摘要: 為解決黏彈性四參數(shù)地基上兩跨連續(xù)修正Timoshenko梁的橫向振動(dòng)問(wèn)題,將兩跨連續(xù)修正Timoshenko梁與黏彈性四參數(shù)地基進(jìn)行組合,建立新的振動(dòng)控制方程,運(yùn)用回傳射線矩陣法,結(jié)合二分法和黃金分割法,分析了黏彈性四參數(shù)地基上兩跨(等跨、不等跨)連續(xù)修正Timoshenko梁與單跨修正Timoshenko梁自振特性之間的聯(lián)系與區(qū)別。結(jié)果表明:對(duì)于黏彈性四參數(shù)地基上的修正Timoshenko梁,單跨梁的各階自振頻率小于兩跨連續(xù)梁的各階自振頻率,單跨梁的偶數(shù)階自振頻率和衰減系數(shù)與兩等跨連續(xù)梁的奇數(shù)階自振頻率和衰減系數(shù)相同,不等跨兩跨連續(xù)梁奇數(shù)階自振頻率小于兩等跨連續(xù)梁奇數(shù)階自振頻率;兩等跨連續(xù)修正Timoshenko梁的偶數(shù)階振型關(guān)于跨中支座對(duì)稱,奇數(shù)階振型關(guān)于跨中支座反對(duì)稱。
關(guān)鍵詞: 黏彈性四參數(shù)地基; 兩跨連續(xù)修正Timoshenko梁; 回傳射線矩陣法; 橫向振動(dòng); 模態(tài)
中圖分類號(hào): TU471+.2; TU348 " "文獻(xiàn)標(biāo)志碼: A " " "文章編號(hào):: 1004?4523(2025)03?0604?08
DOI:10.16385/j.cnki.issn.1004?4523.2025.03.017
Analysis of transverse free vibration characteristics of two?span continuously modified Timoshenko beams on viscoelastic four?parameter foundation
LIU Wei1, WANG Guobing2
(1.School of Civil Engineering, Lanzhou University of Information Science and Technology, Lanzhou 730300, China;
2.Geotechnical Engineering Research Institute, Xi’an University of Technology, Xi’an 710048, China)
Abstract: In order to solve the transverse vibration problem of two?span continuously modified Timoshenko beam on viscoelastic four?parameter foundation, a new vibration governing equation is established by combining two?span continuously modified Timoshenko beam with viscoelastic four?parameter foundation. By using the echo matrix method, bisection and golden section method, the relation and difference between the natural vibration characteristics of two?span (equal?span, unequal?span) continuously modified Timoshenko beam and single?span modified Timoshenko beam on viscoelastic four?parameter foundation are analyzed. The results show that for the modified Timoshenko beam on the viscoelastic four?parameter foundation, the natural frequency of each order of the single?span beam is less than that of the two?span continuous beam, the even?order natural frequency and attenuation coefficient of the single?span beam are the same as the odd?order natural frequency and attenuation coefficient of the two equal?span continuous beam, and the odd?order natural frequency of the unequal?span two?span continuous beam is less than that of the two?span continuous beam. The even?order mode shapes of two equal?span continuous modified Timoshenko beams are symmetrical with respect to the supports in the middle of the span, and the odd?order modes are antisymmetric with respect to the mid?span.
Keywords: viscoelastic four?parameter foundation;two?span continuously modified Timoshenko beam;return ray matrix method;transverse vibration;mode
地基梁作為眾多工程構(gòu)件的基本模型,被廣泛應(yīng)用于鐵路軌道[1]、公路路面[2]、輸水渠道[3]、隧道管棚[4]、建筑工程中的地下條形基礎(chǔ)[5]等,其動(dòng)力學(xué)特性分析在工程領(lǐng)域及學(xué)術(shù)界備受關(guān)注。目前梁的彎曲振動(dòng)已有多種理論,其中初等梁理論因求解方便、應(yīng)用最廣而成為經(jīng)典,但其對(duì)梁的高階振動(dòng)、高度局部承載、高跨比較大等情況,存在靜力問(wèn)題計(jì)算撓度偏小和動(dòng)力問(wèn)題高估振動(dòng)頻率等不足[6]。Timoshenko梁理論的振動(dòng)控制方程解耦后存在撓度關(guān)于時(shí)間的4階導(dǎo)數(shù)項(xiàng)物理意義不明確、第二頻譜、截面剪切修正系數(shù)多解等問(wèn)題[6?8]。為解決這些問(wèn)題,陳镕等[7]對(duì)經(jīng)典的Timoshenko梁運(yùn)動(dòng)方程進(jìn)行修正,指出考慮梁剪切變形引起的轉(zhuǎn)動(dòng)慣量后,時(shí)間的4階導(dǎo)數(shù)項(xiàng)自然會(huì)消失,解決了經(jīng)典Timoshenko梁理論一個(gè)振型對(duì)應(yīng)兩個(gè)振動(dòng)頻率的問(wèn)題。夏桂云[8]利用固有頻率和臨界頻率的關(guān)系論證了Timoshenko梁產(chǎn)生第二頻譜的理論原因,通過(guò)實(shí)例驗(yàn)證Timoshenko梁第二頻譜的存在,并從理論上預(yù)測(cè)存在第二頻譜現(xiàn)象的其他結(jié)構(gòu)。王家樂(lè)等[9]指出當(dāng)?shù)鼗簽樯盍夯蛘哂?jì)算結(jié)構(gòu)高階自振頻率時(shí)(諸如沖擊等問(wèn)題),應(yīng)采用修正Timoshenko梁理論。修正Timoshenko梁理論考慮了梁的剪切變形及其所引起的轉(zhuǎn)動(dòng)慣量的影響,在深梁和高頻振動(dòng)特性的分析方面優(yōu)于經(jīng)典Timoshenko梁理論?;谛拚齌imoshenko梁理論,余云燕等[10]運(yùn)用回傳射線矩陣法求解了三種經(jīng)典邊界條件下變截面修正Timoshenko梁的自振頻率。吳曉等[11]應(yīng)用Timoshenko梁修正理論求解了泡沫鋁合金梁的自振頻率表達(dá)式及其在簡(jiǎn)諧荷載作用下強(qiáng)迫振動(dòng)的解析解。LI等[12]推導(dǎo)出分?jǐn)?shù)階標(biāo)準(zhǔn)固體黏彈性地基上修正Timoshenko梁的振動(dòng)控制方程,分析土體參數(shù)對(duì)修正Timoshenko梁波速、自振頻率的影響。但以上研究以單跨梁為主。
對(duì)兩跨連續(xù)梁自振特性的研究,多以彈性地基模型和簡(jiǎn)單梁理論為主。鄭仰坤等[13]利用MIDAS有限元分析軟件和DASP設(shè)備分別對(duì)連續(xù)梁進(jìn)行模態(tài)分析,得出不同跨徑比下連續(xù)梁的前3階振型及其頻率。張盼等[14]采集連續(xù)梁振動(dòng)視頻并將其轉(zhuǎn)化為數(shù)字圖像,經(jīng)MATLAB程序讀取整個(gè)振動(dòng)過(guò)程梁邊緣的數(shù)據(jù),通過(guò)DASP系統(tǒng)對(duì)觀測(cè)數(shù)據(jù)進(jìn)行模態(tài)分析,得到兩跨連續(xù)梁的前2階頻率和振型。吳晶等[15]用DASP軟件對(duì)兩跨連續(xù)梁進(jìn)行試驗(yàn)?zāi)B(tài)分析,得出該連續(xù)梁的前2階頻率及其阻尼比。周盛林等[16]基于Bernoulli?Euler梁理論,采用模態(tài)分析法獲得雙跨梁的頻率方程,由此求解出雙跨梁的自振頻率。LI等[17]采用模態(tài)展開(kāi)法研究了兩跨梁的自由振動(dòng)特性,對(duì)中間支座錯(cuò)位的兩跨梁振動(dòng)特性進(jìn)行分析。由以上研究可以看出,對(duì)兩跨連續(xù)梁的研究采用有限元分析和試驗(yàn)分析的居多。余云燕等[18]求解了黏彈性Pasternak地基上Timoshenko梁在不同約束條件下單跨及兩跨連續(xù)地基梁的自振頻率、衰減系數(shù)和模態(tài),但未考慮地基水平摩阻的影響,也未分析跨度比對(duì)黏彈性四參數(shù)地基上兩跨連續(xù)修正Timoshenko梁的影響。
回傳射線矩陣法(MRRM)不僅能精確計(jì)算復(fù)雜桿系結(jié)構(gòu)的初期瞬態(tài)響應(yīng),而且能準(zhǔn)確計(jì)算結(jié)構(gòu)的自振頻率和振型,尤其在高階自振頻率和振型的計(jì)算上更有優(yōu)勢(shì)。故本文基于修正Timoshenko理論,建立黏彈性四參數(shù)地基上修正Timoshenko梁的橫向振動(dòng)控制方程,運(yùn)用回傳射線矩陣法解耦后,結(jié)合二分法和黃金分割法,分析黏彈性四參數(shù)地基上兩跨(等跨、不等跨)連續(xù)修正Timoshenko梁與單跨修正Timoshenko梁自振特性之間的聯(lián)系與區(qū)別。
1 振動(dòng)控制方程及方程的解
黏彈性四參數(shù)地基上兩跨連續(xù)修正Timoshenko梁的力學(xué)模型如圖1(a)所示,土體與修正Timoshenko梁的相互作用采用黏彈性Pasternak地基模型,并考慮地基水平摩阻的影響,建立整體坐標(biāo)系(x,v),將地基梁劃分為2個(gè)單元3個(gè)節(jié)點(diǎn),節(jié)點(diǎn)的編號(hào)如圖1(a)所示,對(duì)地基梁的每個(gè)單元JK引入2個(gè)對(duì)偶局部坐標(biāo)系〖(x,y)〗^JK和〖(x,y)〗^KJ,原點(diǎn)分別在節(jié)點(diǎn)J和節(jié)點(diǎn)K,如圖1(b)所示。
對(duì)圖1(a)取微段隔離體,其受力情況如圖2所示。圖中,ys表示梁中心軸至梁頂端的距離。黏彈性地基梁在彎曲變形時(shí),梁一般繞其中性軸轉(zhuǎn)動(dòng),造成梁底相對(duì)于地基有一定的水平變形,如果地基與梁間的接觸面比較粗糙,將對(duì)梁有水平約束作用。設(shè)梁底寬為w,地基與梁底的水平剪切系數(shù)為τ,梁中性軸距底邊為y_x,梁的截面轉(zhuǎn)角為?,梁底相對(duì)滑動(dòng)為y_x ?。
在如圖3所示的極坐標(biāo)ρ?o?θ下,令ρ=√(ω ?_n^2+δ_n^2 ),θ=arctan(δ_n/ω ?_n),由dρ和dθ組成的每個(gè)局域運(yùn)用二分法和黃金搜索法進(jìn)行迭代求解,即用二分法搜索出行列式實(shí)部和虛部變號(hào)的點(diǎn),再用黃金分割法搜索行列式模的極小值點(diǎn),adj|I-R(ω)|的每一個(gè)非零列可看作黏彈性地基梁自由振動(dòng)時(shí)非零出射波幅向量d(ω)。當(dāng)行列式的模小于預(yù)先給定的誤差時(shí),停止迭代,則結(jié)構(gòu)的自振頻率ω ?_n=ρcosθ,衰減系數(shù)δ_n=ρsinθ,可通過(guò)式(20)求得a,則結(jié)構(gòu)中任意點(diǎn)處的橫向位移可由式(7)求出,將各點(diǎn)位移歸一化后即可得到結(jié)構(gòu)的振型曲線。實(shí)際計(jì)算中,利用MATLAB語(yǔ)言編制相關(guān)程序。
2 算例分析
黏彈性四參數(shù)地基上兩跨連續(xù)修正Timoshenko梁的力學(xué)計(jì)算模型如圖1(a)所示,修正Timoshenko梁的計(jì)算參數(shù)如表1所示,土體的計(jì)算參數(shù)參考文獻(xiàn)[19]中的數(shù)值,如表2所示。
黏彈性四參數(shù)地基上兩跨連續(xù)修正Timoshenko梁的各項(xiàng)物理參數(shù)采用表1和2中的數(shù)值,7種工況下各跨梁的長(zhǎng)度計(jì)算參數(shù)如表3所示。其中,工況1為單跨梁,工況2~6為不等跨兩跨連續(xù)地基梁,工況7為兩等跨連續(xù)地基梁。根據(jù)回傳射線矩陣法求解各工況下結(jié)構(gòu)的前8階自振頻率和衰減系數(shù)的數(shù)值解如表4所示。
由表4可知,工況1的各階自振頻率明顯小于其他工況下的各階自振頻率,表明黏彈性四參數(shù)地基上單跨修正Timoshenko梁的各階自振頻率小于兩跨連續(xù)修正Timoshenko梁的各階自振頻率,但其隨階數(shù)增長(zhǎng)的速度明顯大于兩跨連續(xù)修正Timoshenko梁。工況1的偶數(shù)階自振頻率和衰減系數(shù)與工況7的奇數(shù)階自振頻率和衰減系數(shù)相同,表明黏彈性四參數(shù)地基上單跨修正Timoshenko梁的偶數(shù)階自振頻率和衰減系數(shù)與兩等跨連續(xù)修正Timoshenko梁的奇數(shù)階自振頻率和衰減系數(shù)相同,這一結(jié)果與文獻(xiàn)[18]的結(jié)論一致。工況2~6的奇數(shù)階自振頻率小于工況7的奇數(shù)階自振頻率,其偶數(shù)階自振頻率大于工況7的偶數(shù)階自振頻率,表明黏彈性四參數(shù)地基上不等跨兩跨連續(xù)修正Timoshenko梁奇數(shù)階自振頻率小于兩等跨連續(xù)修正Timoshenko梁奇數(shù)階自振頻率,但其偶數(shù)階自振頻率大于兩等跨連續(xù)修正Timoshenko梁偶數(shù)階自振頻率。針對(duì)工況2~6,第1階自振頻率隨著跨度比的增大而增大,衰減系數(shù)隨著跨度比的增大而減小,從第2階開(kāi)始,隨著跨度比的增大,自振頻率和衰減系數(shù)的變化并沒(méi)有統(tǒng)一的規(guī)律,但在不同跨度比時(shí)結(jié)構(gòu)的自振頻率和衰減系數(shù)差異十分明顯,表明跨度比對(duì)黏彈性四參數(shù)地基上兩跨連續(xù)修正Timoshenko梁自振特性的影響較大。
圖4為各工況下結(jié)構(gòu)的前8階模態(tài)。由圖4可知,工況3、工況5和工況7的第5階自振頻率和衰減系數(shù)完全相同,工況4和工況7的第3階及第7階自振頻率和衰減系數(shù)也完全相同,但其對(duì)應(yīng)的振型曲線未完全重合,表明即使跨度比變化時(shí)結(jié)構(gòu)具有相同的自振頻率和衰減系數(shù),但振型曲線受跨度比的影響不完全相同。針對(duì)工況7的結(jié)構(gòu)振型曲線,其偶數(shù)階關(guān)于跨中支座對(duì)稱,其奇數(shù)階關(guān)于跨中支座反對(duì)稱,工況1的奇數(shù)階振型關(guān)于跨中支座對(duì)稱,偶數(shù)階關(guān)于跨中支座反對(duì)稱,而工況2~6的結(jié)構(gòu)振型曲線變化較為復(fù)雜,并沒(méi)有統(tǒng)一的規(guī)律,表明跨度比對(duì)黏彈性四參數(shù)地基上兩跨連續(xù)修正Timoshenko梁的模態(tài)影響較大。同時(shí),黏彈性四參數(shù)地基上兩等跨連續(xù)梁的振型曲線與單跨梁的振型曲線有本質(zhì)的區(qū)別。
3 結(jié) "論
將回傳射線矩陣法推廣至黏彈性四參數(shù)地基上兩跨連續(xù)修正Timoshenko梁的振動(dòng)分析中,結(jié)合二分法和黃金分割法,分析了黏彈性四參數(shù)地基上兩跨(等跨、不等跨)連續(xù)修正Timoshenko梁與單跨修正Timoshenko梁自振特性之間的聯(lián)系與區(qū)別。得出以下結(jié)論:
(1)對(duì)于黏彈性四參數(shù)地基上修正Timoshenko梁,單跨梁的各階自振頻率小于兩跨連續(xù)梁的各階自振頻率,但其隨階數(shù)增長(zhǎng)的速度明顯大于兩跨連續(xù)梁;單跨梁的偶數(shù)階自振頻率和衰減系數(shù)與兩等跨連續(xù)梁的奇數(shù)階自振頻率和衰減系數(shù)相同。
(2)黏彈性四參數(shù)地基上不等跨兩跨連續(xù)修正Timoshenko梁奇數(shù)階自振頻率小于兩等跨連續(xù)修正Timoshenko梁奇數(shù)階自振頻率,但其偶數(shù)階自振頻率大于兩等跨連續(xù)修正Timoshenko梁偶數(shù)階自振頻率。
(3)黏彈性四參數(shù)地基上,隨著跨度比的增大,不等跨兩跨連續(xù)修正Timoshenko梁的高階自振頻率和衰減系數(shù)變化并沒(méi)有統(tǒng)一的規(guī)律,但在不同跨度比時(shí)結(jié)構(gòu)的自振頻率和衰減系數(shù)差異十分明顯。
(4)雖然跨度比變化時(shí)結(jié)構(gòu)具有相同的自振頻率和衰減系數(shù),但是振型曲線受跨度比的影響并非完全重合。
(5)黏彈性四參數(shù)地基上,兩等跨連續(xù)修正Timoshenko梁的偶數(shù)階振型關(guān)于跨中支座對(duì)稱,其奇數(shù)階振型關(guān)于跨中支座反對(duì)稱,單跨修正Timoshenko梁的奇數(shù)階振型關(guān)于跨中支座對(duì)稱,偶數(shù)階振型關(guān)于跨中支座反對(duì)稱,隨著跨度比的變化,不等跨兩跨連續(xù)修正Timoshenko梁的振型曲線變化較為復(fù)雜,并沒(méi)有統(tǒng)一的規(guī)律。
(6)本文計(jì)算方法可求解任意邊界條件下及復(fù)雜參數(shù)結(jié)構(gòu)的橫向自振特性,從而為工程實(shí)踐提供理論基礎(chǔ)。
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通信作者: 柳 "偉(1990―),男,碩士,講師。E?mail: 279339776@qq.com