MMFX Steel Alternative to Post-Tensioning for Pier Cap

By

Amgad M.F. Girgis, Ph.D., S. E., Adam Sevenker, E.I.T.,

and Maher K. Tadros, Ph.D., P.E.

eConstruct.USA, LLC, Omaha, Nebraska

This article summarizes a study conducted by eConstruct.USA, LLC for MMFX Technologies

Corporation. Our company has recently completed the design of a bridge with a precast concrete

pier cap requiring both bottom pretensioning and top post-tensioning. The results of the study

given below demonstrate that use of ASTM A1035 steel reinforcing bars supplied by MMFX

result in several advantages including simplified construction steps, reduced pier cap weight and

concrete quantities and reduced reinforcement cost.

There are commonly two types of concrete reinforcing steel used in the US, grade 60 ksi mild

reinforcement, per ASTM A615 Standard, and Grade 270 ksi prestressing seven-wire strand, per ASTM

A416 Standard. When mild rebar is used as the primary flexural reinforcement, the structural member

is expected to be cracked under service loading conditions. Cracking must be controlled

through certain detailing conditions, in an effort to minimize the potential for corrosion of the

rebar.

When strand is used, it is generally employed in an active way to pre-compress the concrete zone

that is expected to go into tension under external loads. Pretensioned prestressed

concrete has been in common practice for several decades with good success. Prestress, requires

that the product be made in a precasting facility and the product shipped to the jobsite. The

strands are stretched to a certain limit, the concrete is placed around them. The prestress is released

to the ends of the member when concrete strength reach the design limit.

Sometimes, as in the application discussed here, prestress in the form of "post -tensioning"

used when pretensioning is not convenient or economical. Post-tensioning as the name implies

involves inserting multi-strand tendons through ducts in the already-hardened concrete, anchoring one

end and tensioning the other end against the concrete member, thus imparting compression on the

member. Thus duct is pressure grouted to "bond" the tendons to the concrete.

Prestressing, while it has the value of keeping concrete from cracking under service loads, requires

additional construction steps. It is harder to design than conventional reinforcing. Pretensioning

requires a factory and the precast products must be shipped, erected and connected together.

Post-tensioning requires specialized jacking equipment and personnel. It also requires careful

grouting with high quality products and processes.

The recent introduction of MMFX corrosion resistant high strength, ASTM A1035, reinforcing steel

may allow in some applications for simpler design and construction than prestressing and for

lower amounts of steel than use of mild reinforcing. It has been recognized by ASTM, ACI and

most recently by AASHTO LRFD Bridge Design Specifications. ACI has recently published ACI ITG

6R-10 as a design guide for designers who wish to design with this material utilizing the high yield

strength of 100 Ksi. This guide is used in the application described here in a simplified form

developed by eConstruct engineering.

To perform flexural strength design, the stress-strain relationship for the steel needs to be defined. A

combination of sources was used by eConstruct to develop what is called the stress -strain Power

Formula relationship. The lower bound values of test results were used, see Fig. 1. Also, the ASTM

A1035 standards specify minimum values for "yield" strength at 0.2 percent proof strain and

ultimate elongation. The design values cannot be assumed in design to exceed these minimums.

The ACI ITG 6R-10 recommends a series of formulas for the stress for a given strain. The Power

Formula given below is valid for the entire range from zero strain to the strain that produces the

ultimate strength. From that point on, the stress is conservatively assumed to be constant.

The Power Formula below is based on work by Mattock (1979) and Devalapura and Tadros

(1992), and is given in the PCI Bridge Design Manual and used extensively by designers. The

coefficients given in Equation 1 were developed using the properties just described for the

Grade 100 ksi, ASTM A1035 steel.

Figure 1. Stress-Strain Curves for ASTM A-1035 compared with ASTM A-615 and ASTM A416

Steel.

The figure shows a plot of the results of Eq. 1 in comparison with the results of the ACI ITG 6

formulas and the lower bound of test results. The figure also shows the curve assumed in

design for Grade 60 mild steel and for Grade 270 prestressing strands.

Because Grade 100 ksi steel does not have a yield plateau, the flexural strength needs to

include iteration that accounts for the strain (and thus stress) in the steel. The same process is

used for f lexural analysis with strands. Such analysis procedure is called "the strain

compatibility method." Refer to Fig. 2. Engineers at eConstruct have developed a spread sheet

that aids in the performance of this analysis and also allows for calculation of the strength reduction

(resistance) factor to be applied to the nominal strength to obtain the available design

strength. For more information, please refer to the PCI Bridge Design Manual where this method

has been adopted.

Figure 2. Strain Compatibility Method Graphs.

Equipped with the tools given above, one can perform analysis of the pier cap example. Input

for this specific example is as follows:

The cap's original design is for a three span bridge, 120-230-120 ft long, 48.5 ft wide. The pier cap

is 8.5 ft deep, supporting two 7.5 ft deep U-shaped beams. A single column having a cross section, 5

ft wide (in the direction of traffic) by 12 ft, is required due to traffic sight distance conditions, See

Figs. 3 and 4. This creates large bending and shearing forces on the pier cap from the

supported U-beams. Longitudinal U-beam post-tensioning must pass through the precast pier cap

and two 2-ft wide cast-in-place closure pours on the two sides of the cap. The cap is double-

cantilevered. It is 42.5 ft long, 8.5 ft deep. Its main reinforcement due to full loads is 8 post-

tensioning tendons, each containing 19-0.6 inch strands. The pier PT must accommodate

the tight space available between the U-beam tendons, refer to Fig. 5. The PT anchors require that

the width of the cap be at least 5 ft- 6 in. at its ends, reduced to 5 ft width as it joins with the U-

beams. The PT, being concentrated near the top, creates undesirable tension in the bottom

fibers before the pier cap is fully loaded. For this reason, additional 300.6 in. pretensioning

strands are provided near the bottom f ibers. In addition to the prestressing, stirrups and

horizontal skin reinforcement are provided in the cap as shown in the figures. The passive

reinforcement is all Grade 60 steel in the original design.

A 3-ft diameter access hole must be provided in the pier cap to allow for internal in-service

inspection. The resulting pier cap weight is nearly 123 tons.

Figure 3. Elevation View of the Prestressed Pier Cap

Figure 4. Side View of the Prestressed Pier Cap

Figure 5. Pier Cap Reinforcement in 3-D

The alternate design explored here is to use an inverted tee cap with the bottom flange width is

the same, 5 ft, as the original design. However, the web can be narrower, controlled by shear

capacity and space available reinforcement. It was determined that a 2-ft wide web would be

sufficient. To accommodate the depth of the U-beams, the bottom flange depth must be only 1 ft

thick. The proposed solution, besides providing lighter cap, has the very significant advantage of

possible elimination of most of the shoring required to support the U-beams before they are

made continuous across the pier cap. The transverse direction must be carefully designed to allow

for the weight of the U-beam before it is made integral with the cap through the closure pour and

the longitudinal post-tensioning. The primary flexural reinforcement is changed from 270 ksi

prestressing strand to ASTM A1035 bars. The shear and horizontal reinforcement is also changed to

the same ASTM A1035 steel, refer to Fig. 6.

For shear design, the AASHTO LRFD specifications currently limit the design strength of the

steel to 75 ksi. In the design for shear, both the vertical and horizontal bars are allowed to contribute to

shear resistance as permitted by the AASHTO LRFD specifications, refer to Fig. 7. Despite the fact that

MMFX Steel conforming to ASTM A1035 is corrosion resistance, this additional value is ignored here

and crack control criteria have been satisfied. An advantage of using ASTM A1035 as flexural

reinforcement in shear resistance that is not intuitively obvious is the fact that the effective shear

depth is larger than that with the PT solution, and the demand for shear reinforcement is

correspondingly reduced. A disadvantage of eliminating prestress is steeper shear angle that does

not allow for as much vertical reinforcement to be engaged, as for the PT solution. The effects

of the larger effective depth and the steeper angles are offsetting each other and the quantities

of steel in the two solutions appear comparable. The reinforcement required for the ASTM A1035

alternate is summarized below:

1) 24 - #11 bars flexural reinforcement near the top fibers.

2) #8 bars at 6 in. spacing vertical and horizontal steel for shear and crack control.

3) Additional reinforcement is needed in the ledge to support the U-beams, in lieu of

shoring, until the beams are made continuous across the cap.

Figure 6. Cross section of the alternative design.

Figure 7. Elevation view of the alternative design

As shown in Fig. 8, one of the important advantages of the alternative design, besides the much

simpler detailing, is that the tension demand on the pier cap bottom fiber at release and

handling is greatly reduced. Thus the pretension strands at the bottom fiber can be eliminated.

Eliminating the pretensioning strands makes the total flexural reinforcement of the alternative

design almost equal to the original design despite the grade difference in steel, refer to Table 1.

Figure 8. Alternative Pier Cap Reinforcement in 3-D

Ledge Reinforcement Was Removed For Clarity

Summary and Conclusions

The alternative design does not have geometrical restrictions compared to the post-tensioning

solution. The alternative design also does not have to accommodate any of the anchorages or

end zone reinforcement required by the original design. This flexibility in the alternative design

allows the pier cap weight to be less than 50% of the original design, refer to Table 1.

The alternative design resulted in a much lighter and prismatic section which will significantly

improve the production and construction cost. Construction safety is also a byproduct of reducing

precast segment weight. This is not to mention eliminating the falsework supporting the tubs,

which is not only a major cost saving but also speeds up construction. This is in line with the

national trend of rapid construction. However, the reinforcement of the ledge must be designed

carefully to ensure safety during construction.

The precast pier cap of the alternative design does not have the temporary high tension

demand on the bottom fiber and therefore the bottom pretensioning can be eliminated. Also,

the use of corrosion resistant high strength steel allows the flexural reinforcement to be

positioned in a manner to create a large lever arm between the tensile forces and the

compression forces. This aids in the flexural capacity of the diaphragm and helps to reduce the

amount of shear reinforcement required.

As a summary and referring to table 1, the savings in steel quantities are not significant but the saving in construction and production costs, time and safety are substantial.

Table 1. Total material quantities of flexural steel and concrete in the two designs.

Item Original Design

Alternative Design

MMFX Conforming to ASTM

A1035

Weight

(tons)

Volume

(Yard3)

Weight

(tons)

Volume

(Yard3)

Concrete 123 61 58 29

Weight

(tons)

Grade

(Ksi)

Weight

(tons)

Grade

(Ksi)

Flexure

Reinforcement 2.8 270 2.7 100

Shear

Reinforcement 5.6 60 3.5 75

Leger

Reinforcement

-

2.2 75