(b)

D2 F2FP F4 F6F5E3 B1 C取截面左侧分析由?Fy?0,F5?2113?FP?0?F3??FP61331FP3A1再由节点法分析A,B节点马上可以求得F1=FP,F1??0.5FP31133?Fx?0,F2?F4?2FP?6FP?13?0?F2??F4?FP取截面右侧,由?MC?0,F2?2d?F4?d?FP?d?0?F4?3FP,F2??2FP再由节点法分析D,E节点马上可以求得FDE=2FP,F3?5FP

3-15 试求图示桁架各指定杆件的内力。 (c)

FPBFPC取图示隔离体，对A点取矩1F12DFPAFP0.5FP1.5FP215a?F1?a?0?F1??FP355再用节点法依次对B,C,D节点进行分析，容易求出F2?MA?0,FP?a?F1?2172FBC=-FP,FCD??FP,F2?FP336

3-16 试作图示组合结构刚架杆件的弯矩图，并求链杆的轴力。 (a)

1取1-1截面左边q A q B A F C G FCXF FCY 2qaD 1E 2qa2qaD FDE1由?MC?0,qa2?FDE?a?2qa?2a?FDE?2qa2再分析节点EFDFFDA不难求得FDED所以弯矩图为12qa21FDA?22qa,FDF??2qa,MFA?qa2212qa2

(b)

D E qa F A 0

1NDE?NAB?qa?NBC,NBF?02q qa12qa8 12qa2B C qa

(c)

C E FP1 D E FP1 FQFA FQFB由对称性,FQFA?FQFB?分析AF区段1FP22A F FP2 B FHG由?MA?01求得FHG?FP221(FP1?FP2)21(FP1?FP2)21(FP1?FP2)22FP2?FGI由节点法,易得FEG?2FP2,FQEC?FQED1FP2a?MI2 1?FP1?FP22?MC?FQEC?2a?(FP1?2FP2)a?MDMH?FQFA?a?FP1a?2FP2a FP1a?2FP2a M图1FP2a21FP2a2

(d)

C qa ?ME F D313?0,qa?2a?FFG??a?0?FFG??2qa422用节点法分析G节点，易得FGE=2qa,FGC?FGD?2qa2考虑DB杆FGDDA 1.25qaB 0.75qa32qaF 20.5qa1.5qa335由?Fx?0?FGD?qa,?FGC?qa?2qa??qa4443Bqa23qa4

32qa4 32qa4