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How to Build a Light Steel Structure Part 1

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    Posted: 24 Jan 2019 at 7:40am

Approach

 

These requirements are based primarily on the American Iron and Steel Institute’s (AISI) Specification for the Design of Cold-Formed Steel Structural Members [1] for steel member strength, on the American Concrete Institutes’s (ACI) Building Code Requirements for Structural Concrete [2], on the Structural  Design of Insulating Concrete Form Walls in Residential Construction [3] for concrete design and specification, and on the Standard for Cold-Formed Steel Framing - Prescriptive Method for One and Two Family Dwellings [29] for steel framing requirements. The provisions for building loads are based on the American Society of Civil Engineers’ (ASCE) Minimum Design Loads for Buildings and Other Structures [4], the International Building Code [5], and the International Residential Code [6].

 

These provisions are intended to represent sound engineering and construction practice, taking into account the need for practical and affordable construction techniques for residential buildings. This document is not intended to restrict the use of sound judgment or exact engineering analysis of specific applications.

 

 

Scope

 

The provisions of this Prescriptive Method apply to the construction of detached one- and two- family dwellings, townhouses, and other attached single-familydwellings in compliance with the general limitations of Table 2.1. The limitations are intended to define the appropriate use of this document for most one- and two- family dwellings. Using insulating concrete forms and cold-formed steel systems with other construction materials in a single structure shall be in accordance with the applicable building code requirements for that material, the general limitations of Table 2.1, and relevant provisions of this document. An engineered design shall be required for applications that do not meet the limitations of Table 2.1.

 

The provisions of the Prescriptive Method shall not apply to irregular structures or portions of structures in Seismic Design Categories C, D , and D . Only such irregular

portions of structures shall be designed in accordance with accepted engineering practice to the extent such irregular features affect the performance of the structure. A portion of the building shall be considered to be irregular when one or more of the following conditions occur:

 

  • Exterior shear wall lines are not in one plane vertically from the foundation to the uppermost story in which they are required (i.e., cantilevers).
  • A section of floor or roof is not laterally supported by shear walls on all edges.
  • An opening in the floor or roof exceeds the lesser of 12 ft (3.7 m) or 50 percent of the least floor dimension.
  • Portions of the floor are vertically offset.
  • Shear walls (i.e. exterior ICF walls) do not occur in two perpendicular directions.
  • Shear walls are constructed of dissimilar systems on any one story level.

TABLE 2.1

APPLICABILITY LIMITS

 

ATTRIBUTE

MAXIMUM LIMITATIONS

General

 

Building Dimensions

60 feet with center bearing wall or beam

32 feet without center bearing wall or beam

 

Number of Stories

 

Design Wind Speed

 

2 stories above grade with a basement

 

130 mph (209 km/h) 3-second gust

 

Ground Snow Load

 

70 psf (3.4 kPa)

Seismic Design Category

A, B, C, D1 and D2 (Seismic Zones (0, 1, 2, 3, and

4)

 

Floors

 

Floor Dead Load

10 psf (0.72 kPa)

First-Floor Live Load

40 psf (1.9 kPa)

Second-Floor Live Load (sleeping rooms)

30 psf (1.4 kPa)

Floor Clear Span (unsupported)

32 feet (9.8 m)

 

Walls - Concrete

 

Unit Weight of Concrete

 

 

 

150 pcf (23.6 kN/m3)

Wall Height (unsupported)

10 feet (3 m)

 

Walls - Cold-Formed Steel

 

Wall Dead Load

 

 

 

10 psf (0.48 kN/m2)

Load Bearing Wall Height

10 feet (3 m)

 

Roofs

 

Roof and Ceiling Dead Load

15 psf (0.72 kPa)

Roof Live Load (ground snow load)

70 psf (3.4 kPa)

Roof Slope

3:12 to 12:12

Attic Live Load

20 psf (0.96 kPa)

Roof Clear Span (unsupported)

32 feet (9.8 m)

 

For SI: 1 foot = 0.3048 m; 1psf = 47.8804 Pa; 1 pcf = 157.0877 N/m3  = 16.0179 kg/m3; 1 mph =

1.6093 km/hr

 



Edited by BSRSA - 24 Jan 2019 at 12:42am
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Definitions

 

The following are definitions of key terms as they are used in this document. Figure

2.1 illustrates certain terms as referenced by these definitions.

 

Accepted Engineering Practice:  An engineering approach that conforms with accepted principles, tests, technical standards, and sound judgment.

 

Anchor Bolt: A bolt, headed or threaded, used to connect a structural member of different material to a concrete member.

 

Approved: Reference to approval by the building code authority having jurisdiction. A rational design by a competent design professional shall constitute grounds for approval.

 

Attic: The enclosed space between the ceiling joists of the top-most floor and the roof rafters of a building not intended for occupancy but sometimes used for storage.

 

Authority Having Jurisdiction: The organization, political subdivision, office, or individual charged with the responsibility of administering and enforcing the provisions of applicable building codes.

 

Axial Load: The longitudinal force acting on a member. Examples are the gravity loads carried by columns or studs.

 

Backfill:  The soil that is placed adjacent to completed portions of a below-grade structure (i.e., basement) with suitable compaction and allowance for settlement.

 

Basement: That portion of a building, which is partly, or completely below grade and which may be used as habitable space.

 

Bearing Stiffener:  Additional material that is attached to the web to strengthen the member against web crippling. Also called a web stiffener.

 

Bond Beam:  A continuous horizontal beam of concrete with steel reinforcement located in the exterior walls of a structure to tie the structure together and distribute loads.

 

Buckling: A kink, wrinkle, bulge, or otherwise loss of the original shape of a member due to compressive, bending, bearing, or shear loads.

 

Building: Any one- or two-family dwelling or portion thereof that is used for human habitation



Building Length: The dimension of a building that is perpendicular to roof rafters, roof trusses, or floor joists (L).
 
Building Width:  The dimension of a building that is parallel to roof rafters, roof trusses, or floor joists (W).
 

Ceiling Joist: A horizontal structural framing member that supports ceiling components and which may be subject to attic loads.

 

C-Shape: A cold-formed steel shape used for structural and non-structural framing members consisting of a web, two (2) flanges and two (2) lips (edge stiffeners).

 

Clip Angle: An L-shaped short piece of metal (normally with a 90-degree bend), typically used for connections.

 

Cold-Formed Sheet Steel: A process where light-gauge steel members are manufactured by (1) press-braking blanks sheared from sheets or cut length of coils or plates, or by (2) continuous roll forming of cold- or hot-rolled coils of sheet steel; both forming operations are performed at ambient room temperature, that is, without any addition of heat such as would be required for hot forming.

 

Compressive Strength: The maximum ability of concrete to resist a compressive load, usually measured in pounds per square inch (psi) or Pascals (Pa). The compressive strength is based on compression tests of concrete cylinders that are moist-cured for 28 days in accordance with ASTM C 31 [7] and ASTM C 39 [8].

 

Concrete Web: A concrete wall segment as per Figure 2.1, a minimum of 2 inches (51 mm) thick, connecting the vertical and horizontal concrete members (cores) of a waffle-grid ICF wall or lintel member. Webs may contain form ties but are not reinforced (i.e., vertical or horizontal reinforcement or stirrups).

 

Crawlspace: A type of building foundation that uses a perimeter foundation wall to create an under floor space which is not habitable.

 

Dead Load:  Forces resulting from the weight of walls, partitions, framing, floors, ceilings, roofs, and all other permanent construction entering into, and becoming part of, a building.

 

Deflection:  Elastic movement of a loaded structural member or assembly (i.e., beam or wall).

 


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Design Professional: An individual who is registered or licensed to practice their respective design profession as defined by the statutory requirements of the state in which the project is to be constructed.

 

Design (or Basic) Wind Speed: Related to winds that are expected to be exceeded once every 50 years at a given site (i.e., 50-year return period). Wind speeds in this document are given in units of miles per hour (mph) by 3-second gust measurements in accordance with ASCE 7 [4].

 

Dwelling: Any building that contains one or two dwelling units for living purposes.

 

Edge Stiffener:  The part of a C-shape framing member that extends from the flange as a stiffening element that extends perpendicular to the flange.

 

Endwall:  The exterior wall of a building which is perpendicular to the roof ridge and parallel to floor framing, roof rafters, or trusses. It is normally the shorter dimension of a rectangular building’s footprint.

 

Exposure Categories:  Reflects the effect of the ground surface roughness on wind loads in accordance with ASCE 7 [4]. Exposure Category B includes urban and suburban areas, or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger. Exposure Category C includes open terrain with scattered obstructions having heights generally less than 30 ft (9.1 m) and shorelines in hurricane prone regions. Exposure D includes open exposure to large bodies of water in non-hurricane-prone regions.

 

Flange: The portion of the C-shape framing member or track that is perpendicular to the web.

 

Flat Wall:  A solid concrete wall of uniform thickness produced by ICFs or other forming systems.

 

Floor Joist: A horizontal structural framing member that supports floor loads and superimposed vertical loads.

 

Form Tie: The element of an ICF system that holds both sides of the form together. Form ties can be steel, solid plastic, foam plastic, a composite of cement and wood chips, a composite of cement and foam plastic, or other suitable material capable of resisting the loads created by wet concrete. Form ties remain permanently embedded in the concrete wall.

 

Foundation:  The structural elements through which the load of a structure is transmitted to the earth.



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Foundation Wall:  The structural element of a foundation that transmits the load of a structure to the earth; includes basement, stem, and crawlspace walls.

 

Grade:  The finished ground level adjoining the building at all exterior walls.

 

Ground Snow Load:  Measured load on the ground due to snow accumulation developed from a statistical analysis of weather records expected to be exceeded once every 50 years at a given site.

 

Horizontal Reinforcement:  Steel reinforcement placed horizontally in concrete walls to provide resistance to temperature and shrinkage cracking. In certain circumstances, horizontal reinforcement is required for additional strength around openings and in high loading conditions such as experienced in hurricanes and earthquakes.

 

In-Line Framing: A framing method where all vertical and horizontal load carrying members are aligned.

 

Insulating Concrete Forms (ICFs): A concrete forming system using stay-in- place forms of foam plastic insulation, a composite of cement and foam insulation, a composite of cement and wood chips, or other insulating material for constructing cast-in-place concrete walls. Some systems are designed to have one or both faces of the form removed after construction.

 

J Bolt: A threaded anchor bolt typically embedded in concrete with threads on one end and a crook in the shank at the other; used to connect a structural member of different material to a concrete member.

 

Lateral Load:  A horizontal force, created by wind or earthquake, acting on a structure or its components.

 

Lateral Support:  A horizontal member providing stability to a column or wall across its smallest dimension.

 

Ledger: A horizontal structural member fastened to a wall to serve as a connection point for other structural members, typically floor joists.

 

Lip: See edge stiffener.

 

Live Load: Any gravity load that is not permanently applied to a structure; typically transient and sustained gravity forces resulting from the weight of people and furnishings, respectively.


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Material Thickness (Steel): The base metal thickness excluding any protective coatings. Thickness is now commonly expressed in mils (1/1000 of an inch).

 

Metallic Coated Steel:  Steel that has a metallic coating for protection against corrosion. The level of protection provided is measured by the weight of the metallic coating applied to the surface area of the steel. Typical metallic coatings are galvanizing, galvalume, or galfan, which are zinc-based.

 

Mil: A unit of measurement equal to 1/1000 of an inch (e.g., 33 mil = 0.033 inch).

 

Multiple Span: The span made by a continuous member having intermediate supports.

 

Non-Structural Walls:  Refer to walls.

 

Post-and-Beam Wall:  A perforated concrete wall with widely spaced (greater than that required for screen-grid walls) vertical and horizontal concrete members (cores) with voids in the concrete between the cores created by the ICF form (i.e., flat, waffle-, or screen-grid as per Figure 2.1).

 

Ridge: The horizontal line formed by the joining of the top edges of two sloping roof surfaces.

 

Roof Snow Load: Uniform live load on the roof due to snow accumulation; roughly equivalent to 70 to 80 percent of the ground snow load in accordance with ASCE 7 [4].

 

Screen-Grid Wall:  A perforated concrete wall with closely spaced vertical and horizontal concrete members (cores) with voids in the concrete between the members created by the ICF form as per Figure 2.1. It is also called an interrupted-grid wall or post-and-beam wall in other publications.

 

Seismic Load:  The force exerted on a building structure resulting from seismic

(earthquake) ground motions.

 

Seismic Design Categories: Designated seismic hazard levels associated with a particular level or range of seismic risk and associated seismic design parameters (i.e., spectral response acceleration and building importance). Seismic Design Categories A, B, C, D1, and D2 (Seismic Zones 0, 1, 2, 3, and 4) correspond to successively greater seismic design loads; refer to the IBC [5] and IRC [6].

 

Sill Plate: A horizontal member constructed of wood, steel, or other suitable material that is fastened to the top of a concrete wall, providing a suitable surface for fastening structural members constructed of different materials to the concrete wall.



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Slab-on-Grade:  A concrete floor, which is supported by, or rests on, the soil directly below.

 

Slump:  A measure of consistency of freshly mixed concrete equal to the amount that a cone of uncured concrete sags below the mold height after the cone-shaped mold is removed in accordance with ASTM C 143 [9].

 

Smoke-Development Rating:  The combustibility of a material that contributes to fire impact through life hazard and property damage by producing smoke and toxic gases; refer to ASTM E 84 [10].

 

Span:  The clear horizontal distance between bearing supports.

 

Stem Wall:  A below-grade foundation wall supported directly by the soil or on a footing. Wall thickness and height are determined as that which can adequately distribute the building loads safely to the earth.

 

Stirrup: Steel bars, wires, or welded wire fabric located perpendicular to horizontal reinforcement and extending across the depth of the member in concrete beams, lintels, or similar members subject to large shear loads.

 

Story: That portion of the building included between the upper surface of any floor and the upper surface of the floor next above, except that the top-most story shall be that habitable portion of a building included between the upper surface of the top-most floor and the ceiling or roof above.

 

Story Above-Grade: Any story with its finished floor surface entirely above grade except that a basement shall be considered as a story above-grade when the finished surface of the floor above the basement is (a) more than 6 feet (1.8 m) above the grade plane, (b) more than 6 feet (1.8 m) above the finished ground level for more than 50 percent of the total building perimeter, or (c) more than 12 feet (3.7 m) above the finished ground level at any point.

 

Strap:  Flat or coiled sheet steel material typically used for bracing and blocking which transfers loads by tension and/or shear.

 

Stud: Vertical structural element of a wall assembly, which supports vertical loads and/or transfers lateral loads.

 

Townhouse: Attached single-family dwelling units constructed in a row with each unit separated by fire walls at property lines.



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Track:  A framing member consisting of only a web and two (2) flanges. Track depth measurements are taken to the inside of the flanges.

 

Truss:  A coplanar system of structural members joined together at their ends usually to construct a series of triangles that form a stable beam-like framework.

 

Vertical Reinforcement:  Steel reinforcement placed vertically in concrete walls to strengthen the wall against lateral forces and eccentric loads. In certain circumstances, vertical reinforcement is required for additional strength around openings.

 

Waffle-Grid Wall: A solid concrete wall with closely spaced vertical and horizontal concrete members (cores) with a concrete web between the members created by the ICF form; refer to Figure 2.1. The thicker vertical and horizontal concrete cores and the thinner concrete webs create the appearance of a breakfast waffle. It is also referred to as an uninterrupted-grid wall in other publications.

 

Wall Height: The clear vertical distance between the finished floor and the finished ceiling. Where a finished floor does not exist (i.e., crawlspace), the wall height is the clear vertical distance between the interior finish grade and the finished ceiling.

 

Walls (steel):

Structural or Load Bearing:  Wall systems subject to loads that exceed the limits for a non-structural system.

 

Non-Structural or Non-Load Bearing:  Wall systems that are limited to a lateral (transverse) load of not more than 5 psf (240 Pa), a superimposed vertical load per member, exclusive of sheathing materials, of not more than

100 lb/ft (1460 N/m), or a superimposed vertical load per member of not more than 200 lbs (890 N).

 

Web: That portion of a framing member that connects the flanges.

 

Web Crippling: The localized permanent (inelastic) deformation of the web member subjected to concentrated load or reaction at bearing supports.

 

Web Stiffener: Additional material that is attached to the web to strengthen the member against web crippling. Also called a bearing stiffener.

 

Wind Exposure: refer to Exposure Categories.

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Wind Load: The force or pressure exerted on a building structure and its components resulting from wind. Wind loads are typically measured in pounds per square foot (psf) or Pascals (Pa).

 

Wind Speed: Wind speed is the design wind speed related to winds that are expected to be exceeded once every 50 years at a given site (i.e., 50 year-return period). Wind speeds in this document are given in units of miles per hour (mph) by “3- second gust” measurements (refer to Table 2.2 to convert to fastest-mile wind speed).




 

TABLE 2.2

EQUIVALENT BASIC WIND SPEEDS (mph)1

 

Fastest Mile

70

75

80

85

90

100

110

3-Second Gust

85

90

100

105

110

120

130

 

For SI: 1 mph = 1.609 km/hr = 0.447 m/sec

1 Linear interpolation is permitted.

Yield Strength: A characteristic of the basic strength of the steel material defined as the highest unit stress that the material can endure before permanent deformation occurs as measured by a tensile test in accordance with ASTM A 370 [11].

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