The pretopped double tee is a very common precast,

Transcription

1 President (7.5 Engineering Manager the interest of designers and precast concrete manufacturers The Shockey Precast Group Winchester, Virginia alike. Furthermore, the design live load of 80 psf (3.83 kpa) for roof tees in some mid-atlantic areas is quite high and is (18.3 to 18.6 m). BACKGROUND Under the current ACT Code, the load combina tion for ultimate strength is 1.2D + 1.6L, and the -factors for flexure and shear are 0.90 and 0.75, respectively. These Charles Wynings, P.E. Engineering Manager are significant changes from previous editions. Since 1971, Tindall Corporation Petersburg, Virginia a number of load tests have been conducted on full-size un 66 PCI JOURNAL omission of web reinforcement in prestressed 1f <lftl 12/), and the interest in omitting web reinforcement for shear by Section have attracted recent ACT Building Code requirements for features, including total omission of shear prestressed concrete product in North America. The load and factors, the impact of bottom tensile stress, es peciajy for the Class T category under Section typical units used in parking structures. Design of reinforcement in the web. The paper concludes with appropriate recommendations related to the The pretopped double tee is a very common precast, tees spanning 62 ft (18.9 m) and representing bottom tensile stress under full live load was in the range of to ii. units in the Class T category of AC! , Section Although the specimens stem dimensions 9-..[j, which places these varied, the double tees shared many common Building Code for load and resistance factors. The the units used heavy live loads and the AC! tees. All specimens were typical production double This paper describes three static load tests that were conducted on full-scale pretopped 12DT30 double concrete double tees of similar geometry. Without Web Reinforcement Load Testing of Prestressed often used in conjunction with a unit length of 60 to 61 ft Concrete Double Tees Alex Aswad, Ph.D., P.E., FPCI Professor of Engineering The Pennsylvania State University Middletown, Pennsylvania Blue Ridge Design, Inc. Ned M. Cleland, Ph.D., P.E., FPCI Winchester, Virginia David Orndorff, P.E. Denver, Pennsylvania Senior Engineer High Concrete Structures, Inc. George Burnley, P.E. at Harrisburg

2

3

4

5

6

7

8

9 /8 in. (9.5 mm) diameter strands. Each end bearing area was reinforced with a special steel plate welded to one No. 6 (19.1 mm), 4.0 ft (1.22 m) long bot tom reinforcing bar. The design live load was 80 psf (3.83 kpa). A listing of properties re sulting from this loading is provided in Table 1. The computer analysis shows a predicted release camber of 0.81 in. (21 mm) and a camber at erec tion of 1.39 in. (35.3 mm). On the test date, the measured camber (using a string line) was approximately 1.7 in. (43 mm). Fig. 10. Specimen 2: Stage 5 loading (near failure). reached nearly simultaneously. Fig. 10 shows the specimen near the end of Stage 5. TEST RESULTS FOR SPECIMEN 3 Specimen 3 consisted of a 12 ft (3.66 m) wide, 30 in. (760 mm) deep double tee cast on October 10, 2002, with a total length of ft (19.1 m). The specimen contained twelve /16 in. (14.3 mm) diameter straight strands, which were tensioned to 75 percent of the UTS. Strands in Row 3 from the bottom were debonded for 6 ft (1.83 m) at each end. In addition to the prestressing reinforcement, one 20 ft (6.1 m) long No. 7 (22.2 mm) Grade 60 ksi (414 MPa) mild steel reinforc ing bar was placed at 7 in. (127 mm) from the bottom in each stem and cen tered within the span. The nominal concrete strengths weref = 3800 psi (26.2 MPa) and f = 6000 psi (41.4 MPa). The actual av erage strength at release wasf = 3938 psi (27.2 MPa). On the test date (November 1), the actual average strength wasf = 7130 psi (49.2 MPa). Welded wire fabric in the deck was supported by four slightly tensioned Loading stages and observations To simulate each of the five loading stages, a series of concrete blocks with different weights were placed in a symmetrical layout at specific loca tions in order to reach the target midspan total moment, MT. The exact positions and equivalent point load magnitudes for Stage 5 are shown in Fig. 11. The comprehensive curve showing the midspan live load mo ment versus the camber change for all stages is shown in Fig. 12. The load test started at 8:15 a.m. Total effective duration of loading to failure was approximately four hours. The weather was cloudy, with temper atures between 40 and 48 F (4 and 9 C). At the end of Stage 1, with a calcu 2x 1.114k BLKS _2x 1.138k BLKS TEE 2.531k BLKS EA. LOC. Ni I I i. I I _.. I I I I in. I- - I- I ini I I Fig. 11. Specimen 3, Stage 5: Required failure load with = CENTERLINES of BEARING 74 PCI JOURNAL

10

11

12