全文获取类型
收费全文 | 4638篇 |
免费 | 340篇 |
专业分类
公路运输 | 1178篇 |
综合类 | 1579篇 |
水路运输 | 1266篇 |
铁路运输 | 665篇 |
综合运输 | 290篇 |
出版年
2024年 | 23篇 |
2023年 | 88篇 |
2022年 | 152篇 |
2021年 | 249篇 |
2020年 | 270篇 |
2019年 | 137篇 |
2018年 | 120篇 |
2017年 | 172篇 |
2016年 | 164篇 |
2015年 | 238篇 |
2014年 | 400篇 |
2013年 | 303篇 |
2012年 | 380篇 |
2011年 | 400篇 |
2010年 | 273篇 |
2009年 | 257篇 |
2008年 | 225篇 |
2007年 | 304篇 |
2006年 | 256篇 |
2005年 | 154篇 |
2004年 | 85篇 |
2003年 | 63篇 |
2002年 | 38篇 |
2001年 | 52篇 |
2000年 | 30篇 |
1999年 | 28篇 |
1998年 | 21篇 |
1997年 | 15篇 |
1996年 | 19篇 |
1995年 | 12篇 |
1994年 | 10篇 |
1993年 | 8篇 |
1992年 | 9篇 |
1991年 | 7篇 |
1990年 | 7篇 |
1989年 | 5篇 |
1988年 | 1篇 |
1985年 | 1篇 |
1984年 | 2篇 |
排序方式: 共有4978条查询结果,搜索用时 15 毫秒
51.
城市公交线网优化的线性模型 总被引:10,自引:1,他引:10
为了对现有公交线网进行优化,有效利用现有交通资源,解决城市交通问题,运用系统科学的思想,通过对城市公交线网优化的主要内容、优化原则、优化目标以及约束条件的分析,兼顾考虑乘客出行时间、公交线网密度和公交企业的利益以及公交线网的布局对整个城市的交通系统的影响,提出了优化目标的函数表达式及相应约束条件的数学表达式,建立了公交线网优化的线性模型,并给出了运用逐步筛选法对所提出的数学模型进行求解的方法。 相似文献
52.
南京长江二桥斜拉桥施工过程稳定性分析 总被引:5,自引:0,他引:5
针对南京二桥南汊钢斜拉桥,考虑结构的非线性和构件的极限承载能力,计入施工过程的变形和应力的叠加效应,用包含梁和索单元的空间组合结构模型,进行了大桥的结构行为分析,着重研究了在施工全过程中的结构稳定性问题。 相似文献
53.
54.
Environmental contours are often applied in probabilistic structural reliability analysis to identify extreme environmental conditions that may give rise to extreme loads and responses. They facilitate approximate long term analysis of critical structural responses in situations where computationally heavy and time-consuming response calculations makes full long-term analysis infeasible. The environmental contour method identifies extreme environmental conditions that are expected to give rise to extreme structural response of marine structures. The extreme responses can then be estimated by performing response calculations for environmental conditions along the contours.Response-based analysis is an alternative, where extreme value analysis is performed on the actual response rather than on the environmental conditions. For complex structures, this is often not practical due to computationally heavy response calculations. However, by establishing statistical emulators of the response, using machine learning techniques, one may obtain long time-series of the structural response and use this to estimate extreme responses.In this paper, various contour methods will be compared to response-based estimation of extreme vertical bending moment for a tanker. A response emulator based on Gaussian processes regression with adaptive sampling has been established based on response calculations from a hydrodynamic model. Long time-series of sea-state parameters such as significant wave height and wave period are used to construct N-year environmental contours and the extreme N-year response is estimated from numerical calculations for identified sea states. At the same time, the response emulator is applied on the time series to provide long time-series of structural response, in this case vertical bending moment of a tanker. Extreme value analysis is then performed directly on the responses to estimate the N-year extreme response. The results from either method will then be compared, and it is possible to evaluate the accuracy of the environmental contour method in estimating the response. Moreover, different contour methods will be compared. 相似文献
55.
56.
57.
58.
FPSO (floating, production, storage and offloading) units are widely used in the offshore oil and gas industry. Generally, FPSOs have excellent oil storage capacity owing to their huge oil cargo holds. The volume and distribution of stored oil in the cargo holds influence the strain level of hull girder, especially at critical positions of FPSO. However, strain prediction using structural analysis tools is computationally expensive and time consuming. In this study, a prediction tool based on back-propagation (BP) neural network called GAIFOA-BP is proposed to predict the strain values of concerned positions of an FPSO model under different oil storage conditions. The GAIFOA-BP combines BP model and GAIFOA which is a combination of genetic algorithm (GA) and an improved fruit fly optimization algorithm (IFOA). Results from three benchmark tests show that the GAIFOA-BP model has a remarkable performance. Subsequently, a total of 81 sets of training data and 25 sets of testing data are obtained from experiment using fiber Bragg grating (FBG) sensors installed on the surface of an FPSO model. The numerical results show that the GAIFOA-BP is capable of predicting the strain values with higher accuracy as compared with other BP models. Finally, the reserved GAIFOA-BP model is utilized to predict the strain values under the inputs of a 10-day time series of volume and distribution of stored oil. The predicted strain results are further used to calculate the fatigue consumption of measurement points. 相似文献
59.
60.
为了解决传统匝道控制车流汇入时车辆需要减速至停止,从而造成延误时间过长的问题,提出了一种智能网联车环境下的高速匝道汇入车辆轨迹优化的两阶段优化模型,其中,第1 阶段优化车辆进入匝道口的时序;第2 阶段基于第1 阶段的最优时序,优化车辆轨迹. 根据所构建的模型设计了一种启发式算法优化车辆通过匝道冲突区域的时序,然后结合 GPOPS工具优化车辆的轨迹.为了验证所提出方法的有效性,将所提出的方法应用到20 min 随机到达的车流,进行仿真实验.实验结果表明,与先进先出的方法相比,本文所提出的方法能够使总延误减少59.7%,总油耗减少10.5%,说明该方法能够实现车辆以较高的速度通过匝道冲突区域,有效地减少了车辆汇入延误,同时也节约了油耗. 相似文献