O que é um/uma Kip-Force (kip)?
Formal Definition
The kip (symbol: kip or klbf) is a US customary unit of force equal to 1000 pounds-force (1000 lbf). The name is a contraction of "kilo-pound" — a hybrid of the Greek prefix "kilo-" (thousand) and the English "pound." One kip equals approximately 4,448.22 newtons or 4.44822 kilonewtons. The kip is not part of the international system of units but is the dominant force unit in US structural engineering.
The kip serves the same purpose in the US customary system that the kilonewton serves in SI: it allows engineers to express forces in the thousands-of-pounds range without writing unwieldy numbers. A column supporting 250,000 lbf is more conveniently described as supporting 250 kips. This convenience has made the kip indispensable in American structural engineering practice.
Related Units
The kip per square inch (ksi = kip/in²) is the standard unit for expressing material strengths in US engineering. Structural steel yield strengths are typically 36–65 ksi, high-strength bolts have tensile strengths of 120–150 ksi, and steel cable breaking strengths are 200–270 ksi. The kip-foot (kip·ft) is used for bending moments in structural members.
Etymology
Origin of the Name
The word "kip" is a contraction of "kilo-pound," combining the Greek prefix "kilo-" (from χίλιοι, thousand) with the English "pound." The blending of metric and imperial terminology reflects the practical origins of the unit — American engineers wanted a thousand-pound unit and created a convenient shorthand. The abbreviation "kip" first appeared in engineering literature in the early 20th century.
The usage is strictly American — British, Canadian, and Australian engineers working in imperial units do not commonly use the kip, preferring to express large forces in kilonewtons or tonnes-force. Within the US, however, the kip is so thoroughly established in structural engineering that it appears in all major building codes and design standards.
Precise Definition
Exact Definition
One kip is defined as exactly 1000 pounds-force: 1 kip = 1000 lbf = 1000 × 4.4482216152605 N = 4448.2216152605 N ≈ 4.44822 kN. Since the pound-force is exactly defined in terms of SI units (through the defined values of the avoirdupois pound and standard gravity), the kip also has an exact SI equivalent.
Key Conversions
1 kip = 1000 lbf = 4448.22 N = 4.44822 kN = 453.592 kgf = 0.5 short ton-force = 0.00444822 MN. For stress: 1 ksi (kip/in²) = 6.89476 MPa.
Derived Units
The kip per square inch (ksi) is the standard stress unit in US structural engineering: 1 ksi = 1000 psi = 6.89476 MPa. The kip per linear foot (klf or kip/ft) is used for distributed loads on beams. The kip-foot (kip·ft) is the standard moment (torque) unit in US structural engineering: 1 kip·ft = 1000 ft·lbf = 1.35582 kN·m.
História
Origins in American Engineering
The kip emerged in the early 20th century as US structural engineering practice matured. As buildings and bridges grew larger, forces measured in individual pounds became impractical. A typical steel column in a multi-story building might support 200,000 to 500,000 pounds — numbers that are cumbersome to write, discuss, and calculate with. The kip provided a clean, manageable alternative.
The American Institute of Steel Construction (AISC), founded in 1921, adopted the kip as a standard unit in its Steel Construction Manual. The American Concrete Institute (ACI) and the American Society of Civil Engineers (ASCE) followed suit. By mid-century, the kip was firmly established as the lingua franca of American structural engineering.
Resistance to Metrication
The US Metric Conversion Act of 1975 encouraged (but did not mandate) the adoption of SI units. While some engineering disciplines made partial transitions, structural engineering in the US remained firmly in the kip-based system. The enormous body of existing standards, design tables, software, and professional knowledge embedded in kips and ksi made conversion impractical.
Attempts to create dual-unit (kip/kN) versions of design standards met with limited success. Engineers found it confusing to work in two systems simultaneously, and the economic incentive for conversion was weak since most US structural engineering serves domestic construction. Today, US structural engineering remains one of the most persistently imperial-unit fields in the world.
Modern Standard
The kip appears throughout all current editions of AISC 360 (steel design), ACI 318 (concrete design), NDS (wood design), and ASCE 7 (minimum design loads). US-educated structural engineers think, calculate, and communicate in kips as naturally as SI-trained engineers use kilonewtons.
Uso atual
Structural Steel Design
In US steel design (AISC 360), all force-related quantities use kips. Column axial loads, beam shear forces, connection capacities, and bolt strengths are expressed in kips. Material strengths use ksi: A36 steel has a yield strength of 36 ksi, A572 Grade 50 steel has 50 ksi, and A992 steel (the most common structural steel) has 50 ksi yield and 65 ksi tensile strength.
Concrete Design
ACI 318 expresses concrete compressive strength (f'c) in psi (typically 3,000–8,000 psi for normal concrete), but design forces in kips and moments in kip-feet. Reinforcing bar strengths are in ksi: Grade 60 rebar has a yield strength of 60 ksi. Foundation bearing pressures are in kips per square foot (ksf).
Bridge Engineering
AASHTO (American Association of State Highway and Transportation Officials) bridge design specifications use kips for all force quantities. Design truck loads are expressed in kips: the HL-93 design truck has axle loads of 8 kips, 32 kips, and 32 kips. Lane loads are 0.64 kips per linear foot. Bridge member capacities are in kips (shear and axial) or kip-feet (moment).
Geotechnical Engineering
Pile capacities are expressed in kips or tons (1 ton = 2 kips). A typical driven steel H-pile might have a capacity of 150–400 kips. Soil bearing pressures are in kips per square foot (ksf) or tons per square foot. Lateral earth pressures are in kips per linear foot of wall.
Everyday Use
Understanding Building Loads
While most people never use kips directly, the unit governs the structural safety of every building they enter. A typical office floor is designed for a live load of about 2.5 kips per 50-square-foot area. A parking garage column might support 500–1000 kips. The cables of a major suspension bridge carry tens of thousands of kips.
Engineering Reports
Homeowners and building owners who review structural engineering reports will encounter kips. An engineer might report that a residential beam has a capacity of 15 kips, or that a foundation must resist a lateral force of 8 kips during an earthquake. Understanding that one kip equals 1,000 pounds helps interpret these values.
Construction Industry
Construction workers, crane operators, and riggers encounter kip ratings on equipment and in lift plans. A crane's load chart might specify maximum capacity at various radii in kips. Rigging hardware (shackles, slings, hooks) may be rated in kips for working load limit or breaking strength.
Academic Context
US engineering students encounter kips throughout their structural engineering courses. Textbook problems routinely express applied loads in kips and distributed loads in kips per foot. Mastering kip-based calculations is essential for passing the Fundamentals of Engineering (FE) and Professional Engineer (PE) licensing exams.
In Science & Industry
Structural Analysis Software
US structural analysis software (SAP2000, ETABS, RISA, RAM) defaults to kip-based units. Input loads are in kips, member forces are output in kips and kip-feet, and design checks reference ksi material strengths. While most software can operate in SI units, the vast majority of American practice uses kip-based settings.
Research and Testing
US structural research facilities measure and report forces in kips. The laboratory at the NEES (Network for Earthquake Engineering Simulation) facilities, university testing labs, and manufacturer testing reports all use kips. A typical large-scale structural test might apply loads of 100–1000 kips using hydraulic actuators.
Seismic Engineering
Seismic base shear — the total horizontal force an earthquake applies to a building's base — is calculated in kips using ASCE 7 provisions. A typical mid-rise building in a high seismic zone might have a design base shear of 500–2000 kips. Story forces, diaphragm forces, and connection forces are all expressed in kips throughout the seismic design process.
Wind Engineering
Wind loads on buildings and structures, calculated per ASCE 7 Chapter 26-31, produce forces in pounds or kips depending on structure size. The total wind force on a tall building might be 1,000–10,000 kips. Wind tunnel testing of building models reports forces in kips after scaling to full-size equivalents.
Interesting Facts
The kip is one of the few engineering units formed by combining a Greek prefix (kilo-) with a non-metric base unit (pound). This hybrid origin reflects the pragmatic nature of American engineering — borrowing whatever works, regardless of system purity.
A typical four-story office building column supports approximately 200–400 kips — the weight of about 100–200 tons pressing down through the structure above. These forces are carried continuously for the life of the building.
The main cables of the Golden Gate Bridge carry a total tension of approximately 200,000 kips (100,000 tons) — the combined weight of the roadway, traffic, and the cables themselves.
US structural engineering students solve hundreds of problems in kips during their education. The PE structural exam requires fluency in kip-based calculations, making it one of the most persistently imperial-unit professional examinations in the world.
A standard A325 high-strength bolt (3/4-inch diameter) has a design shear capacity of about 17 kips in single shear. A typical beam-to-column connection might use 4–8 such bolts, providing 68–136 kips of connection capacity.
The AISC Steel Construction Manual — the 'bible' of US steel design — contains thousands of tables listing member capacities in kips and kip-feet. Converting these tables to SI units would require republishing the entire manual.