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Article # 0013
Design of a Slab-on-Grade Foundation with Conventional Reinforcement on Expansive Clay
By James A. Lacy, P.E.
August, 2005
Abstract
This paper describes the design of a slab-on-grade foundation for a light frame building on Houston Black Clay. The design follows the Post-Tensioned Institute method to a solution, and then converts that solution to a conventional reinforced design. Conventional reinforcing bar in substitution for tensioned cables is a viable design and implementation alternative when field conditions prevent effective post-tensioning.
Disclaimer
The design and fabrication of the foundation described in this paper is designed only for use at the author’s location and its peculiar conditions. The author of this report is not responsible for any other use or reliance on this paper, errors, omissions, or damages arising out of the use of this information.
Problem
The author desired to build a light frame building capable of housing two vehicles. The desired location was in an agricultural field in north Texas without hard surface road or electricity. The soil at the site was identified as Houston Black Clay. Time of construction was to be in the winter.
Soil condition observations made in the year previous to construction confirmed that the soil was highly expansive. During dry periods the soil would crack with deep crevices. In wet weather the soil would cling and adhere, exhibiting high clay content. The author observed a neighboring light residential structure which had concrete foundation piles to a depth of twenty feet. The piles had been lifted, indicating that the active zone could be quite deep.
Construction in the winter carried a risk of delays due to rain. Work had to be halted for a period of a week to ten days each time it rained. This was due to the inability to access the site and to work in the sticky mud.
Vehicle housing made concrete flooring attractive. It was customary in the area to use post-tensioned slabs. However, the author felt that adding a vendor (for tensioning) and the necessity of tensioning the slabs at a specific time added risk to the project.
Theory
A 24-foot by 24-foot building would be adequate for the intended use. With a small, light building a mat or ribbed foundation would work provided that it would resist shear and moments due to soil expansion and contraction. One of the main considerations was uneven moisture content in the soil, where exposed soil outside of the building and soil under the foundation do not maintain even moisture. This leads to either edge lifting or center lifting of the concrete slab. The local conditions of climate at the site tended to conditions of low rainfall for extended periods during high heat leading to the edge falling (center lifting).
The introduction in the last century of poured concrete slab-on-grade foundations in Texas and Louisiana made apparent the difficulties of expansive soils. Foundation cracking and failure mushroomed as new construction dropped pier and beam foundations in favor of less expensive concrete slab-on-grade. In the mid-1970’s the Post-Tensioning Institute (PTI) was organized and initiated research efforts at Texas A&M University towards design procedures. Rational design procedures and parametric analysis resulted in the standard design methodology known as the PTI method.
The PTI method has the advantage of an accepted engineering practice and a savings in concrete and steel reinforcement over a conventionally reinforced concrete slab. However, it does require proper placement of the reinforcing cables and a machine to tension the cables after placing the concrete.
Methodology
The author followed the PTI method using Design and Construction of Post-Tensioned Slabs-on-Ground, Post-Tensioning Institute, Second Edition, 1996, as the initial design method. Based on that design solution, the author then converted to a conventional reinforced design, adding an analysis of a cracked section which the post-tensioned design does not consider.
Soil type was identified by direct observation, aided by a US Department of Agriculture county soil survey. Hand-dug pits showed that the soil was consistent through the depths of interest. Soil attributes were based on the identified Houston Black Clay.
Moisture was based on the geographic location and Thornwaite moisture index charts in the PTI publication. The site had no trees, springs, or other anomalies.
Slab thickness was set at six inches and beam width at 12 inches. The slab was designed as a monolithic pour with outside stem-wall; if the stem-wall had not been chosen, a slab depth of 5.5 inches to accommodate a nominal 2x6 form board would have been chosen. Beam width was set with regard to conserving concrete, practicality of shovel width, and room for reinforcement.
Four beams spaced at eight feet, running in both directions, were set as the initial design point. Initial response from the model led to setting beam depth at 16 inches depth for the iterative process.
Allowable deflection was set at L/480, due to the light wood-frame construction. Had brick or masonry been an architectural element, L/960 or better would have been considered.
Design procedure continued for shear and moments in both directions for both center lift and edge lift conditions. The procedure converged to a solution with post-tensioned cables and beam depths of 16 inches.
At this point the PTI slab was converted to a conventionally reinforced slab. The calculated deflections and moments of inertia were used to set a starting point for analysis of a conventional cracked section. After iteration, the new beam depths were calculated to be 24 inches deep, containing four #6 reinforcing bars. #6 was the largest bar readily available from vendors.
While the calculated deflection was greater with conventional reinforcing bar than with post-tensioned cables at the beam depths, calculated deflection was still within allowable limits using either solution.
Calculated Design
A spreadsheet of calculation results is presented in the Appendix. Terminology is defined in the PTI reference. Detailed formulas are not shown; they are also the PTI standard equations and method. Section A soil numbers are taken from the PTI reference. Sections B through K follow the PTI equations and recommendations. Section L converts the PTI solution to a conventional reinforced concrete design; a traditional transformed cracked section and moment of inertia lead to deflection calculations. Section L follows methodology in Ringo and Anderson Designing Floor Slabs on Grade.
Implementation
One bright spot about the clay soil was that its cohesiveness worked to advantage when excavating the foundation beams. Actual concrete placement quantity was almost exactly the calculated quantity.
Top slab reinforcement was #3 reinforcing bar, 12 inches on center in both directions. The bar was supported on plastic chairs. The author prefers this over wire mesh, as the reinforcing bar may be stepped on without problems while the concrete is placed. Stepping on mesh deforms it down into the beams and to the bottom of the slab which negates reinforcement and shrink control.
Placement was made on a cool day, but with high wind. While the author would have preferred not to place on a windy day, it was the only opening between rainy wet spells and an impending freeze. A double layer of plastic sheeting protected the curing concrete from freezing conditions which came a few days after placing.
Results
In this case, the choice of conventional reinforcement turned out to be the right choice for this project. After concrete placement, wet weather prevented access to the site. Cable stressing could not have been accomplished between three and ten days after concrete placing, as is recommended by PTI.
The author would use this method again when constrained by field conditions. Although material costs for concrete and reinforcement were higher, and the beam forms were dug deeper, costs were saved on the lack of cable tensioning and schedule slip, and no trucks were stuck in the mud.
About the Author
James A. Lacy is a registered professional engineer in Texas. His publications include Systems Engineering Management: Achieving Total Quality, McGraw-Hill, 1992.
List of Works Consulted
Brown, Robert Wade, ed. Practical Foundation Engineering Handbook. McGraw-Hill. 1996.
Parker, Harry. Simplified Design of Reinforced Concrete. Wiley. Second Edition. 1960.
Post-Tensioning Institute. Design and Construction of Post-Tensioned Slabs-on-Ground. Post-Tensioning Institute. Second Edition. 1996.
Ringo, Boyd C., and Anderson, Robert B. Designing Floor Slabs on Grade. The Aberdeen Group. Second Edition. 1996.
Appendix
Analysis of Slab-on-Grade Foundation | PTI Method | ||||||
A. Design Data | |||||||
1500000 |
Ec, creep mod, concrete | ||||||
1000 |
Es, soil, psi | ||||||
Short = |
24 |
feet | |||||
3000 |
fprimec, 28 day concrete | ||||||
psi compression | |||||||
11200 |
Assume live load lb | ||||||
Long = |
24 |
feet |
6 |
Slab thickness, inches | |||
12 |
Beam width, inches | ||||||
Clay is montmorillonite. | |||||||
30 |
Thornthwaite moisture index, from Fig A.3.3.a. | ||||||
3.2 |
Soil suction pf, Fig A.3.6 | ||||||
0.7 |
Estimated Moisture velocity max inches per month. | ||||||
5.5 |
em edge moisture variation distance, Table, feet edge lift. | Conservative | |||||
4.5 |
em center moisture variation distance, Table, feet center lift. | Conservative | |||||
70% clay, Houston Black Clay. | |||||||
7 |
Z depth to constant suction, feet. | ||||||
0.7 |
By chart, ym estimated differential swell for center lift, inches. | ||||||
1 |
By Chart, ym estimated differential swell for edge lift, inches. | ||||||
B1. Preliminary beam depth, center lift. | |||||||
Long direction | |||||||
24 |
Beam length, feet | ||||||
8 |
Beam spacing, feet |
4.00 |
Number of beams, long | ||||
480 |
Perimeter load, pounds per foot. | ||||||
10 |
Beta assumed to be 10 feet. | ||||||
24 |
Smaller of L (beam length) or 6Beta. | ||||||
480 |
C delta, Table 6.2 | relaxed | |||||
0.6 |
delta allowable in inches | ||||||
8.04 |
hlong, approximate depth of stiffening beam, inches | ||||||
16.00 |
Assumed beam depth, inches | ||||||
Short direction | |||||||
24 |
Beam length, feet | ||||||
8 |
Beam spacing, feet |
4.00 |
Number of beams, short | ||||
480 |
Perimeter load, pounds per foot. | ||||||
10 |
Beta assumed to be 10 feet. | ||||||
24 |
Smaller of L (beam length) or 6Beta. | ||||||
480 |
C delta, Table 6.2 | relaxed | |||||
0.6 |
delta allowable in inches | ||||||
8.04 |
hshort, approximate depth of stiffening beam, inches | ||||||
16.00 |
Assumed beam depth, inches | ||||||
B2. Preliminary beam depth for edge lift condition. | |||||||
Long direction | |||||||
24 |
Beam length, feet | ||||||
8 |
Beam spacing, feet |
4.00 |
Number of beams, long | ||||
480 |
Perimeter load, pounds per foot. | ||||||
10 |
Beta assumed to be 10 feet. | ||||||
24 |
Smaller of L (beam length) or 6Beta. | ||||||
480 |
C delta, Table 6.2 | ||||||
0.6 |
delta allowable in inches | ||||||
12.80 |
hlongedge, preliminary beam depth, inches | ||||||
16.00 |
Assumed beam depth, inches | ||||||
Short direction | |||||||
24 |
Beam length, feet | ||||||
8 |
Beam spacing, feet |
4.00 |
Number of beams, long | ||||
480 |
Perimeter load, pounds per foot. | ||||||
10 |
Beta assumed to be 10 feet. | ||||||
24 |
Smaller of L (beam length) or 6Beta. | ||||||
480 |
C delta, Table 6.2 | ||||||
0.6 |
delta allowable in inches | ||||||
12.80 |
hshortedge, preliminary beam depth, inches | ||||||
16.00 |
Assumed beam depth, inches | ||||||
Max assumed beam depth | |||||||
16.00 |
Max assumed beam depth, long, inches | ||||||
16.00 |
Max assumed beam depth, short, inches | ||||||
C. Check soil bearing | |||||||
43200 |
Slab weight | ||||||
24000 |
Beam weight | ||||||
46080 |
Perimeter load | ||||||
11200 |
Live load | ||||||
124480 |
Total, pounds | ||||||
176 |
Beam bearing area, sq ft | ||||||
707.27 |
Soil Pressure, pounds/square foot | ||||||
D. Calculate section properties | |||||||
Long direction | |||||||
Area, in2 |
y, in |
Ay, in3 |
Ay2 |
Io |
|||
Slab |
1440 |
-3 |
-4320 |
12960 |
4320 |
||
Beams |
768 |
-8 |
-6144 |
49152 |
16384 |
||
2208 |
-10464 |
62112 |
20704 |
||||
20704 |
|||||||
82816 |
|||||||
-4.74 |
yt, axis, inches | ||||||
33,226 |
I, Gross moment of inertia, inch4 | ||||||
7,011 |
St, Section modulus with respect to top,in3 | ||||||
2,951 |
Sb, Section modulus with respect to Bottom, in3 | ||||||
Short direction | |||||||
Area, in2 |
y, in |
Ay, in3 |
Ay2 |
Io |
|||
Slab |
1440 |
-3 |
-4320 |
12960 |
4320 |
||
Beams |
768 |
-8 |
-6144 |
49152 |
16384 |
||
2208 |
-10464 |
62112 |
20704 |
||||
20704 |
|||||||
82816 |
|||||||
-4.74 |
yt, axis, inches | ||||||
33,226 |
I, Gross moment of inertia, inch4 | ||||||
7,011 |
St, Section modulus with respect to top,in3 | ||||||
2,951 |
Sb, Section modulus with respect to Bottom, in3 | ||||||
E. Prestressing steel requirements | |||||||
Number of tendons required for minimum average prestress: | |||||||
Stress in tendons immediately after anchoring: | |||||||
270 |
Prestressing steel - 1/2" - 270ksi | ||||||
0.153 |
Aps, cross sectional area of one tendon, in2 | ||||||
189 |
fpi, stress permitted per tendon, in ksi | ||||||
159 |
fe, stress in tendon after losses, ksi | ||||||
24.327 |
Force per tendon, kips | ||||||
4.54 |
Nt(long), number of tendons | ||||||
4.54 |
Nt(short), number of tendons | ||||||
Number of tendons required to overcome slab-subgrade friction on polyethylene: | |||||||
67200 |
Weight of beams and slab | ||||||
0.75 |
mu, coefficient of friction, .75 | ||||||
1.04 |
Nt, number of tendons, friction, each direction | ||||||
Total number of tendons to provide 50 psi minimum and overcome friction: | |||||||
Long: | |||||||
5.57 |
Nt(long) | ||||||
5.57 |
Nt(short) | ||||||
9 |
Nt(long) | ||||||
9 |
Nt(short) | ||||||
Recheck: | |||||||
8.80 |
Nt(long) | OK | One every 5 feet, slab. | ||||
8.80 |
Nt(short) | OK | One every 5 feet, slab. | ||||
Calculate Pr | |||||||
218.94 |
Prlong, resultant prestress force after all losses, kips | ||||||
218.94 |
Prshort, resultant prestress force after all losses, kips | ||||||
2.00 |
Reinforcement cover, long, top, inches | ||||||
3.00 |
Reinforcement cover, long, bottom, inches | ||||||
2.00 |
Reinforcement cover, short, top, inches | ||||||
3.00 |
Reinforcement cover, short, bottom, inches | ||||||
2.74 |
elong, top, prestress eccentricity, inches | ||||||
8.26 |
elong, bottom, prestress eccentricity, inches | ||||||
2.74 |
eshort, Top, prestress eccentricity, inches | ||||||
8.26 |
eshort, bottom, prestress eccentricity, inches | ||||||
0.33 |
ft, allowable concrete tensile stress, ksi | ||||||
1.35 |
fc, allowable concrete compressive stress, ksi | ||||||
F. Design Moments | |||||||
Center Lift | |||||||
Long Direction | |||||||
0.520894 |
Ao, constant | ||||||
4.5 |
em |
1 |
|||||
1 |
B, constant |
0 |
|||||
0 |
C, constant | ||||||
3.353 |
MLong, moment in long direction, foot kips / foot | ||||||
Short Direction | |||||||
3.353 |
Mshort, moment in short direction, foot kips / foot | ||||||
Edge Lift | |||||||
Long Direction | |||||||
4.260214 |
Mlong, moment in long direction, edge lift, foot kips / foot | ||||||
Short Direction | |||||||
4.260214 |
Mshort, moment in short direction, edge lift, foot kips / foot | ||||||
G. Compare actual and allowable service load stresses | |||||||
Center lift | |||||||
Long direction | |||||||
Tension in the top fiber: | |||||||
-470.675 |
Pr e, inch-kips | ||||||
8.05 |
Moment allowable, top, foot-kips / foot | ||||||
3.35 |
Design moment, foot-kips / foot | ||||||
OK | Design less than allowable? | ||||||
Compression in bottom fiber: | |||||||
-470.675 |
Pr e, inch-kips | ||||||
10.45 |
Moment allowable, top, foot-kips / foot | ||||||
3.35 |
Design moment, foot-kips / foot | ||||||
OK | Design less than allowable? | ||||||
Short direction | |||||||
Tension in the top fiber: | |||||||
-470.675 |
Pr e, inch-kips | ||||||
6.87 |
Moment allowable, top, foot-kips / foot | ||||||
3.35 |
Design moment, foot-kips / foot | ||||||
OK | Design less than allowable? | ||||||
Compression in bottom fiber: | |||||||
-470.675 |
Pr e, inch-kips | ||||||
9.27 |
Moment allowable, top, foot-kips / foot | ||||||
3.35 |
Design moment, foot-kips / foot | ||||||
OK | Design less than allowable? | ||||||
Edge Lift | |||||||
Long direction | |||||||
Tension in bottom fiber: | |||||||
-470.675 |
Pr e, inch-kips | ||||||
6.75 |
Moment allowable, bottom, foot-kips / foot | ||||||
4.26 |
Design moment, foot-kips / foot | ||||||
OK | Design less than allowable? | ||||||
Compression in top fiber: | |||||||
-470.675 |
Pr e, inch-kips | ||||||
32.81 |
Moment allowable, top, foot-kips / foot | ||||||
4.26 |
Design moment, foot-kips / foot | ||||||
OK | Design less than allowable? | ||||||
Short direction | |||||||
Tension in bottom fiber: | |||||||
-470.675 |
Pr e, inch-kips | ||||||
7.93 |
Moment allowable, bottom, foot-kips / foot | ||||||
4.26 |
Design moment, foot-kips / foot | ||||||
OK | Design less than allowable? | ||||||
Compression in top fiber: | |||||||
-470.675 |
Pr e, inch-kips | ||||||
34.00 |
Moment allowable, top, foot-kips / foot | ||||||
4.26 |
Design moment, foot-kips / foot | ||||||
OK | Design less than allowable? | ||||||
H. Differential Deflection, Edge Lift | |||||||
Edge Lift | |||||||
Allowable differential deflection, long direction, edge lift | |||||||
7.00 |
Beta, long direction, edge lift, feet | ||||||
42.01 |
6 Beta | ||||||
24.00 |
Min of Long or 6Beta | ||||||
0.60 |
Delta allowable long, inches | ||||||
Expected differential deflection, long direction, edge lift | |||||||
0.37 |
Delta expected, long, inches | ||||||
OK | Deflection in long direction | ||||||
Allowable differential deflection, short direction, edge lift | |||||||
7.00 |
Beta, short direction, edge lift, feet | ||||||
42.01 |
6 Beta | ||||||
24.00 |
Min of Long or 6Beta | ||||||
0.60 |
Delta allowable long, inches | ||||||
Expected differential deflection, short direction, edge lift | |||||||
0.37 |
Delta expected, short, inches | ||||||
OK | Deflection in short direction | ||||||
I. Shear Calculations, edge lift | |||||||
Long direction | |||||||
1.93 |
Expected shear force, long direction, edge lift condition, kips / foot | ||||||
82.2 |
Allowable shear stress, psi | ||||||
60.44 |
Total design shear stress, long direction, edge lift, psi | ||||||
OK | Shear stress in long direction | ||||||
Short direction | |||||||
1.93 |
Expected shear force, short direction, edge lift condition, kips / foot | ||||||
82.2 |
Allowable shear stress, psi | ||||||
60.44 |
Total design shear stress, short direction, edge lift, psi | ||||||
OK | Shear stress in short direction | ||||||
J. Differential deflection, center lift | |||||||
Center Lift | |||||||
Long Direction | |||||||
Allowable differential deflection, long direction, center lift | |||||||
7.00 |
Beta, long direction, center lift, feet | ||||||
42.01 |
6 Beta | ||||||
24.00 |
Min of Long or 6Beta | ||||||
0.60 |
Delta allowable long, inches | ||||||
Expected differential deflection, long direction, center lift | |||||||
0.29 |
Delta expected, long, inches | ||||||
OK | Deflection in long direction | ||||||
Short direction | |||||||
Allowable differential deflection, short direction, center lift | |||||||
7.00 |
Beta, short direction, center lift, feet | ||||||
42.01 |
6 Beta | ||||||
24.00 |
Min of Short or 6Beta | ||||||
0.60 |
Delta allowable short, inches | ||||||
Expected differential deflection, short direction, center lift | |||||||
0.29 |
Delta expected, short, inches | ||||||
OK | Deflection in short direction | ||||||
K. Shear Calculations, center lift | |||||||
Long direction | |||||||
0.63 |
Expected shear force, long direction, edge lift condition, kips / foot | ||||||
82.2 |
Allowable shear stress, psi | ||||||
19.81 |
Total design shear stress, long direction, center lift, psi | ||||||
OK | Shear stress in long direction | ||||||
Short direction | |||||||
0.81 |
Expected shear force, short direction, edge lift condition, kips / foot | ||||||
82.2 |
Allowable shear stress, psi | ||||||
25.32 |
Total design shear stress, short direction, edge lift, psi | ||||||
OK | Shear stress in short direction | ||||||
L. Conventionally reinforced slab conversion | |||||||
Edge lift | |||||||
0.60 |
Post tensioned slab long direction allowable differential deflection, inches | ||||||
0.37 |
Post-tensioned slab long direction expected differential deflection, inches | ||||||
33,226 |
Previous moment of inertia | ||||||
20,762 |
Minimum acceptable I by ratio: | ||||||
27,614 |
Starting point I, inch4; cracked section | ||||||
Long direction | |||||||
Choose a section for the long direction approximating the desired moment of | |||||||
inertia. | |||||||
Add rebar in place of cable. | Bar no | Bar area | |||||
Develop transformed steel area. |
4 |
0.2 |
|||||
9 |
n, transform area. |
5 |
0.31 |
||||
24 |
New beam depth, inches |
6 |
0.44 |
||||
4 |
Number of bars in a beam |
7 |
0.6 |
||||
6 |
Bar size by number |
8 |
0.79 |
||||
0.44 |
Area of bar, in2 |
9 |
1 |
||||
10 |
1.27 |
||||||
11 |
1.56 |
||||||
14 |
2.25 |
||||||
Determine neutral axis and section properties: | |||||||
Area, in2 |
y, in |
Ay, in2 |
|||||
Concrete slab |
1728 |
3 |
5,184 |
||||
Steel (transformed) |
63.36 |
20.5 |
1,299 |
||||
1791.36 |
6,483 |
||||||
3.62 |
ybar, distance from top fiber to neutral axis, inches | ||||||
Determine moment of inertia: | |||||||
Io |
d |
Ad2 |
TI |
||||
Concrete slab |
5184 |
0.62 |
662 |
5,846 |
|||
Steel (transformed) |
0 |
16.88 |
18,056 |
18,056 |
|||
23,902 |
Total | ||||||
Check chosen section for allowable deflection, long direction: | |||||||
6.45 |
Beta, new, feet | ||||||
38.69011 |
6Beta | ||||||
0.97 |
Delta allowable, inches | ||||||
Determine expected differential deflection, long direction: | |||||||
33,226 |
I of post tensioned section | ||||||
23,902 |
I of cracked conventional section | ||||||
0.37 |
Previous expected differential deflection, post tensioned, inches | ||||||
0.52 |
Expected deflection of conventional reinforced section, inches | ||||||
OK | New expected is under new allowable? | ||||||
Short direction: | |||||||
0.60 |
Previous allowable deflection, short direction, inches | ||||||
0.37 |
Previous expected deflection, short direction, inches | ||||||
33,226 |
Previous I, short direction, in4 | ||||||
20,762 |
Minimal acceptable I by ratio | ||||||
27,614 |
Design starting point, I, in4 | ||||||
Proceed with same depth of new beams and steel as long direction | |||||||
Determine neutral axis and section properties: | |||||||
Area, in2 |
y, in |
Ay, in2 |
|||||
Concrete slab |
1728 |
3 |
5,184 |
||||
Steel (transformed) |
63.36 |
20.5 |
1,299 |
||||
1791.36 |
6,483 |
||||||
3.62 |
ybar, distance from top fiber to neutral axis, inches | ||||||
Determine moment of inertia: | |||||||
Io |
d |
Ad2 |
TI |
||||
Concrete slab |
5184 |
0.62 |
662 |
5,846 |
|||
Steel (transformed) |
0 |
16.88 |
18,056 |
18,056 |
|||
23,902 |
Total | ||||||
Check chosen section for allowable deflection, short direction: | |||||||
6.45 |
Beta, new, feet | ||||||
38.69011 |
6Beta | ||||||
0.97 |
Delta allowable, inches | ||||||
Determine expected differential deflection, short direction: | |||||||
33,226 |
I of post tensioned section | ||||||
23,902 |
I of cracked conventional section | ||||||
0.37 |
Previous expected differential deflection, post tensioned, inches | ||||||
0.52 |
Expected deflection of conventional reinforced section, inches | ||||||
OK | New expected is under new allowable? | ||||||
Article # 0013 TEST QUESTIONS:
1. The use of conventional reinforcing bar rather than post-tensioned cables is a viable design and implementation alternative when ....?
Field conditions prevent effective post-tensioning.
Reducing the quantity of concrete is most important.
Reducing the quantity of reinforcement is most important.
All of the above
2. In this article, what does PTI stand for?
Procedure for Tensioning In situ
Parametric Technology Incorporated
Post-Tensioning Institute
Pittsburg Technical Institute
3. A disadvantage of the PTI method is ...?
A machine is required to tension the cables after placing the concrete.
Typically, more concrete is required with the post-tensioned method.
The PTI method provides savings in steel reinforcement over conventional methods
All the equations that must be solved.
4. In this example, the soil type at the north Texas site was identified as ...?
Amarillo sandy loam
Midland red clay
Purves clay
Houston Black Clay
5. PTI recommends that cable stressing should be accomplished ...?
Three to ten weeks after concrete placing.
Three to ten days after concrete placing.
Three to ten hours after concrete placing.
Before calling the local code inspector.
6. Why does the author prefer bar over mesh reinforcement for the top slab?
Reinforcing bar may be stepped on without problems while the concrete is placed.
Reinforcing bar is more economical and easier to adjust as the concrete is placed.
Reinforcing mesh floats to the top of the slab as the concrete is placed.
All of the above
7. For this example, the PTI method converged on a solution with beam depths of ____ inches?
24
22
18
16
8. When the design was converted to a conventionally reinforced slab, the beam depths ...
decreased to 16 inches.
did not change.
increased to 16 inches.
increased to 24 inches.
9. In this example and due to the light wood-frame construction, the allowable deflection was set to ...
3.0 inches
0.3 inches
L/960
L/480
10. What were the considerations is setting the beam width at 12 inches?
Shovel width.
Conserving concrete.
Allowing room for reinforcement.
All of the above
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