【正文】 earth should be pacted and thoroughly moistened to about 6 in. in depth to avoid absorption of the moisture present in the wet concrete. Concrete should always be placed in horizontal layers which are pacted by means of high frequency powerdriven vibrators of either the immersion or external type, as the case requires, unless it is placed by pumping. It must be kept in mind, however, that over vibration can be harmful since it could cause segregation of the aggregate and bleeding of the concrete. 華北水利水電學(xué)院畢業(yè)設(shè)計(jì) 土木工程專業(yè) 3 Hydration of the cement takes place in the presence of moisture at temperatures above 50176。 struck capacity ( 25 m 179。 and the articulated type of about m 179。 elastic or plastic deformation or creep that cause a substantial change of the geometry of the structure。 and corrosion. Computational methods used to verify structures with respect to the different safety 華北水利水電學(xué)院畢業(yè)設(shè)計(jì) 土木工程專業(yè) 6 conditions can be separated into: (1)Deterministic methods, in which the main parameters are considered as nonrandom parameters. (2)Probabilistic methods, in which the main parameters are considered as random parameters. Alternatively, with respect to the different use of factors of safety, putational methods can be separated into: (1)Allowable stress method, in which the stresses puted under maximum loads are pared with the strength of the material reduced by given safety factors. (2)Limit states method, in which the structure may be proportioned on the basis of its maximum strength. This strength, as determined by rational analysis, shall not be less than that required to support a factored load equal to the sum of the factored live load and dead load ( ultimate state ). The stresses corresponding to working ( service ) conditions with unfactored live and dead loads are pared with prescribed values ( service limit state ) . From the four possible binations of the first two and second two methods, we can obtain some useful putational methods. Generally, two binations prevail: (1)deterministic methods, which make use of allowable stresses. (2)Probabilistic methods, which make use of limit states. The main advantage of probabilistic approaches is that, at least in theory, it is possible to scientifically take into account all random factors of safety, which are then bined to define the safety factor. probabilistic approaches depend upon : (1) Random distribution of strength of materials with respect to the conditions of fabrication and erection ( scatter of the values of mechanical properties through out the structure )。 (2)Number of human lives which can be threatened by this failure。 (3) Cost of physical and material damage due to the failure of the construction。將可塑的混凝土拌合物注入到模板內(nèi)，并將其搗實(shí)，然后進(jìn)行養(yǎng)護(hù)，以加速水泥與水的水化反應(yīng)，最后獲得硬化的混凝土。 由于鋼筋混凝土截面在均質(zhì)性上與標(biāo)準(zhǔn)的木材或鋼的截面存在著差異，因此，需要對(duì)結(jié)構(gòu)設(shè)計(jì)的基 本原理進(jìn)行修改。如果拌制混凝土的各種材料配合比恰當(dāng)，則混凝土制成品的強(qiáng)度較高，經(jīng)久耐用，配置鋼筋后，可以作為任何結(jié)構(gòu)體系的主要構(gòu)件。一般情況下，除使用混凝土泵澆筑外，混凝土都應(yīng)在水平方向分層澆筑，并使用插入式或表面式高頻電動(dòng)振搗器搗實(shí)。為了保證水泥的水化作用得以進(jìn)行，必須具備上述條件。鋼筋混凝土通常是現(xiàn)場(chǎng)澆注的合成材料，它與在工廠中制造的標(biāo)準(zhǔn)的鋼結(jié)構(gòu)梁、柱等不同，因此對(duì)于上面所提到的一系列因素必須予以考慮。因此，當(dāng)試算截面選定后，每次設(shè)計(jì)都是對(duì)截面進(jìn)行復(fù)核。那是大部分土方是采用窄軌鐵路運(yùn)輸，在這目前來(lái)說(shuō)是很少采用的。 土方工程或土方挖運(yùn)工程 指的是把地表面過(guò)高處的土壤挖去（挖方），并把它傾卸到地表面過(guò)低的其他地方（填方）。它將幫助他在最短的時(shí)間內(nèi)獲得最好的方案。推土機(jī)和正鏟挖土機(jī)都能做到這點(diǎn)。卸都不準(zhǔn)確。正鏟挖土機(jī)不能挖比其停機(jī)平面低很多的土，而深挖堅(jiān)實(shí)的土壤時(shí)，反鏟挖華北水利水電學(xué)院畢業(yè)設(shè)計(jì) 土木工程專業(yè) 10 土機(jī)最適用，但其卸料半徑比起裝有正鏟的同一挖土機(jī)的卸料半徑則要小很多。一旦鏟運(yùn)機(jī)裝滿，助推拖拉機(jī)就回到開挖的地點(diǎn)去幫助下一臺(tái)鏟運(yùn)機(jī)。），由 430 馬力的牽引發(fā)動(dòng)機(jī)驅(qū)動(dòng)。特殊型式的翻斗車包括容量為 4 m179。翻斗車車斗向前傾翻而司機(jī)坐在后方卸載，因此有時(shí)被稱為后卸卡車。 破壞不僅僅指結(jié)構(gòu)的整體破壞，而且還指結(jié)構(gòu)不能正常的使用，或者，用更為確切的話來(lái)說(shuō)，把破壞看成是結(jié)構(gòu)已經(jīng)達(dá)到不能繼續(xù)承擔(dān)其設(shè)計(jì)荷載的“極限狀態(tài)”。器例子包括結(jié)構(gòu)失穩(wěn)之前的過(guò)大變形和位移；早期開裂或過(guò)大的裂縫；較大的振動(dòng)和腐蝕。 （ 2）極限狀態(tài)法，在這種方法中，結(jié)構(gòu)的工作狀態(tài)是以其最大強(qiáng)度為依據(jù)來(lái)衡量的。根據(jù)前兩種方法和后兩種方法的四種可能組合，我們可以得到一些實(shí)用的計(jì)算方法。概率法取決于： （ 1）制作和安裝過(guò)程中材料強(qiáng)度的隨機(jī)分布（整個(gè)結(jié)構(gòu)的力學(xué)性能數(shù)值的分散性）； （ 2）截面和結(jié)構(gòu)幾何尺寸的不確定性（由結(jié)構(gòu)制作和安裝造成的誤差和缺陷而引起的）； 對(duì)作用在結(jié)構(gòu)上的活載和恒載的預(yù)測(cè)的不確定性； 所采用的近似計(jì)算方法有關(guān)的不精確性（實(shí)際應(yīng)力與計(jì)算應(yīng)力的偏差）。但是，應(yīng)該考慮到進(jìn)行全概率分析的困難。第一種方法對(duì)材料和荷載采用不同的安全