【正文】 Preliminary Selection of Plate Girder Webs and FlangesThe ASDS, Section B10, states that, in general, plate girders should be proportioned by the momentofinertia method. This approach requires the selection of a suitable trial cross section that would then be checked by the momentofinertia method.As was mentioned previously, the total girder depth should generally range from 1/8 to 1/12 of the span length, depending on load and span requirements. Therefore, the web depth may be estimated to be from 2 to 4 in. less than the assumed girder total depth. The web thickness may then be selected on the basis of permissible depththickness ratios as established in the ASDS. These ratios are based on buckling considerations. The web must have sufficient thickness to resist buckling tendencies that are created by. girder curvature under load. As a girder deflects, a vertical pression is induced in the web due to the ponents of the flange stresses, the result of which constitutes a squeezing action. The buckling strength ofthe web must be capable of resisting this squeezing action. This is the basis for the ASDS criteria (Section G1) that the ratio of the clear distance between flanges to the web thickness must not exceed h/t14000/ ADSD EP.(G11)where h = clear distance between flanges t= web thickness F= specified minimum yield stress of the flange (ksi) It is allowed for this ratio to be exceeded if transverse intermediate stiffeners are providedwith a spacing not in excess of times the distance between flanges. The maximum permissible 0 ratio then bees maximum h/ t=2000/ ASDS EP.(G12)TABLE54 Maximum h/ t Rationsh/t F 36ksi 42ksi 46ksi 50ksi 14000/2000/322 282 261 243333 309 295 283 Resulting values for the preceding two expressions, as functions of various F values, are shown in Table 54. In addition, web buckling considerations may require a reduction of the allowable bending stress in the pression flange. According to the ASDS, Section G2, when the web depththickness ratio exceeds 760/, the maximum bending stress in the pression flange must be reduced to a value that may be puted from ASDS Equation (G2l). Plate girder webs that depend on tension field action (to be defined shortly), as discussed in ASDS. Section G3, must be proportioned so that the web bending tensile stress due to moment in the plane of the web does not exceed （）F ASDS Ep(G51)where f=puted average web shear stress (total shear divided by web area) (ksi) F=allowable web shear stress according to ASDS Equation (G3l) (ksi)This expression in effect constitutes an allowable bending stress reduction due to the interaction of concurrent bending and shear stress (ASDS, Section G5).After preliminary web dimensions are selected, the required flange area may be determined by using an approximate approach as follows. With reference to Figure 521, the moment of inertia of the total section with respect to axis xx is I=I+INeglecting the moment of inertia of the flange areas about their own centroidal axes and assuming that h=(dt),an approximate gross moment of inertia may be expressed as I= th/12+2A(h/2) Expressing this in terms of the section modulus (S) and also assuming that hd: S= th/12/h/2+2A(h/2)/h/2= th/6+AhFIGURE 521 Girder nomenclature. The required S = M/F。ASDM的第2部分，提供一般說明，從45到92英寸的名義深度，設(shè)計(jì)實(shí)例和焊接板梁屬性表是一個(gè)范圍廣泛的部分，此表作為選擇經(jīng)濟(jì)比例焊接板梁的指導(dǎo)?？偹郊羟辛ΓㄇО趺烤€性英寸）可從下列表達(dá)式（任何材料的強(qiáng)度）得出 注：=翼緣相對(duì)梁中性軸的靜矩（）V=最大剪力（KI）I=梁截面的總慣性矩（）當(dāng)負(fù)載直接作用于翼緣（無支承加勁肋存在），可考慮每英寸的垂直剪切力和矢量的水平剪力，以確定腹板和翼緣之間產(chǎn)生的剪切力。這焊縫可設(shè)計(jì)為間歇性的角焊縫，支承加勁肋到腹板的焊縫應(yīng)是連續(xù)的，腹板翼緣到腹板的焊縫也應(yīng)是連續(xù)的。翼緣板到腹板的連接：翼緣板到腹板的連接為了防止產(chǎn)生總水平剪切梁的彎曲力。支承加勁肋到腹板的連接：由于支承加勁肋是承載的要素，連接焊縫通常是一個(gè)在每個(gè)加筋板兩側(cè)的連續(xù)角焊縫。下面表示提供的最低值 如果實(shí)際的腹板剪切力基于 小于ASDS方程（G31）允許剪切力的基礎(chǔ)上，然而，轉(zhuǎn)移的剪切力（f）可能會(huì)成正比減少。只有腹板翼緣焊縫以內(nèi)的加勁肋被視為有效的支承，（ASDS，J8）。有效列的長度不應(yīng)小于計(jì)算長細(xì)比加勁肋長度的四分之三（ASDS，）。圖523 列支承加勁肋支承加勁肋應(yīng)緊密接觸，大約延伸到翼緣邊緣，如圖522所示。如果加載的翼緣不受制于偏轉(zhuǎn)以及該值然后在這里R=最大反力或集中荷載（K）L=最大的側(cè)向無支撐長度沿任意翼緣的負(fù)載點(diǎn)（英寸）B=翼緣寬度（英寸）d=d2k=腹板角焊縫（英寸）凈高，或腹板是受均勻分布負(fù)載，方程（K16）和（K17）不需要檢查。當(dāng)壓縮載荷超過下列值，就需要支承加勁肋時(shí)。）如果提供加勁肋并延長了至少h/2，ASDS方程（K14）和（K15）就需要檢查。k維，在腹板屈服方程，（K12）和（K13）中使用，采取從外表面的翼緣角焊縫和梁腹板交界處的距離。除了使反作用力或集中荷載轉(zhuǎn)移到腹板，支承加勁肋還可以防止腹板局部屈服，以及一般的腹板損壞和側(cè)移腹板屈曲，在這篇課文的第4章討論。一般來說，中間加筋肋在梁受拉翼緣的截?cái)喔鶕?jù)ASDS要求，加勁肋產(chǎn)生附加焊縫的最小長度，（ASDS，第G4）。當(dāng)f面板中的,要求的總面積可能因?yàn)楸壤齠v/而減?。ˋSDS，G4部分）。因此，在設(shè)計(jì)時(shí)只需根據(jù)張力場(chǎng)的作用。 A=1C/2[a/h(a/h) /]YDht ASDS方程（G42）注： C，a，h和t如先前定義 Y=腹板鋼的屈服應(yīng)力與加筋鋼的屈服應(yīng)力比 D= = =當(dāng)加勁肋成對(duì)布置，如何確定該區(qū)域的總面積。為了提供足夠的側(cè)向支撐腹板，ASDS，G部分4要求所有中間加勁肋（不管是一對(duì)或一個(gè)）參考腹板平面軸I的慣性矩，如： I(h/50) ASDS方程（G41）加強(qiáng)板，還必須滿足ASDS方程（G42）規(guī)定的最小截面積。然后確定加勁肋的大小，一般而言，焊接板梁的加勁肋交替焊接在腹板的每一側(cè)。ASDS方程（F42）或表l36和150，可用于確定允許設(shè)計(jì)剪應(yīng)力，或根據(jù)設(shè)計(jì)的張力場(chǎng)作用，ASDS方程（G31）或，ASDM第2部分的表236，表250基礎(chǔ)上來確定。這必須根據(jù)ASDS公式（F4 2）或表136和150的ASDM的第2部分，因?yàn)檫@個(gè)面板必須在沒有任何張力場(chǎng)作用下設(shè)計(jì)（ASDS 部分G4）。a/h（有時(shí)也被稱為長寬比）的比例不得超過所給出的值a/h(260/h/t) ASDS方程（F51）最大間距為梁腹板高度H的三倍。當(dāng)存在張力場(chǎng)的作用，梁以外的混合梁（假設(shè)提供適當(dāng)?shù)闹虚g加勁肋），36KSI屈服應(yīng)力鋼和50KSI屈服應(yīng)力鋼各自允許剪應(yīng)力可能取自ASDS方程（G3L），或ASDM的第2部分，表236和250中。如果腹板的h/t比率是小于260（以及比在表54中規(guī)定的限制少）并且最大的腹板剪應(yīng)力小于ASDS方程（F42）所允許的，那么可以不考慮中間加筋肋和張力場(chǎng)的作用，注 最大=V/h t允許剪應(yīng)力=(C)/ ASDS方程（F42）注:C=45000k/ (h/ t) 當(dāng)C=190 /( h/ t) 當(dāng)Ck=+(a/h) 當(dāng)a/h=+(a/h) 當(dāng)a/h注：t=腹板厚度a=中間加勁肋之間的距離h=翼緣之間的距離36KSI屈服應(yīng)力鋼和50KSI屈服應(yīng)力鋼各自允許的剪切應(yīng)力，在ASDS方程（F42）的基礎(chǔ)上，可能從ASDM的第2部分，表136和表150中得到。當(dāng)這種情況發(fā)生時(shí)，屈曲的腹板承受著對(duì)角線的張力和中間加勁肋壓力。橫向中間加勁肋橫向中間加勁肋的作用主要是對(duì)屈曲深，薄梁腹板加勁的目的。翼緣板理論過渡點(diǎn)的測(cè)定與板梁蓋板理論截止點(diǎn)的測(cè)定類似。改變翼緣板是最好的方法，通過改變板的厚度，寬度，或兩者都改變，在兩個(gè)翼緣板的兩端加入下降槽的對(duì)接焊縫。翼緣板，在其最大彎矩的基礎(chǔ)上確定大小，這樣會(huì)延長梁的全長。在完成梁腹板和翼緣的初步選