ARCH 5605 – HIGH RISE STUDIO STUDIO RESEARCH STUDIES LOUIS KHAN & KIRSTEN TUDOR
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 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.
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 structural steel that was ductile with fewer fewer impurities. It was at this point that steel steel became widely used as a structural element in i n building construction.
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.
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 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
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 Supported by bearing walls o Least expensive concrete framing system o for shot spans and light loads Popular for multiple dwelling building types o 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 The addition of beams and slabs to the o construction can increase the load capacity and span range of the system and eliminate the need for regularity The increased complexity makes it one of o the more expensive of all sitecast concrete systems to construct. Whenever possible, beam depth should o 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. Standard Pan witdth: 20-30 in o Standard Joist Width: 5-9 in o Economical for spans up to 40ft o Two-Way Flat Plate One of the most economical framing o systems Can span farther than one-way slabs o Plain form makes it simple to construct o and easy to finish •
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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. Column bays are most efficient when square o Span lengths should not differ by 1/3 of the o longer span Two-Way Slab & Beam Uses beams to support the slab between o the columns High construction cost o Economical only for long spans and heavy o loads Best for heavy industrial applications o Waffle Slab Is an economical system for long spans o or heavy loads Desirable for the waffle-like appearance o underneath slab. Standard: 19in domes w/ 5in ribs to create o 24in module Economy of system depends on maximum o repetition of standard forms and sizes. o
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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 Commonly used in hotels, multifamily dwellings, o commercial structures, hospitals, schools, and parking structures. Sitecast concrete is poured over precast o slabs to: Increase structural performance Increase fire resistance Allow integration of electrical communications services into the floor
Provide a more level and smoother floor surface Solid and hollow core slabs may be combined o with other spanning elements to create several variations of floor systems. Single and Double Tees Can span farther than precast slabs o Commonly used in such building types o as commercial structures, schools, and parking garages Tees may be combined with other elements o to create framing systems or spread systems Tees are erected with spaces between them o than are bridged with precast or hollow core slabs where sitecast concrete is poured as a part of the topping. Systems can increase the economy of long o span structures
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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
90-99% x 4ksi (Structure typical) + 1-10% x 60ksi Slow Concrete Pump $↑ exponentially / floor Short (create formwork)
Labor $ Material $ Connecting structural Connecting Other Manufacturing risk Bracing 2nd Order Magnification Code Design Style
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)
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
Moment-based Typical 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
The Architects Studio Companion – Edward Allen/Joseph Iano Reinforced Concrete Mechanics & Design (Fourth Edition) – James MacGregor & James Wright Building Structures – James Ambrose
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
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.
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"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
