Sustainability Advantage of Post-tensioning in Buildings

®

Dennis Reilly Arch/SE
POSTEN Engineering Systems

In the last 100 years, the
Population of the World
has grown from 2 billion
to 6.5 billion people.

With the highest standard
of living in the World, the
United States consumes 25%
of the World’s Natural Resources.

If the rest of the World lived
by our standards (which they
trying to do), we would need
5 planet Earths to sustain us.
Population growth over the last 2,000 years

  New Mandatory Regulations:
Across the country, Cities, Counties and States are
creating (& in some cases already mandating)
Regulations, which require compliance with GREEN
Building or LEED standards.

It just makes sense:
  Sustainable Design of Post-tensioned Structures
saves money and increases overall building efficiency;
  While Sustainable Design of other structural building
materials normally requires new construction methods
or structural systems that lack a track record &, as a
result, increase Professional Liability Risk,
Sustainable Post-tensioning means simply designing
efficiently to get the most from a well tested system,
with a 40 year track record.

  Reduce the slab or beam thickness to it’s
minimum;
  Reduce the amount of steel used; and

  With that in mind, as much as possible,
promote the use of Moment Frame Structures,
instead of Shear Wall Structures, which saves
concrete & steel.

  Even an inefficient Post-tensioned structure uses less
concrete & steel than Reinforced Concrete. So,
shouldn’t Post-tensioning already be Sustainable?
  To be honest - No!! As shown in the first slide, the
Standard of Care is NOT Sustainable. Because of the
cost of steel and concrete, Post-tensioning is already
the Standard of Care.
  What is required for Sustainability is to reduce the use
of concrete & steel & maximize the efficiency of the
building.

Carbon Footprint
(How much Carbon Dioxide is
released to the Environment in the
manufacture, delivery of materials
& long term use of the building)

Solid Waste
Resource Use
Energy Use (aka Embodied Energy)
Water Polution
Air Pollution
Carbon Footprint of Materials

Concrete:
If you could reduce the thickness of a typical 150 ft x 300 ft
concrete slab by just 1”, you would reduce the Carbon
Emissions from it’s manufacture by the same amount as is
produced by 4 automobiles in one year.
Steel:
Steel is the real culprit, making Conventionally Reinforced
Concrete Buildings & Especially Steel Frame Buildings
inherently Non-Green. Reducing Steel Use is Paramount!

Let’s look at
a typical
Interior Bay
Flat Plate

To illustrate the stark difference between Post-
tensioned Concrete & Reinforced Concrete:
Compared to an
Efficiently designed 8.5” thick Post-tensioned
slab,
the comparable Conventionally Reinforced
Concrete Slab is 12” thick with a lot of rebar.

Span 2 – Reinf. Conc. Flat Slab Output

12” CONC. SLAB W/480 FT
OF #5 REBAR PER SPAN

This is why we Post-tension in
the first place!

Now - The Path to Sustainable Design:
Stage One: Reduce the Amount of Concrete used.
Using POSTEN Multistory,
The “Auto Depth Option” automatically determines
the thinnest slab section possible based upon the
minimum Effective Pre-stress at the specific “User
selected” spans &, with that, provides a full design.

POSTEN
Multistory Design Procedures 1, 2 & 3
Computer
Automatically produce an
Input Efficient Design of the
Rebar, Tendons & Drapes

We will use Design Procedure 1,
using
Allowable Tensile Strength
as our Control.

POSTEN
Multistory
Computer
Input

The Auto Depth Option
determines the
thinnest Concrete
section possible &
Proceeds with a full
design.

POSTEN
Multistory
Computer
Input

To improve efficiency,
we will allow the
program to add
pre-stress at the
outer spans, if
necessary.

POSTEN
Multistory
Computer
Input

We will start out with a
9.5” thick slab and see
if we can reduce the
Thickness of this slab.

Minimum
Thickness
Output

The Output shows that POSTEN reduced the slab thickness
from 9.5” to 8.5”
(A savings of 1” in slab thickness) and proceeded with the
design of the thinner slab.

Span 2 - Minimum Thickness Output

Span 2 -
Minimum
Thickness
Output

Now that we have reduced our Carbon
Footprint, by minimizing the thickness of the
slab,

the Next Step,

Reduce the amount of Steel used.

From analyzing Post-tensioning designs, we learned
that at interior spans and cantilevers there are usually
residual compressive stresses remaining at the
tension faces.

By analyzing & Balancing the Stresses in the sections,
the efficiency of the tendons can be maximized,
resulting in less steel.

POSTEN Multistory’s
“Drape & Pre-stress Optimization” algorithms starts
out by performing an Efficient Proportional Load
Balancing Design. Once this design is completed, the
program immediately proceeds with 10 cycles of
balancing the stresses in the sections, thereby
creating the most efficient design.

Since “Drape & Pre-stress Optimization” starts out
with Proportional Load Balancing, the program knows
how much steel was required by Proportional Load
Balancing, and as a result, prints out the amount of
steel saved in the process of Stress Balancing.
Sometimes the savings is significant and sometimes the
savings is minor. Normally, there is a savings.

Using the same computer input from Example 2,
we need only turn off the “Auto Depth Option”, turn on
the “Drape & Pre-stress Optimization Option” and re-
run the program to get the Sustainable Design (with
both the minimum concrete & steel).

Clear Auto Depth Option

POSTEN
Multistory
Computer
Input

POSTEN
Multistory
Computer
Input

Select
“Drape & Pre-stress
Optimization”
& run the design.

This output shows the percentage of savings (9.2%) of steel that was saved by
Performing Stress Balancing (above & beyond the Efficient Design produced by
POSTEN’s Proportional Load Balancing).

Minimum Steel Output

Our Sustainable Design Resulted from:
  Determining the Thinnest Section of Slab or Beam
(saving 1” in the post-tension design or saving 3.5”
when compared to Conventional Reinf. Conc.)
  Determining the least amount of Steel through Stress
Balancing (saving an additional 9% of the steel)

POSTEN’s Automatic LEED Documentation
SLAB OR FLAT PLATE SCHEDULE
(SEE TYPICAL DETAIL FOR NOTATION)
NAME SPAN WIDTH DEPTH TOP LEFT TOP RIGHT BOT "BM" END DIST BOT "BX" ANCH'D
s1 30.00 192.00 12.00 6#5 17#5 8#5 "D.F.L." 3#5 ENDS?
NOMINAL LENGTH, "NL" 5.2 FT 9.8 FT 18.4 FT 3.6 FT 27.3 FT NONE

s2 32.00 192.00 12.00 17#5 15#5 6#5 "D.F.L." 2#5 ENDS?

s3 R. 32.00 192.00 12.00 15#5 17#5 6#5 "D.F.L." 2#5 ENDS?

END 30.00 192.00 12.00 17#5 42#5 8#5 "D.F.L." 3#5 ENDS?


BAR LENGTHS SHOWN ARE NOMINAL LENGTHS, PRIOR TO ADDING ANCHORAGE FOR FULL BAR DEVELOPMENT.

NAME SPAN WIDTH DEPTH TOP LEFT TOP RIGHT BOT "BM" END DIST BOT "BX" ANCH'D BOT LEFT BOT RIGHT PRE- TENDON ORDINATES
L1 30.00 288.00 8.50 4#5 5#5 "D.F.L." ENDS? STRESS L. MID. R.
NOMINAL LENGTH, "NL" 5.3 FT 9.8 FT NONE 514.K 4.2 1.4 7.1



END 30.00 288.00 8.50 4#5 4#5 "D.F.L." ENDS? STRESS L. MID. R.

BAR LENGTHS SHOWN ARE NOMINAL LENGTHS, PRIOR TO ADDING ANCHORAGE FOR FULL BAR DEVELOPMENT.




END 30.00 288.00 8.50 4#5 4#5 "D.F.L." ENDS? STRESS L. MID. R.


Minimum
Thickness
Output

&
Ultimately
Minimum
Steel
Output

Using POSTEN Multistory,
Post-tensioned Moment
Frames Preserved the
Historic Fabric of this
National Historic
Landmark.

®

When designing for Wind or Seismic Forces, the
Post-tensioned Concrete Moment Frame
Structure is the Sustainable Alternative.
  Using less Steel and Concrete; and
  Significantly improving overall building
efficiency by eliminating shear walls.

To do this:
  The columns must be accurately designed
simultaneously with the Post-tensioned floors and roof,
using the correct columns stiffness’s (not
approximations);
  The correct wind or seismic lateral forces (along with the
P-delta magnification factors) must be inputted into the
Post-tensioned floors & roof designs; and
  Correct design procedures must be used to design the
Moment Frame

To obtain the correct Lateral (Wind or Seismic)
Forces, the Magnification Factors for P-Delta
and/or the correct Column Stiffness’s (floor by
floor) – we recommend using a Multistory
Concrete Frame Analysis Program, such as:
  ETABS by CSI or
  EZframe by POSTEN Engineering Systems
We strongly recommend against Finite Element Analysis

POSTEN
Multistory
Computer
Input

In this example, each floor level is designed, one level
at a time, based upon the lateral forces & P-delta
magnification factors from a 2nd order Multistory
Frame Analysis for the full structure.

POSTEN
Multistory
Computer
Input

Moment Frames are
designed by
Activating
Q5 – First Order Design
Q7 - Gravity Force Design
or
Q9 – Second Order Design

POSTEN
Multistory
Computer
Input
Two additional Input
Screens appear to
include the Column
Properties, Design Criteria,
Lateral Forces &
Magnification Factors
For Post-tensioned
Moment Frame Design.

POSTEN
Multistory
Computer
Input

Span 1 Beam – Moment Frame Design

Columns – Moment Frame Design

POSTEN
Multistory
Computer
Graphic Output

Span 1 of 3
Post-tensioned
Moment Frame

POSTEN
Multistory
Computer
Graphic Output

Span 2 of 3
Post-tensioned
Moment Frame

POSTEN
Multistory
Computer
Graphic Output

Span 3 of 3
Post-tensioned
Moment Frame

Sustainable Post-tensioning Advantages
  Less Weight
  Less Steel
  Lower Building Height or Higher Building
Volume
  Lower Construction Cost
  Lower Carbon Footprint
  Lower Embodied Energy, Waste & Pollution

Despite Post-tensioning’s inherent advantages
over Reinforced Concrete & Steel Frame,
Sustainability additionally requires:
  Minimizing Materials (i.e. conc. & steel);

  Maximizing Efficiency (thin sections, stress
analysis &/or moment frames); and
  The Proper Documentation to back it up.

Post-tensioning can provide it all – Like No Other.

Thank you for listening.

®

Dennis Reilly Arch/SE
POSTEN Engineering Systems
510-275-4750
sales@postensoft.com
www.postensoft.com
www.postensoft.blogspot.com

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