148 -2714 -203 2415 -159 -916 -185 -617 -196 2118 -143 1119 -59 -920 -13 -1421 -50 -72322 210 -155623 185 -149724 -206 -67425 -246 -5626 -170 -3527 -424 -14528 -88 12229 -134 -14330 -159 -11831 -181 -10032 -207 -10033 -232 14334 -152 13735 -204 -11336 -227 2737 -214 9538 -67 3139 -37 -5340 -6 -5941 93 -105742 234 -127843 231 -118444 10 -82945 -10 -8646 -523 -1147 427 -28248 381 -54949 400 -51950 370 -17951 -374 -15552 -388 -16253 -437 -48254 -356 -48255 -398 -19056 -468 -15357 -44 -29058 -53 -7459 15 -9260 4 -13261 -39 -122562 105 -116763 151 -110164 231 -72565 -326 -92166 -794 -93067 -635 -45068 -503 -47569 -461 -43770 -508 -31271 -503 -23172 -500 -28773 -436 -37674 -462 -36875 -569 -27776 -721 -62677 -310 -42178 -141 -179 6 -19480 -14 -19581 -23 -130782 -78 -111283 -2 -100884 -31 -111285 -636 -119686 -572 -84487 -651 -66388 -594 -51589 -559 -44690 -559 -37391 -564 -32792 -538 -33293 -517 -35694 -535 -36195 -582 -41896 -517 -47297 -617 -65598 -33 -40499 -98 -181100 1 -253101 24 -1312102 108 -1179103 292 -913104 -655 -1032105 -517 -1014106 -581 -901107 -596 -727108 -613 -597109 -605 -498110 -598 -431111 -588 -390112 -572 -376113 -556 -380114 -547 -402115 -526 -433116 -523 -487117 -489 465118 -658 -320119 234 -106120 45 -377121 8 -1240122 55 -1153123 206 -1110124 -716 -1208125 -559 -981126 -546 -903127 -584 -782128 -606 -657129 -618 -558130 -616 -486131 -605 -440132 -587 -419133 -563 -418134 -532 -433135 -501 -458136 -473 -464137 -503 -422138 -678 -562139 196 -369140 37 -321141 -15 -1134142 -75 -1058143 -214 -1081144 -275 -1130145 -465 -1086146 -537 -939147 -576 -815148 -605 -704149 -622 -607150 -624 -532151 -615 -483152 -593 456153 -561 -451154 -519 -462155 -169 -178156 -420 -487157 -370 -522158 -189 -474159 -131 -327160 -16 -156161 -15 -1075162 -73 -1014163 -220 -1050164 -283 -1114165 -472 -1088166 -545 -955167 -584 -841168 -673 -737169 -628 -642170 -632 -567171 -621 -513172 -600 -483173 -563 -475174 -513 -482175 -451 -499176 -569 -522177 -252 -519178 -152 -451179 -80 -296180 -28 -57181 7 -1061182 53 -1031183 204 -1110184 -747 -1156185 -587 -993186 -573 -953187 -612 -859188 -635 -755189 -639 -666190 -639 -590191 -630 -531192 -609 -497193 -572 485194 -528 -489195 -461 -504196 -368 -521197 -257 -516198 -147 -452199 -56 -292200 7 -19201 19 -1018202 89 -973203 260 -803204 -718 -1003205 -570 -1034206 -632 -986207 -652 -856208 -679 -753209 -675 -681210 -623 -608211 -639 -542212 -603 -496213 -608 -484214 -567 -175215 -509 -485216 -432 -498217 -330 -509218 -192 478219 -72 -331220 -18 -29221 9 -915222 22 -821223 -84 -813224 -47 -1034225 -718 -1237226 -648 -971227 -745 -850228 -715 -727229 -708 -654230 -743 -660231 -395 -551232 -700 -505233 -663 -439234 -626 -447235 -586 -449236 -543 -453237 -486 -463238 -381 -501239 -150 -443240 -15 -105241 -3 -783242 -22 -773243 -5 -786244 110 -575245 -414 -976246 -899 -1130247 -780 -700248 -700 -741249 -705 -702250 -829 -443251 0 0252 -792 -330253 -718 -424254 -682 -414255 -666 -411256 -667 -400257 -689 -383258 -723 -395259 -686 -579260 -73 -369261 -5 -671262 -44 -701263 -24 -665264 -131 -630265 -73 -207266 -673 -297267 -653 -602268 -660 -865269 -773 -822270 -751 -519271 -401 -176272 -758 -491273 -725 -398274 -716 -400275 -723 -389276 -759 -367277 -846 -329278 -1027 -269279 -1393 -253280 -2176 -1069281 -7 -572282 -38 -627283 -107 -624284 -180 -511285 -386 -402286 -459 -121287 -500 -510288 -511 -199289 -611 -182290 -623 -448291 -660 -433292 -650 -422293 -706 -419294 -722 -397295 -743 -391296 -783 -379297 -869 -351298 -1049 -299299 -1404 -289300 -2179 -1101301 -320 -505302 -93 -582303 -165 -588304 -284 -537305 -408 -476306 -461 -449307 -490 -392308 -602 -304309 -611 -283310 -626 -346311 -651 -392312 -671 -394313 -687 -399314 -711 -404315 -733 -409316 -757 -429317 -765 -447318 -787 -483319 -739 -682320 -97 -476321 -21 -507322 -158 -554323 -232 -593324 -460 -480325 -460 -480326 -504 -436327 -556 -380328 -608 -326329 -643 -303330 -655 -315331 -659 -341332 -675 -363333 -685 -383334 -695 -408335 -705 -435336 -716 -465337 -705 -556338 -498 -640339 -261 -600340 -27 326341 36 -595342 196 -530343 -745 -656344 -568 -479345 -534 -479346 -557 -439347 -588 -380348 -625 -327349 -654 -292350 -671 -283351 -677 -298352 -678 -328353 -679 -368354 -677 -412355 -677 -462356 -670 -501357 -691 -517358 -807 -701359 162 -516360 26 -414361 44 -571362 231 -281363 -733 -455364 -563 -525365 -596 -521366 -589 -447367 -613 -386368 -636 -319369 -663 -267370 -682 -242371 -687 -251372 -679 -291373 -667 -350374 -659 -418375 -646 -478376 -650 -554377 -598 -560378 -734 -467379 -221 -237380 -46 -423381 -3 -376382 -714 -305383 -62 -502384 -715 -714385 -597 -516386 -625 -485387 -607 -398388 -634 -314389 -671 -230390 -680 -187391 -682 -193392 -678 -247393 -645 -338394 -622 -425395 -639 -512396 -604 -544397 -705 -729398 -60 -499399 -106 -261400 1 -270401 -20 -256402 -16 -272403 98 -63404 -421 -388405 -761 -658406 -587 -483407 -554 -469408 -657 -306409 -653 -161410 -647 -130411 -647 -133412 -654 -171413 -657 -323414 -548 -485415 -575 -500416 -732 -669417 -397 -401418 -107 -67419 -8 -237420 -17 -187421 -3 -151422 -16 -127423 -127 -119424 -77 118425 -517 -29426 -468 -683427 -677 -683428 -604 -160429 -582 -106430 -570 -78431 -568 -79432 -578 -110433 -600 -166434 -679 -684435 -411 -680436 -470 -51437 -49 99438 -104 -117439 -6 -105440 0 -113441 -12 -57442 -65 -65443 -127 -13444 -271 -8445 -297 -100446 -257 -254447 -496 -224448 -481 -126449 -456 -67450 -437 -32451 -432 -32452 -443 -66453 -465 -125454 -485 206455 -189 240456 -241 -110457 -227 -16458 -99 -13459 -51 -54460 -9 -42461 -11 -11462 -57 -12463 -150 -2464 -252 -18465 -362 0466 -536 49467 -519 43468 -370 14469 -382 -26470 -244 -6471 -235 -6472 -255 -25473 -352 -15474 -454 39475 -466 46476 -295 -1477 -199 -2478 -116 -2479 -42 -10480 -8 -8AVERAGE -397.925 -452.246AVEDEV 271.1039 256.7913COMBIN. -425AVERAGE:______________________________________ TABLE 2______________________________________STRESS VALUES (BY ELEMENT) ORIGINAL LOOPCONFIGURATION Sx-x Sy-y______________________________________1 -138 -1382 -1307 -22533 -1506 -21044 -895 -365 -632 -26 -513 -117 -431 -18 -371 -39 -327 -310 -299 -411 -289 -712 -309 -2813 -383 -1514 -526 -4115 -539 -4816 -366 017 -257 -1818 -155 -119 -60 -1220 -12 -1221 -2251 -130722 -1227 -122923 -888 -120124 -1064 -57925 -846 -9726 -667 -4227 -557 -2728 -485 -2029 -441 -1930 -420 -2331 -420 -3432 -434 -7433 -469 -21934 -469 -21935 -240 -25036 -283 -10037 -262 -638 -123 -1039 -64 -6440 -12 -6041 -2101 -150742 -1200 -88843 -849 -84844 -793 -59645 -79 -29946 -695 -13447 -607 -8348 -547 -6249 -510 -5650 -494 -6051 -496 -8352 -511 -12053 -538 -17654 -616 -69655 -423 -69356 -482 -3157 -53 -12158 -113 -11359 -11 -12360 -2 -15561 -36 -89862 -580 -106463 -597 -79164 -599 -59765 -620 -40266 -617 -25067 -584 -16568 -551 -12769 -530 -11370 -521 -11771 -523 -13972 -536 -18373 -550 -33274 -461 -49275 -510 -49976 -702 -66577 -380 -38378 -122 -5379 -6 -26180 -18 -25881 -2 -63782 -98 -84683 -300 -78984 -403 -61785 -465 -46586 -502 -34087 -515 -25388 -513 -20489 -509 -18290 -507 -18391 -509 -20592 -510 -26493 -186 -35394 -479 -43395 -518 -50996 -513 -52697 -656 -70798 -28 -48199 -00 -281100 -1 -367101 -11 -520102 -42 -667103 -433 -690104 -247 -613105 -333 -503106 -392 -404107 -430 -328108 -451 -279109 -461 -255110 -466 -254111 -467 275112 -459 -318113 -449 -375114 -448 -436115 -451 -482116 -488 -535117 -485 -516118 -685 -420119 -253 -234120 -49 -539121 -1 -440122 -26 -555123 -79 -599124 -156 -577125 -236 -516126 -301 -446127 -350 -387128 -382 -345129 -402 -323130 -41 -321131 -412 -336132 -406 -367133 -389 -448134 -389 -448135 -391 -487136 -401 -499137 -472 -468138 -686 -617139 223 -467140 42 -523141 -3 -383142 -17 -479143 -54 -532144 -108 -539145 -171 -513146 -232 -471147 -282 -43148 -318 -398149 -342 -381150 -353 -380151 -355 -191152 -349 -413153 -39 -440154 -327 -470155 -327 -495156 -310 -509157 -160 -532158 -160 -532159 -128 -445160 -14 -371161 -2 -339162 -13 -427163 -39 -485164 -80 -509165 -130 -505166 -182 -485167 -229 -459168 -265 -418169 -289 -428170 -300 -429171 -302 -419172 -296 -451173 -284 -471174 -265 -491175 -241 -511176 -209 -533177 -147 -535178 -100 -494179 -66 -410180 -26 -287181 -2 -306182 -11 -391183 -31 -453184 -63 -488185 -104 -499186 -148 -493187 -191 -478188 -227 -465189 -250 -461190 -259 -469191 -256 -480192 -252 -488193 -240 -495194 -216 -507195 -181 -522196 -137 -529197 -95 -513198 -58 -464199 -24 -378200 -5 -249201 -1 -281202 -9 -367203 -26 -413204 -53 -476205 -88 -496206 -127 -500207 -166 -493208 -201 -477209 -211 -477210 -229 -498211 -328 -520212 -220 -515213 -216 -506214 -180 -513215 -141 -526216 -101 -524217 -63 -498218 -32 -442219 -11 -451220 -1 -211221 -1 -261222 -8 -354223 -24 -425224 -50 -472225 -83 -497226 -120 -501227 -455 -501228 -185 -189229 -209 -460230 -247 -516231 -194 -579232 -240 -532233 -190 -481234 -158 -518235 -421 -528236 -84 -520237 -49 -489238 -22 -430239 -6 -341240 -1 -227241 -1 -227242 -9 -151243 -27 -427244 -55 -476245 -89 -501246 -126 -509247 -161 -503248 -189 -493249 -209 -477250 -220 -261251 0 0252 -208 -271253 -186 -499254 -156 -519255 -122 -526256 -84 -519257 -49 -489258 -22 -429259 -6 -340260 -1 -226261 -2 -248262 -14 -359263 -17 -418264 -61 -486265 -109 -509266 -149 -510267 -185 -501268 -214 -485269 -237 -452270 -276 -510271 -211 -568272 -263 -520273 -213 -472274 -180 -510275 -443 -524276 -103 -521277 -65 -496278 -32 -440279 -12 -350280 -2 -230281 -6 -256282 -27 -179283 -64 -460284 -103 -501285 -147 -516286 -192 -507287 -227 -490288 -256 -469289 -283 -465290 -275 -482291 -273 -500292 -263 -493293 -260 -486294 -224 -495295 -486 -517296 -141 -525297 -97 -509298 -60 -460299 -25 -174300 -5 -245301 -27 -288302 -68 -408303 -405 -488304 -155 -525305 -254 -495306 -254 -495307 -281 -473308 -307 -453309 -321 -443310 -324 -445311 -318 -451312 -313 -457313 -301 -466314 -280 -482315 -250 -502316 -214 -526317 -150 -529318 -101 -489319 -66 -405320 -26 -281321 -15 -370322 -130 -440323 -165 -524324 -312 -545325 -320 -497326 -329 -480327 -346 -455328 -361 -427329 -373 -408330 -377 -400331 -375 -403332 -368 -415333 -356 -436334 -341 -462335 -324 -485336 -316 -501337 -307 -549338 -161 -526339 -128 -439340 -14 -366341 -42 -518342 -221 -459343 -689 -608344 -478 -459345 -411 -488346 -405 -474347 -407 -435348 -418 -397349 -428 -365350 -434 -348351 -132 -349352 -424 -369353 -412 -402354 -401 -439355 -400 -477356 -406 -491357 -473 -162358 -686 -613359 223 -461360 48 -533361 48 -533362 252 -232363 -689 -417364 -490 -509365 -495 -526366 -462 -470367 -162 -424368 -465 -366369 -177 -317370 -486 -288371 -484 -289372 -473 -320373 -459 -369374 -456 -471375 -456 -471376 -489 -528377 -484 -513378 -684 -417379 253 -229380 48 -530381 0 -362382 -100 -277383 -30 -477384 -660 -702385 -517 -519386 -523 -500387 -486 -423388 -495 -344389 -520 -263390 -521 -218391 -520 -219392 -516 -264393 -489 -345394 -477 -421395 -513 -498396 -509 -521397 -651 -708398 -31 -481399 -101 -274400 0 -357401 -18 -254402 -6 -258403 -120 249404 -384 -378405 -707 -660406 -508 -492407 -459 -483408 -542 -324409 -531 -183410 -521 -150411 -521 -150412 -529 -183413 -539 -324414 -447 -479415 -489 -487416 -668 -665417 -400 -394418 -114 -16419 -9 -254420 -18 -249421 -1 -153422 -11 -120423 -113 -109424 -55 -127425 -488 -23426 -439 -691427 -576 -688428 -508 -174429 -486 -120430 -475 -90431 -475 -90432 -187 -118433 -512 -171434 -587 -678435 -389 -676436 -438 -10437 -66 -134438 -117 -107439 -12 -117440 -2 -149441 -12 -59442 -63 -62443 -63 -62444 -260 -2445 -283 -94446 -246 -250447 -415 -224448 -408 -135449 -386 -75450 -370 -38451 -370 -38452 -387 -71453 -413 -130454 -431 -210455 -207 -246456 -264 -90457 -249 -0458 -118 -9459 -62 -61460 -12 -57461 -12 -12462 -59 -12463 -152 -1464 -250 -16465 -351 -0466 -510 -48467 -187 -12468 -325 -14469 -235 -29470 -201 -8471 -200 -7472 -233 -27473 -319 -15474 -472 -40475 -500 -48476 -344 -2477 -245 -16478 -148 -1479 -57 -12480 -11 -11AVERAGE -293 -375AVEDEV: 194 176COMBIN -334AVERAGE:______________________________________

As can be seen from Tables 1 and 2, a considerable improvement has been achieved with the new concrete building panel configuration. First, there has been a reduction in variation in compressive stress values between the Y and X axes from 28% to 14%. At the same time the increase in the average pre-compressive force with the new cable configuration is a 27% increase (from 334 psi to 425 psi) over the pattern achieved in the concrete building panel of U.S. Pat. No. 4,432,175. The increased pre-compression force further enhances the concrete building panel in the following areas:

1. Allowable load capacity in flexure--ACI 18.7.1 2. Resist punching shear--ACI 11.12.2.2 3. Reduced deflection--ACI 9.5.4.1 4. Enhanced Autogenous Healing (Neville' publication Properties of Concrete, Fourth Edition)

While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.

Claims

1. A post tensioned prefabricated concrete building panel comprising a concrete slab, a cable member, and a pair of anchors, the cable member being preformed in a perimeter loop pattern, a portion of the cable extending between opposing sides of said loop pattern and bisecting the concrete slab to form a first section and a second section of the concrete slab, the second section being adjacent the first section, said anchors being secured to opposite ends of the cable member so that the cable member is maintained under a predetermined tension.

2. The post tensioned prefabricated concrete building panel recited in claim 1 wherein the cable member starts from one corner of the concrete slab to a point where the cable member enters the slab at the one corner to form the perimeter loop pattern and at a point turning 90.degree. to follow the cable member to a point midway up a Y axis of the concrete slab and then turning 90.degree. across a X axis of the concrete slab to bisect the slab and cross an opposite parallel portion of the cable member to exit out an adjacent side of the concrete slab.

3. The post tensioned prefabricated concrete building panel recited in claim 2 further comprising a first anchor placed in said one corner of the concrete slab and a second placed in said adjacent side of the concrete slab, said first anchor and said second anchor each having a hole through which said cable member passes and is anchored.

4. The post tensioned prefabricated concrete building panel recited in claim 1 wherein the predetermined stress in the cable of approximately 0.7 F.sub.u or 270 kips per square inch (ksi).

5. The post tensioned prefabricated concrete building panel recited in claim 1 further comprising a sheath encasing the cable member within the concrete slab.

6. The post tensioned prefabricated concrete building panel recited in claim 1 wherein the cable member is lubricated along its length within the concrete slab.