Xc nh trng tm ca mt ct ngang

Bi 5: Dm n gin chu lc tp trung, lc phn b u v moment, tit din hnh ch I khng i xng.

1. Xc nh trng tm ca mt ct ngang

Gi C l trng tm ca mt ct ngang th

Xc= Sy/A,Yc = Sx/A

- i vi mt ct c 2 trc i xng th trng tm C chnh l giao cu 2 trc i xng

- i vi hnh ch c 1 trc i xng th trng tm s nm trn trc v ta ch cn i tm 1 ta cn li

- i vi cc hnh phc tp th ta c th chia hnh ra thnh nhiu hnh c bit khc nhau v t tm trng tm ca hnh

V d tm trng tm ca hnh bn vi cc kch thc c th cho ( n v mm)-y l 1 hnh c 1 trc i xng, v vy trng tm ca hnh chc chn s nm trn trc i xng y.

-Chn h trc Oxy nh hnh v vi Oy trng vi trc i xng

Ta ch cn i xc nh tung ca trng tm C

-Chia mt ct thnh 3 hnh ch nht nht nh hnh v.k hiu hnh 1, hnh 2 v hnh 3 c cc kch thc nh hnh v : cc kch thc c tnh bng n v mmXt hnh 1: ta trng tm ca hnh 1 l C1(0,75)

Xt hnh 2: ta trng tm ca hnh 2 l O(0,0)

Xt hnh 3: ta trng tm ca hnh 3 l C3(0,-75)

Tung ca trng tm mt ct c xc nh bng cng thc

Yc =

Vy ta trng tm cua mt ct bn l C(0,-18.75)

`2. xc nh v v biu ng sut- Xc nh momen qun tnh IxDa vo cng thc tnh Ix ca HCN v cng thc chuyn trc song song ta tnh Ix ca mt

ct nh sau

Ix = Ix1+Ix2 +Ix3 =

- xc nh ng sut php cc i: i cc gi tr kch thc t mm sang cm th ta tnh c cc gi tr nh sau

=

3 . xc nh ng sut tip v v biu ng sut tipCng thc chung tnh ng sut tip

EMBED Equation.KSEE3 \* MERGEFORMAT t =

Trong cc i lng l nh sau:

Qy: l lc ct

Sxc: l momen tnh ca phn gch cho di trc trung ha

Ix: l momen qun tnh ca mt ct i vi rc x

bc : l b rng ca thn mt ct

- tnh momen tnh ca phn gch cho i vi trc trung ha

Phn gch cho cch trc trung ha 1 on l y

Sxc = 200.50.56,75 + 50. (31,75 - y). (31,75 - y) /2

=567500 + 25. (31,75 - y)2Nhn vo bieur thc ta thy: biu ng sut ip s l 1 ng cong bc 2

Gi tr ng sut ln nht l ti truc trung ha khi y = 0

Gi tr ng sut nh nht l tai im tip xuc gia than v phn cnh tc l y= 31,25

i vi phn trn trc trung ha cch trc trung ha 1 on l y

Sxc = 100.50. 93,25 + 50.(68,25 - y). (68,25 - y) /2

= 466250 + 25.(68,25 - y)2

Ta c cc biu ng sut nh sau:

_1413375561.unknown

_1234567895.unknown

_1234567897.unknown

_1413375560.unknown

_1234567896.unknown

_1234567890.unknown