Shallow Continuous Inverted Tee Beam for Hollow Core Slab
PCI Daniel P. Jenny Graduate Research Fellowship
Amgad Girgis, Ph. D.,
Research Assistant Professor
University of Nebraska-Lincoln
402 554 2820
Maher K. Tadros, Ph. D., P.E.
The Charles J. Vranek Distinguished Professor
University of Nebraska-Lincoln
402 554 4842
402 554 3288
Augusto AlbuquerqueNickolas Reiser
M.S.C.EPh. D. Student
402 554 2820
A number of systems have been used in Europe to achieve shallow floor height1, as illustrated
later in this proposal. Shallow Continuous Inverted Tee Beam for Hollow
Core Slab SupportFloors
The A key economic criterion for multistory office buildings and similar types of construction is
to minimize floor building height. Post-tensioned concrete flat slab floor systems can be built
with a span to structural depth ration of 45. This results in a depth of 8” for 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 AME, and can aallow for additional floors for the same
building heightlevels. As an example, in a 12-story building, saving 12 inches in every story
allows for extra story. If the extra story is not necessary, the The AME savings is about 75 to
90% of the total building cost and any small savings in these systems would have a significant
impact on the overall project economics.are huge in comparison to the floor structural system
cost. Although the beam cost may be higher than that of a deeper beam, Another advantage is the
savings from walls and columns due to the reduction of lateral loads.
the cost of the structural frame may still be lower because of reduced hollow core plank spans, as
a relatively wide beam would be necessary, and as the lateral loads would be reduced for the
same number of floors.
In order to achieve the goal of minimizing floor heightIn the US, , the available floor systems
need to be carefully investigated.1 tThere are three main flooring floor framing systems in the
United States. Each system has its advantages and disadvantages. These alternatives are as
1. Cast-in-pPlace post-tensioning tensioned slab system
2. Open web steel joists
3. Hollow core slabs system
The first system has higher per square foot cost due to post-tensioning. In addition, this system
does not have the durability of precast concrete. One of theThe main advantages of this system is
that it is the shallowest system available, with a span to depth ratio of 45. tThere is no need for
additional beams to support the slab. Also this system has a span to depth ratio of 45 which
results in 8 in. slab for 30 ft span., consequently This leads to minimizing the floor height for the
CIP post-tensioning system. Also, the joints between the columns and walls and the slab has
higher durability as they are monolithically cast.The slow cast-in-place construction, the cost of
forming and the cost of post-tensioning are drawbacks.
The second system, which is the open web steel joist, is very attractive economically. For this
system, a 24”-30” deep open web steel joist is used. Metal decking is generally used to form a
2.5-45” thick composite slab. The utilities can pass through the joist openings, saving the height
needed for the utilities. However, again has higher per square foot cost due to steel prices
continue to climb; however. A ceiling is required to cover the unattractive framing system,
resulting in a large total floor height. the utilities can pass through the joist, saving the height
needed for the utilities.
The third system, which utilizes hollow core slabs supported by inverted tee prestressed concrete
beams which are in turn supported on column corbels. , is the most widely known forIt
providesing economical, fire-resistant and efficient floor and roof system, with excellent
deflection and vibration characteristicss. The top surface can be prepared for installation of a
floor covering by feathering the joints with latex cement, installing thin non-structural
cementitious leveling topping, composite fill concretes ranging from ½ in . to 2-3 in. concrete
thick depending composite topping on the material used, or be casting a composite structural
The hollow core slabs are designed to be supported by beams which are supported by the
building columns and walls. This supporting e beam is a weak point in the system as it generally
projects below the planks by about 12 to 14 inches and does not have openings in the projecting
depth below the planks to adds to the floor thickness and does not allow utilities to pass through.
Thus, the design of these supporting beams is extremely challenging as the depth of these beams
needs to be as close to the hollow core as possible to optimize the project costs while minimizing
the difference between the camber and the deflection from the hollow core weight. Also
minimizing the hollow core slab system depth increases the system’s chance to compete with the
Cast-in-Place post-tensioning system as this is the major strength point of the later system.
TSome of the important criteria for designing design of the Inverted Tee precast prestressed
bBeam to support hollow core slabs includeare:
1. Reduce Small the structural depth. This criterion represents minimum total building cost.
to building height for the same floor height
2. CReasonable camber to offset the deflection due to slabplank weight
3. Small live load deflection Formatted: Bullets and Numbering
3.Satisfactoryy the required structural resistanceflexural and shear strengths for the beam Formatted: Bullets and Numbering
4. Formatted: Bullets and Numbering
4.5.Adequate Provide ledges to adequately support ofto the hollow core slabplanks Formatted: Bullets and Numbering
6. Fire protection Formatted: Bullets and Numbering
7. Corrosion protection
8. Consistent with prevailing erection techniques
Using the European PracticesSteel Beams Option for the Hollow Core Support Beam iin
Europe and the US
Europeans realized the fact that reducing the floor system thickness of the hollow core results in
a cost-effective and durable slab system, in addition to the merits mentioned above, hence the
use of steel structures as shown in Figure 1. Figure 1 shows some of the designssteel beam
shapes that have been used in Europe1. for Hollow Core support beams.
(a) (b) (c) (a) (b) (c)
Figure 1 European practices in designing hollow core supporting beams
The first two designs shapes are built-upplate girder (built up) steel sections and the third is a
rolled steel rolled section. These systems should satisfy provide the targeted minimum depth
requiredcriterion. , They will be carefully examined in this study for efficiencies relative to the
other criteria.however these designs are associated with big deflections due to the small beam
stiffness, in addition to the high costs due to steel prices.
Formatted: Tab stops: 2.25", Left
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 special steel beam shape, Figs. 2 and 3, similar to a bulb tee section has
been useddeveloped by Girder-Slab Technologies LLC. Thehe steel beam was used as an interior
girder supporting the precast slabplanks are supported on its bottom flange of the steel beam. The
maximum span capacity is shown in the literature for this system to be 16 ft which is much
shorter than 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 above. This resulted in minimum floor-to-floor heights. The
web and top flange were concealed within the plane of the slab as shown in Figures 2 and 3. The
first two designs are built-up steel sections and the third is a steel rolled section. These systems
provide the targeted minimum depth required, however these designs are associated with big
deflections due to the small beam stiffness, in addition to the high costs due to steel prices.
As mentioned earlier, this design cannot be adopted for large spans because the steel section has
low stiffness. This low stiffness and the large Hollow Core loads result in large deflections. This
is in addition to the high costs of the steel prices.
Using Precast Prestressed Concrete Beam Options Hollow Core Support BeamUS practices
and the proposed solution
The hollow core slab in the United States practices has been predominantly supported by the
precast prestress inverted tee beam system, which is in turn supported by the column and the
wall corbels. Precast beams This system has been proven to be economicalare the most dominant
product the US due to their overall superiority in meting the various design criteria. However, its
depth is not as shallow as the other systems. One of the aspects to be optimized in this system is
increasing the camber to offset the large deflection due to the large hollow core weight.
Minimizing the difference between the camber and the deflection reduces the change of the
topping thickness and the associated costs.
The European solution has some merits but a concept suitable for the US market and that
improves the current design practices needs to be developed. For example, in the US, significant
cost savings can result from having the beam section as close to the slab depth as possible, while
providing the ledges for the hollow core support. Moreover, uUnlike the European practices, the
useUse of built-up steel sections in the US would require that the section be purchased from as
steel fabricator which is not the most desirable approach. Most precasters have welding
capabilities but are not equipped to perform may not be efficient, as the US precasters are not
adept in continuous welding for plate girders. An alternative is for US precasters to buy these
steel built-up sections from the steel fabricators, which may not be a cost-effective option.
Precast products tend to be more cost effective when nonfabricated materials such as steel rolled
sections, WWR, rebar, or strands are utilized. On the other hand, materials that require
fabrication such as built up sections with continuous welding are not as cost-effective. many
However, US precasters have shown efficient cy in welding stud welding capabilitess.
The design of the prestress precast support beam should take into account the prevailing erection
practices in the US. is based on the current construction practices in the US. It is desirable
favored by the precasters to completely erect the all the precast elementsframe before in a short
period of time in dry conditions an cast-in-place operations commence. The topping and grouting
is poured in later time. In this practice support beams are installed on the columns and the walls.
Shortly after that the Hollow Core slabs are installed. When all the precast items are installed the
process of pouring the toppings and the grouting begins. Also, it is desirable to use multi-story
columns. Thus the beam pieces should be shorter than the clear space between columns.
Proposed Concept: Innovative Solution
The purpose of this proposal is develop to investigate an effective approach for reducing thea
beam with a total depth to be 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 equal capacity to simple span deep beam is
to increase the beam width and to make it continuous for as much of the load as possible. and
compensating this loss in strength by increasing the beam width and creating continuity over the
column by using a TSS section.Continuity details, with no cast-in-place operations during
erection will be very challenging. Another, 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. One of the
improvements that will be investigated in this study is to simplify continuity detail so that it can
fixed in a very short time before installing the hollow core slab in order not to affect the
construction cost by having all the precast elements installed in the shortest time possible.
Figure 2 4 Shallow Continuous Hollow Core Beam with Concrete Ledgedetails for Bryan
Figure 2 4 shows the beam details that were developed toconsidered for be used in Bryan
Hospitala project in Lincoln, Nebraska. They were not used due to lack of time and lack of
supporting technical backing. The proposed solution minimizes the structural depth of the
inverted tee to be as close as possible to the hollow core depth. At the same time, tThis
solutionconcept would allow offers continuity to for continuity in the beams by passing
reinforcing bars through column through the columnsopenings, which improves the structural
efficiency of the system and helps in reducing the overall structural depth. Another advantage of
this proposed system is eliminating the need forfor the concrete under-beam having corbels are
replaced here with a in the columns, which results in cost savingshollow structural steel tube that
is inserted in a reinforced column opening and welded to the column in the precast plant. This
system also allows for the flexibility of having the camber close to the deflection caused by the
heavy weight of the hollow core, which eliminates the need to vary the topping thickness to
avoid this defect.
Figure 5 Shallow Continuous Beam with Steel Ledge Hollow Core Slab More Efficient Detail
Figure 5 shows a more ambitious detail in which the supporting beam depth is equal to the
hollow core slabplanks depth. With this detail the hollow core slabplank will match the Cast-In-
Place post-tensioning slab system’s strong point, in addition to the precast system advantagesis
supported on structural steel shelf angles. The angles would not be continuous to save steel and
to prevent loss of prestress. 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 maximum beam span in this project is 36 ft. By having this solution work with the 36 ft
modules this system can be easily adapted for the standarda typical office space areas bay size
of 30 ft x 30 ft.
Improving the economy of the hollow core system by minimizing the hollow core Formatted: Bullets and Numbering
supporting beam depth, and making the supporting beam continuous. Also eliminating
the corbels helps optimize the economy of the system.
Increasing the camber of the supporting beam to offset the large deflection from the heavy
hollow core weight to eliminate the difference in the topping thickness.
Impact and Anticipated Benefits
Minimizing the hollow core overall system depth increases the chance for its competition against
the CIP post-tensioning slab and open steel joist systems. The benefits of this study will extend
to parking structures and floor framing in commercial buildings.
1. Literature review: Literature review of previous research and practices on hollow core
systems will be collected from the United States and Europe.
2. Develop detailing and construction sequence to serve the objectives: Details for hollow core
supporting beams will be completed and will cover all applications that the hollow core
structure will be utilized in.
3. Analysis of Camber, Deflection and Continuity: Complete analysis will be done to make
sure the details suggested in task 2 are applicable and provide the required strength for the
purpose that the hollow core systems will be utilized in.
5.System Optimizationing the system by consultingfor production, erection and other criteria Formatted: Bullets and Numbering
precasters and contactors; the system will be examined and compared with other US and
foreign systems for the various criteria listed above. mMeetings will be held with precasters
and contactors, in which the proposed details will be discussed for feasibility and further
refinementsfurther optimization. Formatted: Font: 14 pt, Bold
4. Formatted: Bullets and Numbering
5. Presentation of findings and cost analysis: The benefits of the proposed system and details in
terms cost savings and construction efficiency will be studied. 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.
The research team from University of Nebraska will invite several experts in the area of
construction of building parking structures and office buildings, and from the precast industry as
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. PCA, Notes on ACI 318 -99 Building Code Requirements for Structural Concrete, Skokie,
4. PCI Bridge Design Manual, Precast/Prestressed Concrete Institute, Chicago, IL, 1997.
5. PCI Design Handbook, Sixth Edition, Prestressed Concrete Institute, Chicago, Illinois, 2005.
Budget and Time Schedule
Table 1 lists the major actions in the proposed project. Assuming a start date of August 2004,
the project will be completed by July 2005. It is expected that the project will require the
Amgad: please make sure this is consistent with the information in the letter from PCAN
(1) Dr. Girgis, 80 hours ($30.00*1.26 benefits*1.47 indirect costs) $4,445.3
(2) Dr. Tadros 40 hours ($82.00*1.26 benefits*1.47 indirect costs) $6,075.2
(3) Mr. Lafferty, Mr. Drews Culp of the Prestress Concrete Association
of Nebraska (Precast Industry Advisors) $10,000
(4) Nicholas P. Reiser (Research Assistant) ($12,000*1.32 benefits) $15,840
(4) Computer resources, publications, and printing $ 21,000640
(5) Travel $ 32,000
Precast Concrete Association of Nebraska (PCAN) $203,360.5000
Precast Concrete Institute fellowship $2018,000
Table 1. Schedule of Research Activities
A S O N D J F M A M J J
1. Literature review
2. Develop detailing
5. Presentations and final report
Dr. Maher K. Tadros is 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. Dr. Tadros specializes in prestressed concrete. He has been an active member of the
Precast/Prestressed Concrete Institute for many years. He is a member of a number of committees
including the Student Education Committee, the Research and Development Council, the Journal
Advisory Committee, and the Committee on Bridges. He has published extensively in the PCI
Journal and other refereed publications. He is principal author of the PCI Bridge Design Manual.
Dr. Tadros has been recognized by the PCI on numerous occasions. Awards include being
elevated to PCI Fellow and selected as the Distinguished Educator of the Year in the respective
years when these awards were inaugurated. He is a four-time award winner (1976, 1990, 1997,
and 2002) of the ASCE T.Y. Lin Award for papers he co-authored in the PCI Journal, one of
which came as a result of work initiated by a PCI Dan Jenny Fellowship. He is also a co-recipient
of several PCI Martin Korn Awards. 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
Dr. Tadros is 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 seven U.S.
and one Canadian patents. Dr. Tadros is a co-founder of Tadros Associates, LLC, and a specialty
structural engineering firm in Omaha, Nebraska with expertise in precast concrete bridges and
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, where is is engaged in managing sponsored research
projects on confined concrete filled structural steel tube arch bridges, and strand bond in self-
consolidated concrete. Dr. Girgis has eleven years of work experience in the field of structural
engineering, including bridges and high rise buildings. His research experience includes
development of design aids for a new spliced concrete I-girder bridge system where the pier
segment is spliced vertically, and modeling of confined high performance concrete. Prior to
joining UNL, he worked for four years at Windsor University in Ontario and Michigan
Technological University, 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.
No: show Nick. They like to see an American kid get trained and stay with the American
precasters after he/she graduates
Augusto Albuquerque Nicholas P. Reiser is actively pursuing hisis M.S.C.E. from the
University of Nebraska-Lincoln, Omaha campus. a Brazilian Ph.D. Student is coming to
Nebraska for training on precast concrete with emphasis on floor framing system. He earned his
B.S.C.E from UNL in May, 2005. Nick is currently working as a graduate research assistant
under Dr. Maher Tadros, and has a particularly high interest in the prestressed concrete industry.
He is a member of several professional and academic organizations, including PCI, ACI, ASCE,
Structural Engineers Association of Nebraska, and Chi Epsilon. Nick was voted the 2005
“Outstanding Senior of the Year” by the faculty and staff of the UNL Department of Civil