Structural load

A structural load or structural action is a force, deformation, or acceleration applied to structural elements.^[1]^[2] A load causes stress, deformation, and displacement in a structure. Structural analysis, a discipline in engineering, analyzes the effects of loads on structures and structural elements. Excess load may cause structural failure, so this should be considered and controlled during the design of a structure. Particular mechanical structures—such as aircraft, satellites, rockets, space stations, ships, and submarines—are subject to their own particular structural loads and actions.^[3] Engineers often evaluate structural loads based upon published regulations, contracts, or specifications. Accepted technical standards are used for acceptance testing and inspection.

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On my mark. Ready. We have been load testing on this exact airplane in a similar way except for the airbag support. That's different. And I don't know that that's ever been done before. We're going to support the CG of the airplane using airbags, so three airbags, one under the left wing, one under the right wing and one under the aft fuselage. [Hammer Pounding] It looks pretty good. That will keep the main gear tires off the floor so the load going through the main gear doesn't vary. And the problem that we found was that as the main gear load varies, as we load the wings up, the load on the gear goes down and that has an influence of it's own on the strain gauges, because it's torqueing the whole wing box from the main gear, so we're going to isolate the main gear. So that's our test. Ron go ahead and start begin inflation of airbags to contact. [Air Hissing] We're going to continue inflation. Right side's good. Left side's good. What we've got here is hydraulics jacks under the wing. We're going to apply known loads to those hydraulic jacks. And so with those known loads and the strain gauge responses, we'll be able to correlate those so that we can derive our own load equations in flight and be able to monitor the loads real time. Establish control there at the load cell. I have control. All right stop and place it. I see it. That looks good. We've supported the aircraft on airbags, we've collected initial data and things are looking good. We want to understand the structural performance of the aircraft while it's flying, and so doing a test of this nature enables us to understand, on the ground, by applying certain loads into the airplane what the loads will be experienced in flight. I see loading on left eight and right eight This test will be to either to validate those equations or to help fix them if we find that they're off. Verify the power parameter feedback for the chiller and the pump are on. Watch for actuator movement. Okay, coming up on L7. And it's always good to test your technical know how and your strengths -- full pressure -- with challenging tests. Having successfully completed this effort then we will be prepared to flight test the airplane, and track the wing load against the wing strength envelope while its flying. OK, Darren, go ahead and zero DACS channels except for LRTs and pressure gauges. Okay roger that. In work. Stand by. Zeroes are completed. If we don't do our job well now they won't be able to do their job well then. Okay go ahead and remove aircraft jacks from underneath both wings. So it's important that we have a small error band and that we know what it is. So the way it takes off will be the same way it lands. It won't be bent when it comes back. We won't cause harm to the airplane by the flight test of the new flap. How does the data look, are you guys happy with data capture? Yea, I think we're happy with the data capture. Okay thanks. Larry, we're good.

Types

In civil engineering, specified loads are the best estimate of the actual loads a structure is expected to carry. These loads come in many different forms, such as people, equipment, vehicles, wind, rain, snow, earthquakes, the building materials themselves, etc. Specified loads also known as characteristic loads in many cases.

Buildings will be subject to loads from various sources. The principal ones can be classified as live loads (loads which are not always present in the structure), dead loads (loads which are permanent and immovable excepting redesign or renovation) and wind load, as described below. In some cases structures may be subject to other loads, such as those due to earthquakes or pressures from retained material. The expected maximum magnitude of each is referred to as the characteristic load.

Dead loads are static forces that are relatively constant for an extended time. They can be in tension or compression. The term can refer to a laboratory test method or to the normal usage of a material or structure.

Live loads are usually variable or moving loads. These can have a significant dynamic element and may involve considerations such as impact, momentum, vibration, slosh dynamics of fluids, etc.

An impact load is one whose time of application on a material is less than one-third of the natural period of vibration of that material.

Cyclic loads on a structure can lead to fatigue damage, cumulative damage, or failure. These loads can be repeated loadings on a structure or can be due to vibration.

Imposed loads are those associated with occupation and use of the building; their magnitude is less clearly defined and is generally related to the use of the building.

Loads on architectural and civil engineering structures

Structural loads are an important consideration in the design of buildings. Building codes require that structures be designed and built to safely resist all actions that they are likely to face during their service life, while remaining fit for use.^[4] Minimum loads or actions are specified in these building codes for types of structures, geographic locations, usage and building materials.^[5] Structural loads are split into categories by their originating cause. In terms of the actual load on a structure, there is no difference between dead or live loading, but the split occurs for use in safety calculations or ease of analysis on complex models.

To meet the requirement that design strength be higher than maximum loads, building codes prescribe that, for structural design, loads are increased by load factors. These load factors are, roughly, a ratio of the theoretical design strength to the maximum load expected in service. They are developed to help achieve the desired level of reliability of a structure^[6] based on probabilistic studies that take into account the load's originating cause, recurrence, distribution, and static or dynamic nature.^[7]

Dead load

The dead load includes loads that are relatively constant over time, including the weight of the structure itself, and immovable fixtures such as walls, plasterboard or carpet. The roof is also a dead load. Dead loads are also known as permanent or static loads. Building materials are not dead loads until constructed in permanent position.^[8]^[9]^[10] IS875(part 1)-1987 give unit weight of building materials, parts, components.

Live load

Live loads, or imposed loads, are temporary, of short duration, or a moving load. These dynamic loads may involve considerations such as impact, momentum, vibration, slosh dynamics of fluids and material fatigue.

Live loads, sometimes also referred to as probabilistic loads, include all the forces that are variable within the object's normal operation cycle not including construction or environmental loads.

Roof and floor live loads are produced during maintenance by workers, equipment and materials, and during the life of the structure by movable objects, such as planters and people.

Bridge live loads are produced by vehicles traveling over the deck of the bridge.

Environmental loads

Environmental loads are structural loads caused by natural forces such as wind, rain, snow, earthquake or extreme temperatures.

Wind loads
Snow, rain and ice loads
Seismic loads
Hydrostatic loads
Temperature changes leading to thermal expansion cause thermal loads
Ponding loads
Frost heaving
Lateral pressure of soil, groundwater or bulk materials
Loads from fluids or floods
Permafrost melting
Dust loads

Other loads

Engineers must also be aware of other actions that may affect a structure, such as:

Foundation settlement or displacement
Fire
Corrosion
Explosion
Creep or shrinkage
Impact from vehicles or machinery vibration
Construction loads

Load combinations

A load combination results when more than one load type acts on the structure. Building codes usually specify a variety of load combinations together with load factors (weightings) for each load type in order to ensure the safety of the structure under different maximum expected loading scenarios. For example, in designing a staircase, a dead load factor may be 1.2 times the weight of the structure, and a live load factor may be 1.6 times the maximum expected live load. These two "factored loads" are combined (added) to determine the "required strength" of the staircase.

The size of the load factor is based on the probability of exceeding any specified design load. Dead loads have small load factors, such as 1.2, because weight is mostly known and accounted for, such as structural members, architectural elements and finishes, large pieces of mechanical, electrical and plumbing (MEP) equipment, and for buildings, it's common to include a Super Imposed Dead Load (SIDL) of around 5 pounds per square foot (psf) accounting for miscellaneous weight such as bolts and other fasteners, cabling, and various fixtures or small architectural elements. Live loads, on the other hand, can be furniture, moveable equipment, or the people themselves, and may increase beyond normal or expected amounts in some situations, so a larger factor of 1.6 attempts to quantify this extra variability. Snow will also use a maximum factor of 1.6, while lateral loads (earthquakes and wind) are defined such that a 1.0 load factor is practical. Multiple loads may be added together in different ways, such as 1.2*Dead + 1.0*Live + 1.0*Earthquake + 0.2*Snow, or 1.2*Dead + 1.6(Snow, Live(roof), OR Rain) + (1.0*Live OR 0.5*Wind).

Aircraft structural loads

For aircraft, loading is divided into two major categories: limit loads and ultimate loads.^[11] Limit loads are the maximum loads a component or structure may carry safely. Ultimate loads are the limit loads times a factor of 1.5 or the point beyond which the component or structure will fail.^[11] Gust loads are determined statistically and are provided by an agency such as the Federal Aviation Administration. Crash loads are loosely bounded by the ability of structures to survive the deceleration of a major ground impact.^[12] Other loads that may be critical are pressure loads (for pressurized, high-altitude aircraft) and ground loads. Loads on the ground can be from adverse braking or maneuvering during taxiing. Aircraft are constantly subjected to cyclic loading. These cyclic loads can cause metal fatigue.^[13]

References

^ ASCE/SEI 7-05 Minimum Design Loads for Buildings and Other Structures. American Society of Civil Engineers. 2006. p. 1. ISBN 0-7844-0809-2.
^ "1.5.3.1". Eurocode 0: Basis of structural design EN 1990. Bruxelles: European Committee for Standardization. 2002.
^ Avallone, E.A.; Baumeister, T. (eds.). Mark's Standard Handbook for Mechanical Engineers (10th ed.). McGraw-Hill. pp. 11–42. ISBN 0-07-004997-1.
^ "2.2.1(1)". Eurocode 0: Basis of structural design EN 1990. Bruxelles: European Committee for Standardization. 2002.
^ "1604.2". International Building Code. USA: International Code Council. 2000. p. 295. ISBN 1-892395-26-6.
^ "2.2.5(b)". Eurocode 0: Basis of structural design EN 1990. Bruxelles: European Committee for Standardization. 2002.
^ Rao, Singiresu S. (1992). Reliability Based Design. USA: McGraw-Hill. pp. 214–227. ISBN 0-07-051192-6.
^ 2006 International Building Code Section 1602.1.
^ EN 1990 Euro code – Basis of structural design section 4.1.1
^ EN 1991-1-1 Euro code 1: Actions on Structures – Part 1-1: General actions – densities, self-weight, imposed loads for buildings section 3.2
^ ^a ^b Bruce K. Donaldson, Analysis of Aircraft Structures: An Introduction (Cambridge; New York: Cambridge University Press, 2008), p. 126
^ Experimental Mechanics: Advances in Design, Testing and Analysis, Volume 1, ed. I. M. Allison (Rotterdam, Netherlands: A.A. Balkema Publishers, 1998), p. 379
^ Bruce K. Donaldson, Analysis of Aircraft Structures: An Introduction (Cambridge; New York: Cambridge University Press, 2008), p. 129

External links

Luebkeman, Chris H., and Donald Petting "Lecture 17: Primary Loads". University of Oregon. 1996^[1]
Fisette, Paul, and the American Wood Council. "Understanding Loads and Using Span Tables". 1997.^{[2] Archived 2015-05-06 at the Wayback Machine}

This page was last edited on 24 April 2024, at 04:19

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