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					                         Research Proposal

      Continuous Shallow Hollow Core Floor Systems

                            Submitted to the

                 Research and Development Committee
                 Precast/Prestressed Concrete Institute

                                  By

                    Maher K. Tadros, Ph. D., P.E.
             The Charles J. Vranek Distinguished Professor

                         Amgad Girgis, Ph. D.,
                      Research Assistant Professor

Nicholas Reiser, M.S.C.E. Student and Augusto Alburquerque, Ph.D. Fellow
                     University of Nebraska-Lincoln


                      mtadros@mail.unomaha.edu
                             402 554 4842
                             402 554 3288




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Background
This proposal address means of optimization of the total framing systems for multi-story office,
commercial, hotel/motel and condominium buildings. It focuses on minimization the depth of the
inverted tee (IT) beams that support the hollow core (HC) planks and on practical methods of
effecting continuity in the HC/IT/Column joints to allow resistance to lateral forces without need
for shear walls.

Buildings are required to resist gravity as well as lateral loads. Often, shear walls are used to resist
the lateral load and the hollow core-beam-column frame is assigned to resistance of gravity loads.
There is an increasing demand for seismic resistant structures, even in the Midwest. Also, owners
and developers prefer the flexibility a beam/column frame offers, as opposed to structural walls that
cannot be moved during remodeling. Precast concrete building systems could gain significant
advantages over steel open web joist system if the HC-beam-column system can be deigned and
detailed to resist lateral loads without aid of shear walls. Further, if the depth of precast system can
come close to the depth of a post-tensioned cast-in-place slab column system, then precast concrete
could be advantageous due to rapid construction and high plant precast quality.

Post-tensioned concrete flat slab floor systems can be built with a span to depth ratio of 45. This
results in a depth of 8” for a 30 ft span floor. A typical precast concrete system would require a 20”
deep inverted tee plus a 2” topping, for a total floor depth of 22”. Minimizing floor height results in
savings in architectural, mechanical and electrical (AME) systems and may allow for additional
floors for the same building height. As an example, in a 12-story building, saving 12 inches in each
story allows for one extra story. Although a shallow beam cost may be higher than that of a deeper
beam, the cost of the structural frame may still be lower, and the cost of the total building would
most likely be lower. The potential lower cost occurs because of reduced hollow core plank spans,
as a relatively wide beam would be necessary, and the lateral loads would be reduced for the same
number of floors. Furthermore, the AME is about 75 to 80% of the total building cost, and any
small savings in these systems would have a significant impact on the overall project economics.

In the US, the three main floor framing systems are:

   1. Cast-in-place slab systems
   2. Open web steel joist floors
   3. Hollow core slab system

The main advantage of the first system is that it is the shallowest system available, especially if
post-tensioning is applied, with a span to depth ratio as high as 45. There is no need for beams to
support the slab. The drawbacks are the slow cast-in-place construction, the cost of forming and the
cost of post-tensioning. This system is often employed in residential applications due to the clean,
flat soffit produced.

The open web steel joist system is economically attractive in commercial applications. A 24”-30”
deep open web steel joist is typically used. Metal decking is generally used to form a 2.5-4.5” thick


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composite slab. The utilities can pass through the joist openings, saving the height needed for the
utilities. However, steel prices continue to climb. A ceiling is required to cover the unattractive
framing system, resulting in a large total floor height.

The third system utilizes hollow core slabs supported by inverted tee prestressed concrete beams
which are in turn supported on column corbels or wall ledges. It provides an economical and fire-
resistant floor system with excellent deflection and vibration characteristics for both residential and
commercial applications. The top surface can be prepared for installation of a floor covering by
placing a thin non-structural cementitious leveling topping, or a composite 2-3 in. concrete
composite topping.2

The beam generally projects below the planks by about 12 to 14 inches and does not have openings
in the projecting depth below the planks to allow utilities to pass through.


Some effort has been made in the past to attempt to minimize precast concrete floor systems.
However, none of these attempts have made a significant impact on the market. Some of the past
results in the US and abroad are listed below, followed by reasons they have not satisfied the
industry needs.


Steel “Inverted Tee” Beams:
Figure 1 shows some of the steel beam shapes used in Europe1.


                                                    Figure-1 European practices in designing hollow
                                                    core supporting beams

     (a)                 (b)             (c)
The first two shapes are plate girder (built up) sections, and the third is a rolled steel section. These
systems should satisfy the minimum depth criterion as the planks are supported on the bottom plate.
The fib bulletin1 indicates through examples that these beams are limited to about 6 m (20 ft) spans.
This span is considerably less that the 30 ft spans generally required for office building applications.
It is slightly shorter than the approximately 24 ft required in hotel applications, and are reasonable
for apartment/condo applications. They may merit further investigation if the fire protection issues
of the underside of the beam can be satisfactorily resolved and if the cost of fabrication is lower
than an equivalent prestressed concrete beam.




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Figure 2- D-BEAMTM, by Girder-Slab Technologies LLC, Cherry Hill, NJ.




                                                                      Figure     3-Building   under
                                                                      construction using Girder-Slab
                                                                      system of the Girder-Slab
                                                                      Technologies LLC, Cherry
                                                                      Hill, NJ.




In the United States, a steel beam shape, Figs. 2 and 3, has been developed by Girder-Slab
Technologies LLC.3,4,5,6 Similar to the European practices, the precast planks are supported on the
bottom flange of the steel beam. The maximum span capacity shown in the literature for this system
is 16 ft which is much shorter than what would normally be required in an office building. It is a
candidate for further consideration in this study to expand its span capability to 30 ft, and to check
its efficiency relative to the other criteria listed below.

Precast Prestressed Concrete Beam Options
Precast beams are the most dominant products in the US due to their overall economic and fire
rating superiority.7,8,9 Use of built-up steel sections in the US would require that the section be
purchased from a steel fabricator which is not the most desirable approach. Most precasters have
welding capabilities but are not equipped to perform continuous welding for plate girders. Many US
precasters have efficient stud welding capabilities.

Tadros et al10,11 developed a shallow beam system. However, that system required a single story
column which is inconsistent with multi-story column construction practices in the US. Also, the
beam weight and the complex design were discouraging to users. Pessiki et al 12 have developed a
system of double tees and inverted tees where the ITs have openings in the gaps between DT stems.



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However, this solution does not address the more dominant hollow core system used in non-parking
applications.

A successful system must take into account the prevailing erection practices in the US, and must be
within the production capabilities of precasters specializing in building products. It is desirable to
completely erect the precast frame and hollow core planks before cast-in-place operations
commence. It is also desirable to use multi-story columns. Thus, the beam pieces should be shorter
than the clear space between columns. Precasters generally require that beam width not exceed 4
feet, prestressing not exceed 30-0.5 in. strands and concrete strength not exceeding 4500 psi at
release and 6500 psi at final conditions. These limitations will be observed by the research team.
Design will be checked by a panel pf precast concrete advisors selected by the Midwest Precast
Association before specimens are made and tested.

The main criteria for the design of the inverted tee beam and beam/column joint include:
   1. Shallow structural depth.
   2. Satisfactory structural resistance
   3. Continuous for as much of the load as possible
   4. Able to resist lateral loads without shear walls up to six story buildings
   5. Adequate support of the hollow core planks
   6. Consistency with prevailing erection techniques
   7. Minimal topping thickness, and thickness variation along beam
   8. Small live load deflection
   9. Fire protection
   10. Corrosion protection


Proposed Concepts:
The purpose of this proposal is to develop a floor system with a total depth as close to the hollow
core depth as possible, while meeting the other criteria listed above. The loss of depth compared to
conventional design would have to be compensated for. The two main methods to achieve adequate
capacity for a beam are to increase the beam width and to make it continuous for as much of the
load as possible. Continuity details, with no cast-in-place operations during erection will be very
challenging. The authors believe it is very possible to use a welded plate system to render the
system continuous for topping weight. A secondary goal is to eliminate the corbel below the beam
in order to provide a soffit as flat as possible. This is a secondary goal because corbels exist in a
limited space and a ceiling is generally provided in office buildings. However, their elimination is
desirable in hotel/motel/apartment applications.

Figure 4 shows beam details that were considered for use in a project in Lincoln, Nebraska. They
were only partially implemented due to lack of time and lack of supporting technical backing. This
concept would allow for continuity in the beams by passing reinforcing bars through column
openings. The corbels under the beam are replaced with a hollow structural steel tube that is
inserted in a reinforced column opening and welded to the column in the precast plant. A major
parameter in this study is how thin the ledge that supports the HC planks can be made while
satisfactorily enclosing the required reinforcement.


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Figure 5 shows a more ambitious detail in which the supporting beam depth is equal to the hollow
core plank depth. With this detail the hollow core plank is supported on structural steel shelf angles.
The angles would not be continuous to save steel and to prevent loss of concrete pre-compression
due to prestressing. Enough angle length would be provided to support the HC planks. Economical
attaching of the angles to the beams would require further investigation in this study.

The beam span for office buildings to be used in this project is 30 ft and for residential buildings is
24 ft.


Tasks

1. Literature review: Literature review of previous research and practices on hollow core systems
   will be collected from the United States and Europe. Most of that review has already been
   completed in preparation for this proposal. The authors have been involved in this topic for the
   past 15 years and are quite familiar with the available systems.
2. Develop optimized floor options: Constraints of various details such as hollow core to beam,
   beam to column, hollow core and beam to perimeter walls, and at floor discontinuities will be
   established. Also, constraints of erection sequencing and pick weight limits will be established.
   The various types of plausible beam shapes and materials will then be determined. These will
   include structural steel built up sections, prestressed concrete sections or a composite of the two
   materials.
3. Analysis and detailing: Complete analysis will be done to make sure the details suggested in
   Task 2 are applicable and provide the required structural performance. It is interesting to note
   that the frequent success of the University of Nebraska Big Beam contest was one of the
   motivations for writing this proposal. The methods of confinement and the strength based
   design for initial prestress will be valuable tools for this project. One of the challenges is to try
   to find a way in which camber at the initial conditions is accounted for in establishing a flat top
   surface, and that there is no sag in the final condition. Costs of various alternates will be
   analyzed.




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Figure 4 - Shallow continuous beam with concrete Ledge




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Figure 5 Shallow continuous beam with steel ledge

4. Review and revision: A report will be submitted to the MPA precasters involved in guiding this
   project. This will be followed by a special meeting of members of the Midwest Precast
   Association, in which the proposed details will be discussed for feasibility and further
   refinements. It is likely that several iterations will be needed before a final system is selected.
5. Full scale testing: The developed system, especially the connection between the beam and the
   column for both positive and negative moment resistance and the connection between the HC
   units and the beam will be tested in the structural laboratory of the University of Nebraska for
   validation of the design assumptions. The specimens will be produced by member companies of
   the Midwest Precast Association (MPA) and will therefore be “tested” for production efficiency
6. Guidelines and design examples: Often good research does not result in rapid implementation
   due to lack of essential details for actual projects. The research team plans to show all the details
   necessary to substitute the new beams for existing inverted tee beams in a project recently
   completed by one of the companies involved in this project. This task will conclude with actual
   production of specimens and observation of various production and structural issues. A fully
   worked out example will be part of this task.
7. Final report and presentations: The findings from this project will be presented at the PCI
   Convention and in a paper that will be submitted to the PCI JOURNAL for possible publication.




References:
1. Board of FIB steering committee, “Special Design consideration for Precast Prestressed Hollow
   Core floors,” federation international du beton, No. 6, October 1999.
2. Manual for the Design of Hollow Core Slabs, Precast/Prestressed Concrete Institute, Chicago,
   IL, 2nd Edition, 1998.
3. Design Guide for Girder-Slab composite steel and precast system, www.girder-slab.com
4. John Cross, “Coordinated Construction,” Modern Steel Construction, July 2003
5. Rimas Veitas, “Slab Solution,” Modern Steel Construction, May 2002



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6. Peter A. Naccarato,” Steel and Precast Slab Construction System for Mid and High Rise
   Residential Buildings,” Modern Steel Construction , May 2001.
7. PCA, Notes on ACI 318 -05 Building Code Requirements for Structural Concrete, Skokie,
   Illinois
8. PCI Bridge Design Manual, Precast/Prestressed Concrete Institute, Chicago, IL, 1997.
9. PCI Design Handbook, Sixth Edition, Prestressed Concrete Institute, Chicago, Illinois, 2005.
10. Low Say-Gunn, Tadros, Maher K., Einea, Amin, and Magana, Rafael, “Seismic Behavior of a
    Six Story Precast Concrete Office Building,” PCI Journal, Vol. 41, No. 6, November/December
    1996, p.56-75.
11. Low, Say-Gunn, Tadros, Maher K., and Nijhawan, Jagdish C., “A New Framing System for
    Multistory Buildings," Concrete International, Vol. 13, No. 9, September 1991, p. 54-57.
12. Thompson and Pessiki, “Behavior and Design of Precast Prestressed Inverted Tee Girders with
    Multiple Web Openings for Service Systems,” ATLSS Report 04-07, Lehigh University, 2004
    pp. 156

Schedule and Budget:
Table 1 lists the major activities in the proposed project. Assuming a start date of August1, 2006,
the project will be completed by July 31, 2007. The proposed project budget is as follows:

              Expenses:                 Direct cost  Benefits   Overhead    Total
1. Matt Reiser assistantship            $     18,200 $    5,824 $    11,291 $            35,315
2. Augusto Albuqurque hourly            $     1,600                             $         1,600
3. Dr. Tadros hourly                    $     2,751   $     770    $    1,655   $         5,176
4. Dr. Girgis hourly                    $     3,000   $     840    $    1,805   $         5,645
5. Labor by MPA members                 $     5,000                             $         5,000
6. IT and publication expenses           $    2,015                $      947   $         2,962
7. Specimens                             $    6,000                $    2,820   $         8,820
8. Instrumentation and equipment        $     2,000                $      940   $         2,940
9. Travel                               $     2,000                $      940   $         2,940
Total                                   $    42,566   $    7,434   $   20,398   $        70,398


Sources of Funds:
1. Precast/Prestressed Concrete Institute Dan Jenny Fellowship                  $      25,000.00
2. Cash contribution by Midwest Precast Association (MPA)                       $      15,000.00
3. In kind labor and specimens by MPA                                               $ 10,000.00
4. University of Nebraska-Lincoln (overhead waived)                                 $ 20,398.13
Total                                                                           $        70,398




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Table 1. Schedule of Research Activities

                                                A S O N D J F M A M J J
         1. Literature review
         2. Develop optimized beam options
         3. Analysis and detailing
         4. Review and revision
         5. Full scale testing
         6. Guidelines and design examples
         6. Final report and presentations
Backgrounds of Researchers

Dr. Maher K. Tadros is the Charles J. Vranek Distinguished Professor of Civil Engineering at the
University of Nebraska-Lincoln (Omaha Campus), where he has been on the faculty since August,
1979. He is a Fellow of the Precast/Prestressed Concrete Institute. He is a member of a number of
PCI committees including Student Education, R. & D., Journal Advisory, and Bridges. He has
published extensively in the PCI Journal and other refereed publications.

Dr. is the principal author of the PCI Bridge Design Manual. He has been recognized by the PCI on
numerous occasions. Awards include the Distinguished Educator of the Year in 1995, Martin Korn
Awards (in 1975, 1989, 1996, and 2001), ASCE T.Y. Lin Awards ( in 1976, 1990, 1997, and
2002). At the 50th Anniversary of the PCI, hew was recognized along with as one of 50 “Titans of
the Industry” for precast prestressed concrete industry altering contributions in its first fifty years.
He and his former student, Lynn Geren, developed the NU I girder which is the standard product
used in Nebraska bridges, as a result of a PCI Dan Jenny Fellowship.

Dr. Tadros is the past president of the Nebraska Chapter of ACI and the Nebraska Section of ASCE.
He was a member of the 1995, 1999 and 2002 ACI 318-Sub G Committee. He holds eleven U.S.
and one Canadian patents. Dr. Tadros is a co-founder of Tadros Associates, LLC, a specialty
structural engineering firm in Omaha, Nebraska with expertise in precast concrete bridges,
construction engineering, and value engineering.

Dr. Amgad F. Morgan Girgis received his Ph.D. in structural engineering from the University of
Nebraska–Lincoln in May 2003. He is currently a research assistant professor in the Department of
Civil Engineering at that University. He is a PI in several national and state sponsored projects, and
engaged in managing several others. Dr. Girgis has thirteen years of work experience in the field of
structural engineering, including bridges and high rise buildings. His research interests include post
tensioning splice precast girder bridges, prestress losses, unconfined and confined concrete
modeling. Prior to joining UNL, he worked at Mc Mater University, Ontario, as well as Lawrence
Technological University, Michigan, where he conducted research on fiber reinforced polymers.
His experience also includes three years of project management for major hotel construction
projects and three years of structural design experience. Dr. Girgis is also the winner of 2001
National Student Concrete Bridge Design Competition, Portland cement Association (PCA),
Illinois, USA, and contributing author in the PCI Bridge Design manual.




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Mr. Nicholas P. Reiser is actively pursuing his M.S.C.E. from the University of Nebraska-Lincoln,
Omaha campus. He earned his B.S.C.E. (with honors) from UNL in December, 2005. Nicholas is
currently working as a graduate research assistant under Dr. Maher Tadros and has a particularly
high interest in the prestressed concrete industry. He desires to attain valuable and marketable
knowledge through this research project and is excited to apply this knowledge in future career
opportunities. Nicholas recognizes the enormous potential the prestressed industry holds for his
future career. He is a member of several professional and academic organizations, including ASCE,
Chi Epsilon (Civil Engineering Honor Society), and Tau Beta Pi (All Engineering Honor Society).




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