I-säde

On olemassa kaksi I-vakiomuotoa:

• Rolled I-beam, formed by kuumavalssaus, kylmävalssaus or ekstruusio (depending on material).

• Levypalkki, formed by hitsaus (or occasionally pulttaus or niittaamalla) lautaset.

I-beams are commonly made of rakenneteräs but may also be formed from alumiini or other materials. A common type of I-beam is the valssattu teräspalkki (RSJ)—sometimes incorrectly rendered as vahvistettu teräspalkki. brittiläinen and eurooppalaiset standardit also specify Universal Beams (UBs) and Universal Columns (UCs). These sections have parallel flanges, as opposed to the varying thickness of RSJ flanges which are seldom now rolled in the UK. Parallel flanges are easier to connect to and do away with the need for tapering washers. UCs have equal or near-equal width and depth and are more suited to being oriented vertically to carry axial load such as columns in multi-storey construction, while UBs are significantly deeper than they are wide are more suited to carrying bending load such as beam elements in floors.

I-palkit—I-beams engineered from wood with kuitulevy and/or laminoitua viilupuuta—are also becoming increasingly popular in construction, especially residential, as they are both lighter and less prone to warping than solid wooden palkit. On kuitenkin ollut jonkin verran huolta niiden nopeasta voiman menetyksestä tulipalossa, jos niitä ei ole suojattu.

Suunnittelu taivutukseen 

Suurimmat jännitykset (${\displaystyle \sigma _{xx}}$) ovat taivutetussa palkin paikoissa, jotka ovat kauimpana neutraaliakselista.

A beam under bending sees high stresses along the axial fibers that are farthest from the neutraali akseli. Vian estämiseksi suurimman osan palkin materiaalista on sijaittava näillä alueilla. Neutraaliakselin lähellä olevalla alueella tarvitaan suhteellisen vähän materiaalia. Tämä havainto on I-palkin-poikkileikkauksen perusta; neutraaliakseli kulkee pitkin rainan keskustaa, joka voi olla suhteellisen ohut ja suurin osa materiaalista voi keskittyä laippoihin.

The ideal beam is the one with the least cross-sectional area (and hence requiring the least material) needed to achieve a given osan moduuli. Since the section modulus depends on the value of the hitausmomentti, tehokkaan säteen materiaalin tulee olla mahdollisimman kaukana neutraalista akselista. Mitä kauempana tietty materiaalimäärä on neutraalista akselista, sitä suurempi on leikkausmoduuli ja siten suurempi taivutusmomentti voidaan vastustaa.

When designing a symmetric I-beam to resist stresses due to bending the usual starting point is the required section modulus. If the allowable stress is ${\displaystyle \sigma _{\mathrm {max} }}$ and the maximum expected bending moment is ${\displaystyle M_{\mathrm {max} }}$, silloin vaadittu leikkausmoduuli saadaan kaavalla³

• ${\displaystyle S={\cfrac {M_{\mathrm {max} }}{\sigma _{\mathrm {max} }}}={ \cfrac {I}{c}}}$

where ${\displaystyle I}$ is the moment of inertia of the beam cross-section and ${\displaystyle c}$ is the distance of the top of the beam from the neutral axis (see säteen teoria for more details).

For a beam of cross-sectional area ${\displaystyle a}$ and height ${\displaystyle h}$, the ideal cross-section would have half the area at a distance ${\displaystyle h/2}$ above the cross-section and the other half at a distance ${\displaystyle h/2}$ below the cross-section.³ For this cross-section

• ${\displaystyle I={\cfrac {ah{2}}{4}};S=0.5ah}$

Näitä ihanteellisia olosuhteita ei kuitenkaan voida koskaan saavuttaa, koska materiaalia tarvitaan rainassa fyysisistä syistä, mukaan lukien nurjahduksen estämiseksi. Leveillä-laippapalkilla leikkausmoduuli on noin

• ${\displaystyle S\approx 0.35ah}$

joka on parempi kuin suorakaiteen muotoisilla ja pyöreillä palkeilla saavutettu.

Ongelmat 

Though I-beams are excellent for unidirectional bending in a plane parallel to the web, they do not perform as well in bidirectional bending. These beams also show little resistance to twisting and undergo sectional warping under torsional loading. For torsion dominated problems, laatikkopalkit and other types of stiff sections are used in preference to the I-beam.

Muodot ja materiaalit (USA) 

Ruosteinen niitattu teräs I-palkki

Yhdysvalloissa yleisimmin mainittu I-palkki on leveä-laippa (W). Näissä palkeissa on laipat, joiden sisäpinnat ovat yhdensuuntaiset suurimmalla osalla niiden pinta-alasta. Muita I-palkkeja ovat American Standard (merkitty S) muodot, joissa sisälaippapinnat eivät ole yhdensuuntaisia, ja H-paalut (merkitty HP), joita tyypillisesti käytetään paaluperustuksina. Leveät-laippamuodot ovat saatavilla ASTM A992 -luokissa,⁴ which has generally replaced the older ASTM grades A572 and A36. Ranges of yield strength:

• A36: 36,000 psi (250 MPa)

• A572: 42,000–60,000 psi (290–410 MPa), with 50,000 psi (340 MPa) the most common

• A588: Similar to A572

• A992: 50,000–65,000 psi (340–450 MPa)

Kuten useimmat terästuotteet, I{0}}palkit sisältävät usein kierrätettyä sisältöä.

Standardit 

Seuraavat standardit määrittelevät I-palkkiteräsprofiilien muodot ja toleranssit:

Eurooppalaiset standardit 

• EN 10024, Hot rolled taper flange I sections – Tolerances on shape and dimensions.

• EN 10034, Structural steel I and H sections – Tolerances on shape and dimensions.

• EN 10162, Cold rolled steel sections – Technical delivery conditions – Dimensional and cross-sectional tolerances

AISC käsikirjamuokata

The American Institute of Steel Construction (AISC) publishes the Steel Construction Manual for designing structures of various shapes. It documents the common approaches, Sallittu vahvuussuunnittelu (ASD) and Kuorma- ja vastustekijäsuunnittelu (LRFD), (starting with 13th ed.) to create such designs.

muu 

• DIN 1025-5

• ASTM A6, American Standard Beams

• BS 4-1

• IS 808 – Dimensions hot rolled steel beam, column, channel and angle sections

• AS/NZS 3679,1 – Australia and New Zealand standard⁵

Nimitys ja terminologia 

Leveä-laippa I-palkki.

• In the Yhdysvallat, steel I-beams are commonly specified using the depth and weight of the beam. For example, a "W10x22" beam is approximately 10 in (254 mm) in depth (nominal height of the I-beam from the outer face of one flange to the outer face of the other flange) and weighs 22 lb/ft (33 kg/m). Wide flange section beams often vary from their nominal depth. In the case of the W14 series, they may be as deep as 22.84 in (580 mm).⁶

• In Kanada, steel I-beams are now commonly specified using the depth and weight of the beam in metric terms. For example, a "W250x33" beam is approximately 250 millimetres (9.8 in) in depth (height of the I-beam from the outer face of one flange to the outer face of the other flange) and weighs approximately 33 kg/m (22 lb/ft; 67 lb/yd).⁷ I-beams are still available in U.S. sizes from many Canadian manufacturers.

• In Meksiko, steel I-beams are called IR and commonly specified using the depth and weight of the beam in metric terms. For example, a "IR250x33" beam is approximately 250 mm (9.8 in) in depth (height of the I-beam from the outer face of one flange to the outer face of the other flange) and weighs approximately 33 kg/m (22 lb/ft).⁸

• In Intia I-beams are designated as ISMB, ISJB, ISLB, ISWB. ISMB: Indian Standard Medium Weight Beam, ISJB: Indian Standard Junior Beams, ISLB: Indian Standard Light Weight Beams, and ISWB: Indian Standard Wide Flange Beams. Beams are designated as per respective abbreviated reference followed by the depth of section, such as for example ISMB 450, where 450 is the depth of section in millimetres (mm). The dimensions of these beams are classified as per IS:808 (as per BIS).^{viite Tarvitaan}

• In the Yhdistynyt kuningaskunta, these steel sections are commonly specified with a code consisting of the major dimension (usually the depth){{0}}x-the minor dimension-x-the mass per metre-ending with the section type, all measurements being metric. Therefore, a 152x152x23UC would be a column section (UC = universal column) of approximately 152 mm (6.0 in) depth 152 mm width and weighing 23 kg/m (46 lb/yd) of length.⁹

• In Australia, these steel sections are commonly referred to as Universal Beams (UB) or Columns (UC). The designation for each is given as the approximate height of the beam, the type (beam or column) and then the unit metre rate (e.g., a 460UB67.1 is an approximately 460 mm (18.1 in) deep universal beam that weighs 67.1 kg/m (135 lb/yd)).⁵

Mobiilisäteet 

Mobiilisäteet are the modern version of the traditional "kasteloitu palkki" which results in a beam approximately 40–60 percent deeper than its parent section. The exact finished depth, cell diameter and cell spacing are flexible. A cellular beam is up to 1.5 times stronger than its parent section and is therefore utilized to create efficient large span constructions.¹⁰