ARCH 5605 – HIGH RISE STUDIO
RESEARCH STUDIES
LOUIS KHAN & KIRSTEN TUDOR
TOPIC:
Concrete vs. Steel Frame Considerations
We explored the different design aspects of concrete and steel trying to convey the
information in a realistic manner that everyone could understand. As such, definitions were
provided as well as excerpts and tables.
CONCRETE HISTORICAL CONTEXT
Concrete has been used for building purposes throughout history. Pieces of concrete
buildings have been found in Mexico and Peru from prehistoric times. In the Italian colonies
of Magna Graecia there exists evidence that the Greeks used it while the Romans employed it
largely in this country as well as others. Roman uses can be traced back as far as 500 BC. As
far as today’s standards, slightly more than half of the low rise buildings in the United States
are constructed from concrete. The first building to be considered high rise concrete
construction was the 6-story Ingalls Building, completed in Cincinnati in 1903. In 1953
buildings taller than twenty stories still rarely existed. They were not economical to lease
because of the massive columns needed to support the structure left too little usable space for
renting. In 1990, the strength of concrete increased from 5000 psi (34 MPa) to 19,000 psi
(131 MPa) which allowed buildings to grow skyward with an optimal amount of rentable
space. Today, ultra-high strength concrete is now produced with strengths of 21,750 psi (150
MPa). During the 1980s, high rise construction exploded in cities like New York, Chicago
and Dallas requiring the millions of tons of concrete in construction.
STEEL HISTORICAL CONTEXT
The history of steel as a structural material within our culture begins with the use of
cast iron. Cast Iron was first used in England starting in 1777 to create bridges. In 1840
Wrought iron replaces Cast iron as a preferred structural material. The cold roll process was
created in 1780 to create “S” and “I” shape steel. In 1855 the Bessemer process was created
to produce structural steel that was ductile with fewer impurities. It was at this point that steel
became widely used as a structural element in building construction.
CONCRETE
DEFINITIONS
CONCRETE
An artificial, stonelike material used for various structural
purposes, made by mixing cement and various aggregates, as
sand, pebbles, gravel, or shale, with water and allowing the
mixture to harden.
REINFORCED
CONCRETE
Concrete containing steel bars, strands, mesh, etc., to absorb
tensile and shearing stresses.
PLAIN CONCRETE
Structural concrete with no reinforcement or with less
reinforcement than the minimum amount specified for
reinforced concrete.
STRUCTURAL CONCRETE All concrete used for structural purposes including plain and
reinforced concrete.
CEMENT
Any of various calcined mixtures of clay and limestone, usually
mixed with water and sand, gravel, etc., to form concrete, that
are used as a building material.
AGGREGATE
Any of various loose, particulate materials, as sand, gravel, or
pebbles, added to a cementing agent to make concrete,
plaster, etc.
PRECAST CONCRETE
Structural concrete element caste elsewhere than it’s final
position in the structure.
TYPES OF CONSTRUCTION
FOOTINGS
The wet concrete is poured directly into trenches dug into the
earth below frost level
FOUNDATIONS
Concrete is placed between supporting wood or metal forms,
which are removed after the concrete has hardened.
LIFT-SLAB
Floors and roof slabs are cast at ground level and then raised
by hydraulic jacks and fastened to columns at the desired
elevation
SLIP FORM
Used to produce vertical shafts for silos and the cores of
buildings. They are moved upward at a rate of 15 to 38 cm (6
to 15 in) per hour while concrete and reinforcements are put
in place.
TILT UP
Generally used in only one- and two-story buildings. Walls are
cast in place on the ground or on the previously laid concrete
floor and tilted into position by cranes. The walls are joined at
the corners or between panels with cast-in-place concrete
columns
SHOTCRETE
Used in the construction of swimming pools, canal linings, and
curved surfaces. In shotcreting, concrete is sprayed under
pneumatic pressure rather than placed between forms. Often
the use of shotcrete eliminates the need for formwork and
permits placement of concrete in confined areas where
conventional forms would be difficult or impossible to
construct.
PRECAST
Precast construction is appropriate for structures in which the
concrete pattern can be repeated; the more times a concrete
shape or panel can be repeated, the greater economy can be
achieved. Precast construction also offers the advantage of
factory control: concrete strength, appearance, and quality
can be tightly monitored and regulated. Load-bearing precast
wall panels-often used for, hotels, hospitals, and
manufacturing facilities-can either be mass-produced in
standard molds at precast plants, or can be formed in molds
custom-designed for individual projects. These panels are
usually prestressed and often contain a layer of rigid
insulation. Precast concrete is commonly used because precast
systems are economical to construct and the material is largely
impenetrable and damage-resistant.
CONCRETE STRENGTH
Concrete used in most construction work is reinforced with steel. When concrete
structural members must resist extreme tensile stresses, steel supplies the necessary strength.
Steel is embedded in the concrete in the form of a mesh, or roughened or twisted bars. A
bond forms between the steel and the concrete, and stresses can be transferred between both
components.
Prestressing concrete has removed many limitations on the spans and loads for which
a concrete structure can be economically designed. The basic function of prestressing is to
greatly reduce the tensile stresses to which crucial areas of concrete structures are subjected.
Prestressing is accomplished by stretching high-strength steel to induce compressive stresses
in concrete. The strengthening effect of compression in concrete acts like horizontally
squeezing a row of books. When you apply sufficient pressure to the books at each end, you
induce compressive stresses throughout the entire row; thus, although the center volumes are
unsupported, you can lift the books and carry them horizontally.1
Concrete strength is measured in PSI (pounds per square inch) or MPa (megapascals).
MPa is the Metric unit of measuring compressive strength of concrete. Conventional concrete
has a strength of 7,000 PSI. High strength concrete has strength of 7,000 and 14,500. The
easiest way to add strength is to add cement. The factor that most predominantly influences
concrete strength is the ratio of water to cement in the cement paste that binds the aggregates
together. The higher this ratio is, the weaker the concrete will be and vice versa. Every
desirable physical property that you can measure will be adversely affected by adding more
water.2
SITECAST CONCRETE STRUCTURAL SYSTEMS
Choosing a sitecast concrete framing system is often based on the desired spans or
column spacing of the structure. It is also based on the expected magnitude of the in-service
loads on the building. The following systems are:
• One-Way Solid Slab
o Supported by bearing walls
o Least expensive concrete framing system
for shot spans and light loads
o Popular for multiple dwelling building types
such as apartments or hotels, where the
regular spacing of bearing walls is easily
coordinated with the layout of the small
uniformly arranged rooms.
• One-Way Beam & Slab
o The addition of beams and slabs to the
construction can increase the load
capacity and span range of the system
and eliminate the need for regularity
o The increased complexity makes it one of
the more expensive of all sitecast
concrete
systems to construct.
o Whenever possible, beam depth should
be sized for the longest spans, and then
same depths should be used throughout.
Beam widths and spacings, slab depths,
and column sizes should also vary as little
as possible.
• One-Way Joist
o An economical construction system for
heavy or relatively long spans.
o Also sometimes for the distinctive
appearance of the underside of the slab
which may be left exposed.
o Standard Pan witdth: 20-30 in
o Standard Joist Width: 5-9 in
o Economical for spans up to 40ft
• Two-Way Flat Plate
o One of the most economical framing
systems
o Can span farther than one-way slabs
o Plain form makes it simple to construct
and easy to finish
•
•
o Commonly used for apartment and hotel
construction where the flexibility of the
column placements makes it easy to plan
for units and an overall layout.
o Column bays are most efficient when square
o Span lengths should not differ by 1/3 of the
longer span
Two-Way Slab & Beam
o Uses beams to support the slab between
the columns
o High construction cost
o Economical only for long spans and heavy
loads
o Best for heavy industrial applications
Waffle Slab
o Is an economical system for long spans
or heavy loads
o Desirable for the waffle-like appearance
underneath slab.
o Standard: 19in domes w/ 5in ribs to create
24in module
o Economy of system depends on maximum
repetition of standard forms and sizes.
PRECAST CONCRETE STRUCTURAL SYSTEMS3
The initial choice of a framing system should be based on the desired spanning
capacity or column spacings of the system and the magnitude of he expected loads on the
structure. For short spans and light loads solid flat and hollow core slabs are better. For
longer spans and heavier loads double tee and single tee systems are better. The economy
depends on the maximum repetition of standard elements and sizes.
•
Precast Concrete Slabs
o Commonly used in hotels, multifamily dwellings,
commercial structures, hospitals, schools,
and parking structures.
o Sitecast concrete is poured over precast
slabs to:
ƒ Increase structural performance
ƒ Increase fire resistance
ƒ Allow integration of electrical
communications services into the
floor
Provide a more level and smoother
floor surface
o Solid and hollow core slabs may be combined
with other spanning elements to create several
variations of floor systems.
Single and Double Tees
o Can span farther than precast slabs
o Commonly used in such building types
as commercial structures, schools, and
parking garages
o Tees may be combined with other elements
to create framing systems or spread systems
o Tees are erected with spaces between them
than are bridged with precast or hollow core
slabs where sitecast concrete is poured as
a part of the topping.
o Systems can increase the economy of long
span structures
ƒ
•
STEEL
A. Steel availability: www.aisc.org/availability
Common Shear, Moment, and Deflection Tables
Simple Span, Point Loaded
Simple Span, Uniformly Loaded and general deflection estimates
Fixed systems, uniform and point loaded
Continuous fixed system
Cantilevered system
PERTINENT POINT CHART
Properties
Labor $
Material $
Connecting structural
Connecting Other
Manufacturing risk
Bracing
2nd Order Magnification
Code
Design Style
Steel
36 ksi (A36 Structural, Typical “L”),
50 ksi (A992 Structural, Typical “W”)
50 ksi (typical)
Fast
Crain Rental $/day
Long (Pre-order Steel)
Pin-based typical,
Moment-based Expensive
Low
High
Easy (weld or bolt)
Easy (weld or bolt)
Low (factory high standers)
Tension only X bracing (typical)
Low
AISC v.12
LRFD (large build), ASD(small build)
High
Low
Pre-Easy (embed rebar); Post-hard
Hard (embed rebar, drill and bolt)
Moderate (mixing control)
Shear wall (typical)
High
ACI 318-05
ASD only
Fire
Insulation
Bomb resistance
Corrosion Maintenance
Requires fireproofing
Low
Low
High
Fire resistant
Moderate to high
High
Low
Size
Texture
$ of custom shape
Building environment
slender
smooth
high
Corrosion potential
Bulky
Rough to smooth
moderate to low
Dust potential
Strength Compression
Strength Tension
Speed of Construction
Special Cost
Initial delay (construction)
Type of Construction
Concrete
90-99% x 4ksi (Structure typical) +
1-10% x 60ksi
Slow
Concrete Pump $↑ exponentially / floor
Short (create formwork)
Moment-based Typical
REFERENCES:
DESIGN
The Architects Studio Companion – Edward Allen/Joseph Iano
Reinforced Concrete Mechanics & Design (Fourth Edition) – James MacGregor & James
Wright
Building Structures – James Ambrose
CODES
AISC V.12: Steel Construction Manual
ACI 318-05: Reinforce Structural Concrete Code
AWS D1.1: Structural Welding Code
RCSC Specification: Specifications for Structural Joints
ASCE/SEI 7-05: Minimum Design Loads for buildings and other structures
IBC 2003: International Building Code
UBC 1997: Uniform Building Code
THOUGHTS:
Our biggest concern was the quantity of information that could be placed into this
section of the “high rise handbook”. As such, we simply tried to cover the basics and give
everyone a brief overview of the two different types of systems. Just know that there is a vast
amount of material that is not covered in this paper that can be researched to apply to high
rise construction.
1
"Concrete (construction)," Microsoft Encarta Online Encyclopedia 2007 Microsoft
Corporation. 1997-2007. http://encarta.msn.com/encyclopedia_76155877
7/Concrete_(construction).html
2”
Cement and Concrete Basics,” Portland Cement Association. Skokie, Illinois: 2007.
http://www.cement.org/basics/concretebasics_faqs.asp
3
Allen, Edward. The Architects Studio Companion. Third Edition. John Wiley & Sons, New
York: 2002. pg 107-135