问题 单项选择题

平面体系几何组成分析中的“多余约束”从()来看是多余的。

A.对体系的自由度是否有影响的角度

B.从对体系的计算自由度是否有影响的角度

C.从对体系的受力和变形状态是否有影响的角度

D.从区分静定和超静定两类问题的角度

答案

参考答案:A

解析:

一般把使体系成为几何不变而必需的约束称为必要约束,把必要约束之外的约束称为多余约束。每一个必要约束都使体系减少一个自由度,而多余约束的存在并不减少体系的自由度。体系的自由度数就等于其各组成部分互不连接时总的自由度数减去体系中的必要约束数;体系的计算自由度是体系各组成部分总的自由度数减去总的约束数。当所有约束全部是必要约束时,体系的自由度就等于体系的计算自由度;当体系的约束中包含有多余约束时,体系的自由度就大于体系的计算自由度。由此可见:体系几何不变的必要条件是体系的计算自由度W≤0;体系几何不变的充分条件是体系各组成部分的总自由度数与必要约束数相等。因此,多余约束是从对体系的自由度是否有影响的角度来看的,就体系的受力和变形状态而言,多余约束又是必要的。另外,依据多余约束不能直接区分 静定和超静定结构。 [点评] 本题考察体系约束的概念,要充分理解“必要约束”和“多余约束”以及它们与体系自由度和计算自由度的关系。

阅读理解

GU Zhaodi, 60, had taken water for granted for years. In fact, no one in the beautiful lake city of Wuxi, Jiangsu Province had bothered to spare a thought on water.

But all that changed dramatically overnight last month when taps in the city ran dry. Due to a blue-green algae(藻类) outbreak in Taihu Lake, China’s third largest freshwater lake and the source of drinking water for 4 million Wuxi residents(居民), water from the city’s taps became dark and smelly.

“I can’t believe there is no water for drinking, let alone cooking and washing,”said Gu.

The lake was blanketed with the algae, giving off a strong smell of rotting meat. A stone thrown into it took a long time to sink. Experts said that algae usually boom because of hot water and rich chemicals like phosphor(磷)in the water.

“High temperature and a lack of rain in the past few months helped the growth of algae,”said Zhang Lijun, an official from the State Environment Protection Administration. “However, pollution from human activity should be most blamed for the disaster.”

Taihu Lake is surrounded by many small factories, whichsprang up in the 1980s.Inthe 1990s,many foreign-funded(外资的) companies joined in. The industries prospered(繁荣),Wuxi has grown into one of the wealthiest cities in East China.

But that came at the price of pollution. The lake was treated like a waste dump(垃圾堆),with factories emptying industrial waste and untreated sewage(污水)into it.

Although, after the government's effort, life in the city has gone back to normal, the one-week crisis(危机)rang a bell for the government and local people.“It's time to rebuild the beauty of nature, or our lives will be at risk,”said Gu.

“A safe environment comes first. Economic growth will be nothing without fresh water to drink and clean air to breathe.” said China Daily.

小题1:The following can help the blue green algae grow______

A.high water temperature

B.rich chemicals in the water

C.a lack of rain

D.plentiful water and sunshine小题2:What can we learn from the passage?

A.The one-week disaster has made the local people realize the importance of building a

beautiful city.

B.It is human activity rather than the algae that is to blame for the water pollution.

C.Only when the environment is safe and friendly can man live a better and healthier life.

D.Environment should be taken into consideration first if a city plans to develop its economic.小题3:Which of the following can be the best title of the passage?

A.Who is to blame, man or algae?

B.Water turned off

C.Water pollution troubled Wuxi

D.Algae threatened Taihu Lake小题4:According to the passage ,Taihu Lake________

A.is the third largest lake in China.

B.is surrounded by many foreign-funded factories and companies.

C.used to be a waste dump.

D.suffered from industrial waste,untreated sewage and algae.小题5:The underlined phrase “sprang up” in the sixth paragraph means______

A.develop quickly

B.produce goods quickly

C.come to life

D.grow up

单项选择题