¿Qué es un/una Kip (kip)?
Formal Definition
The kip is a unit of force (and sometimes mass) equal to 1,000 pounds-force (4,448.222 newtons) or 1,000 avoirdupois pounds of mass (453.592 kilograms). The name "kip" is a portmanteau derived from "kilo-pound" — "kip" combining "ki" from "kilo" and "p" from "pound." It is used almost exclusively in American structural and civil engineering.
As a unit of force, 1 kip = 1,000 lbf = 4,448.222 newtons (N) = 4.448222 kilonewtons (kN). As a unit of mass, 1 kip = 1,000 lb = 453.59237 kg. The kip is not recognized by the SI or by any international standards body — it exists purely as a practical engineering convenience within the US customary system.
Engineering Context
The kip simplifies engineering calculations involving large forces and loads. Instead of writing "150,000 pounds-force," an engineer can write "150 kips" — a more manageable number that reduces the chance of arithmetic errors and makes structural calculations more readable. The related unit "ksi" (kips per square inch) is widely used for specifying material strength: structural steel commonly has a yield strength of 36 ksi or 50 ksi (36,000 or 50,000 psi).
Etymology
A Modern Coinage
The word "kip" is one of the few measurement unit names coined in the modern era from English roots. It combines the first two letters of "kilo" (from Greek "chilioi," meaning thousand) with the letter "p" from "pound." The result is a compact, distinctive word that is easy to speak, write, and distinguish from other units.
The term emerged in American engineering practice in the early to mid-20th century, though its exact date of coinage is uncertain. It appears in engineering textbooks from the 1930s onward. The kip was never formally adopted by any standards body — it simply became standard practice in US structural engineering through widespread voluntary use.
Linguistic Note
The word "kip" should not be confused with the currency unit of Laos (also called "kip"), or with the British slang term "kip" (meaning a nap or a cheap lodging). In engineering contexts, the meaning is always clear from the technical context.
Precise Definition
Definition as Force
The kip as a unit of force is defined as exactly 1,000 pounds-force (lbf). Using the standard gravitational acceleration g = 9.80665 m/s², one pound-force equals exactly 4.4482216152605 N, so 1 kip = 4,448.2216152605 N = 4.448222 kN.
Definition as Mass
The kip as a unit of mass is defined as exactly 1,000 avoirdupois pounds. Using the international pound of 0.45359237 kg, 1 kip = 453.59237 kg exactly.
Derived Units
The most important derived unit from the kip is ksi (kips per square inch), a unit of stress and pressure. 1 ksi = 1 kip/in² = 1,000 psi = 6.894757 MPa. This unit is ubiquitous in American structural engineering for specifying material strengths, allowable stresses, and design pressures. Another derived unit is the kip-foot (kip·ft), a unit of bending moment: 1 kip·ft = 1,000 lb·ft = 1,355.818 N·m.
Historia
Origin in American Engineering
The kip emerged from the practical needs of American structural engineers in the early 20th century. As buildings, bridges, and other structures grew larger, the forces involved — measured in pounds-force — became unwieldy numbers. A steel beam in a high-rise building might resist 250,000 pounds of force; a bridge cable might carry 1,500,000 pounds of tension. Working with such large numbers increased the risk of arithmetic errors and made calculations tedious.
Engineers began informally using "kilopounds" as a shorthand, which was quickly abbreviated to "kips." The term appears in engineering textbooks by the 1930s and was firmly established in practice by mid-century. The American Institute of Steel Construction (AISC) Steel Construction Manual uses kips throughout, which cemented the unit's place in American engineering education and practice.
Relationship to SI Adoption
The kip's persistence is directly related to the United States' incomplete adoption of the SI system. In countries using SI units, structural engineers work with kilonewtons (kN) and megapascals (MPa), and the kip serves no purpose. In the US, where the customary system dominates construction, the kip provides the same convenience — reducing large numbers to manageable sizes — that kN provides in the metric world. One kip (4.448 kN) is roughly comparable in magnitude to one kilonewton, which has made some engineers argue that switching to SI would not be difficult conceptually, though the practical cost of converting existing codes, standards, and practices would be substantial.
Modern Codification
The kip is used throughout the AISC Steel Construction Manual, the ACI Building Code Requirements for Structural Concrete, the AASHTO Bridge Design Specifications, and other key American engineering codes. These documents define their examples, tables, and design equations in kip-based units. The kip is thus codified not by a standards body but by the professional codes that govern American structural engineering practice.
Uso actual
Structural Engineering
The kip is used daily by structural engineers in the United States. Every steel beam, concrete column, bridge cable, and foundation design involves kip-based calculations. Structural analysis software (SAP2000, ETABS, RISA, etc.) in the US defaults to kips for force units. Design checks, load combinations, and capacity calculations are all expressed in kips and ksi.
Material Specifications
Steel grades in the US are specified using ksi yield strength. ASTM A36 steel has a yield strength of 36 ksi. ASTM A992 (the most common structural steel for wide-flange shapes) has a yield strength of 50 ksi. High-strength bolts are rated at 120 ksi (A325) or 150 ksi (A490) tensile strength. These ksi values are fundamental to American structural design.
Bridge Engineering
AASHTO bridge design specifications use kips for all force-related quantities. Bridge loads — including truck loads, lane loads, and impact factors — are specified in kips. The standard AASHTO HL-93 design truck has axle loads of 8 kips, 32 kips, and 32 kips.
Education
American engineering schools teach structural analysis and design using kips. Textbooks by authors such as Hibbeler, McCormac, and Salmon use kips in their examples and problems. Students learn to think in kips from their first structural engineering course, and this unit system follows them throughout their professional careers.
Everyday Use
Not an Everyday Unit
The kip is not used in everyday life. It is exclusively a professional engineering unit. The average person never encounters kips in daily activities, shopping, or household contexts. Even within engineering, only structural and civil engineers use kips routinely — mechanical, electrical, and chemical engineers are more likely to encounter newtons or pounds-force directly.
Professional Context
For the approximately 275,000 licensed professional engineers in structural and civil engineering in the United States, kips are an everyday professional reality. A structural engineer reviewing building plans might note that a column carries 500 kips, a beam moment is 200 kip-feet, or a connection must resist 75 kips of shear force. These numbers are as natural to a US structural engineer as kilonewtons are to their counterparts in Europe or Asia.
Construction Documents
Kips appear on structural engineering drawings, calculations, and specifications that are part of construction documents for buildings and bridges. Contractors and construction workers may encounter kips in load tables and structural details, though they typically think in pounds for practical field measurements.
Testing and Inspection
Material testing reports in the US express strengths in ksi. Concrete cylinder compression tests yield results in psi that are discussed in ksi terms. Steel tensile test results are reported in ksi. Building inspectors and testing laboratories in the US work with kip-based units as a matter of routine.
In Science & Industry
Engineering Research
In American civil and structural engineering research, the kip is standard. Research papers published in the ASCE Journal of Structural Engineering, ACI Structural Journal, and Engineering Structures (when authored by US researchers) commonly use kips and ksi. Experimental results for structural tests — beam bending, column buckling, connection capacity — are typically reported in kips.
Finite Element Analysis
Finite element analysis (FEA) in American engineering practice uses kips as the standard force unit. Commercial FEA software packages offer kip-inch and kip-foot unit systems alongside SI options. Academic and industry FEA models of US structures are almost always built in kip-based units.
Seismic Engineering
In earthquake engineering, base shear forces and story forces are calculated in kips. The seismic design provisions of ASCE 7 (Minimum Design Loads for Buildings and Other Structures) produce forces in kips when used with customary units. Shake table test results in US laboratories are often reported in kips.
Not Used in Physics or Basic Sciences
Outside engineering, the kip has no role. Physicists, chemists, and biologists use newtons for force and kilograms for mass. The kip is entirely an applied engineering unit with no presence in fundamental science.
Interesting Facts
The name 'kip' is a portmanteau of 'kilo' and 'pound' — 'ki' from kilo plus 'p' from pound. It is one of the few modern measurement units coined from English roots rather than Latin or Greek.
The AISC Steel Construction Manual — the 'bible' of American steel design — uses kips throughout. Every table, chart, and example in the manual is expressed in kip-based units.
One kip equals approximately 4.448 kilonewtons (kN). This near-equivalence means that US and metric structural engineers work with numbers of similar magnitude, just in different units.
The derived unit 'ksi' (kips per square inch) is so common in American engineering that some engineers forget it stands for 1,000 pounds per square inch. ASTM A992 steel's 50 ksi yield means 50,000 psi.
The standard AASHTO design truck used for US bridge engineering has axle loads of 8 kips (front axle) and 32 kips (each rear axle) — totaling 72 kips or 72,000 pounds.
The kip was never formally adopted by any standards organization. It became standard through widespread voluntary use in the American engineering community, primarily driven by AISC publications.
In structural engineering education, students often confuse kips (force) with kips (mass). In most structural contexts, 'kip' means kip-force (1,000 lbf), not kip-mass (1,000 lb), because structural loads are forces.
A typical single-family house in the US might impose a total load of 100-200 kips on its foundation. A 50-story skyscraper might impose 100,000+ kips on each major column.
Regional Variations
United States Only
The kip is used exclusively in the United States. No other country uses this unit in engineering practice, education, or standards. American engineers working on international projects must convert kip-based calculations to kilonewtons and megapascals.
Within the US
Within the US, kip usage is concentrated in structural and civil engineering. Mechanical engineers sometimes encounter kips when working on structural components but generally prefer pounds-force or newtons. Aerospace engineers use pounds-force. Only structural and civil engineers use kips as their primary force unit.
International Engineering
International structural engineering uses kilonewtons (kN) and megapascals (MPa) for the same purposes that kips and ksi serve in the US. The Eurocode structural design standards use kN. British Standards use kN. Australian, Japanese, Chinese, and Indian structural codes all use SI units. American engineers working on international projects or collaborating with international firms must be fluent in both unit systems.
Academic Context
American engineering textbooks use kips. International textbooks use kN. This creates a challenge for international students studying in the US (and vice versa), who must learn an unfamiliar unit system alongside the engineering concepts themselves.