r...: w l (3‘ t .33. 9a . .0 ‘ O An Analytical Comparison of Military and State Highway Bridges A Thesis Sabmitted to The Faculty of MICHIGAN STATE COLLEGE of AGRICULTURE AND APPLIED SCIENCE R. E. LCffel hug-M Candidate for the Degree of Bachelor of Science June 1938 ACKNOI‘ELECDG IIZEI‘ET I wish to take this Opportunity to thank Professor C. L. Allen for his valuable aid and advice, and also the Michigan State College Coast Artillery Officers and the Michigan State Highway Department Oficials for their splendid c00peration. R. Ernest Leffel 1.1807. - I. Introduction This paper is designed to cover the fundamental prin- ciples involved in the design of two types of bridges by two different sets of specifications. The two types of bridges to be designed will be stringer type bridges sup- ported by trestles and truss type bridges. With military bridges as with most military structures, the really important element is that concerned with time of construction. The structure usually is built for a def- inite purpose which gives rise to comparatively short per— iod of life. The question of materials and their ease of procurement enters largely into the design. For these rea- sons military bridges are usually built without extremely detailed plans but with dependence placed upon experience and practice supplemented by approximate charts and tables. The factor of safety is allowed to take care of unknown and indeterminate flaws in the structure, to provide for load- ings larger than foreseen and for defects in design and in construction. In civil practice, economy of labor and material and the life of the structures are of first importance. Their bridg s are designed by carefully following the laws of mechanics and strength of materials Which calls for mater- ials that are constantly held to a high and uniform stand- ard. Statutes as to maximum loading of bridges allows for the use of smaller factors of safety. II. Design of va‘irwger Bridge. A. 5:1 MHH’ar/Lj Spacit‘ncai'iohs 910' I. leznt~ l I I Live Load 3 30 Ton Tank [ I V ] Impac+ = 50% or” Ln've 5pan=lé ff. Stresses Material 3 Douglas Fl'l" .16“ Deep Clear Wid+h Road = IO‘—oo“ 2. Tim bars For Live, Load Maximum bending momzrnL occurs under” “the Following loading. IST. . -T I 33 '6 J r d . 8'~o" _L 6-0" " ’f‘ 2‘ 9—2. 3M2,” BMIe—steg— - |5x435~ =0 BI: 12.89 Tons I21: 30 “12.89 = l7.” Tons Max. Ex+eknal Mamet/1+ occurs at a. Ma=12.m(e742)= Essa H. tons Ma: 177,200’1" Impaci- =_._KE_,_E_D_Q. Tofal M Zfléégoo'fl MdY. ln+zknal Resisting Moment M: 5 EL 5 = [”200 9°70” é: ébhz M= Laoo x 3’; bh” ‘ 515100 b =Qe535’ooma b: 62.3 {moi-n65 Timbers are availabte Rn BuwiA-Hns '- Use gig-“*3: 7.7? 0% c9 Timber/J (a; 54/40/615 m” ”"0; nag/#1) 3. Fr‘om M'I‘H‘arfl experience} incoming 35 arbitrarilfl made 5'inohe6 «Wick. For a one lane bridge ‘Hne wicJ‘Hq = H tee-t. S+ahdard gum—cl rafls are ewe". 4‘. TKYHbZFS ‘For Dead} Loac‘ Wit of Doug\as Fir- =34 #/eu.t+. W+. a? guard kails = leexTEngx34 $2.72“- WJr. OP Hoowin/g = lexuxgx34 eye/:80 Wt o? j’rrihfler‘s = XX‘GX‘FiXJFEZX3QT-3J‘5é’o Total Dead WlL; 5/6/414- 0r 97%}- : 4‘3.25#/Hh. H. Dead Load noelmzrnl at a = Ma Ma = (5. aoe)(e§—) ~(4 «3.2 we g—flé) Ma =12,9é»éftlbs. EquaJrSn/Q Ex+ernal 1'0 Eefiishnfi Momen‘f +0 —Pina| +he acidH’ional wiJHq nee” @55arfl :— M 3(1—QZQAXbXIeF =12fléo ma b:- e" w: 3-1.7: L3" 010 wid'H’I n0+ all/26133] +abien car-e 0?. Thin in negfleOLGC‘ bfl mHH’ak/Ij engineerj since H4631 anw 50°70 tor impac/Jr. 5 Shear Maximum under the ‘FoUow'nhfl hadn't/lg? I51: |5T _ “5' 4.5' w J r 1 I. .1 92 =[I5 + 15(1,'—'é'—5)]eooo -= 51,563“— Maximum shear 2 U :32 + .5 81+ 12; Deaal Load 51,5“; +25,7sl+ 3,306 0‘ l\ \l .. .. = agesoit on each sin nflzr ; 10,0801: UnhL Vex/‘Hca\ shear :yojogongxl—é: qq 1%.. Un'nL horiZon+al shear=92~x7q = \\8.5*F/e~ fiate since mot/o- ]: aHoooec‘ in shear _§or mileanM brflc‘fqes. 6. TresHes AesuMe 8“1(5" pas-t5 avai‘able Then,aecona|'mg +0 military standards, 2.4 teewL is +he may. height. Sate unhL working s’rr‘ees as, 5 :- S(’__l-:_) ___._ ' 200(._ZQKIZ. I 60d ! cox? 5.: 420 *Vn“ P3+o¥od bearing Powek Per eoiumn P= e.A =eeoxexs=3onzo Pounae Cohxmvw load :\m‘9ae+ +L'we +Ueacl .. u :qu, shear=80,650* No. ot co|umn5 neeclecl = M226 30,720 Use 3 coiumns 3n each beh‘l’. 7.77%; a 5+eel Siringer‘ Maximum \enqux is changed 15mm \6' to 22' withoufi losing oii’t‘tnces. Max. mom em‘ occurs under We 90!— lowing \oad‘1ng'.— '5T 15T V 9.675 4: 4.5' f‘ 1625' 1 L 6' g V 11.0' g]! 110' 0,0 1e. Ea __ 12.125 +1625 _—,- E1” 15( 22 13.4e T' M0l :- 13.4%: x 9.875 21623 «0+. Tons Ma = 265,500 F1: lbs. Imp. == V52, ‘1 00 395, 7:0 “9*. \b0. X12: 4,764,400 1“. lbs. Equaatih/gv ex+ernm anal reeie+ing mom- en+s:— . .. - l- I“ 4,784/100 m. lbst’Sc— 16000 C LL;- 26m C From mHH'ar‘fq tables 4‘18"?)54‘: iii-Beams or 6-l5"42.# 135001045 would be SOHS‘Faciorfl Wt added +0 2,155 X4x11=244zfi= Ma = 14.60 m. 875 x 2000 =2qu 300 H1199. Ma2289,800 x12: 3,467/600 Impae+ = l,733,800 5,2011400 in. lbs. = 5201,400 ,. Jlé,000 — 325 4'48" 554* I“ Beams 5+3H are 04K» 0H 8. B14 5+a+e HighWaM SPeCiPn'cqi'ions—a l. Given:— ' H’ZO loading N0 impae’r on Wooden members \6"dee|o, Douglas Fir IO' clear roadwafl 16' Span 2. Maximum bending Won/10m} occurs when “’10 real" axle ‘oad 01“ 320004‘ is on fine midd|e 0P ’rhe spama). 32000 A 1 8‘ A: 8’ 1 10000 \6000 Ma=‘6000x8=128,000 F+. lbs. which ac- Cor‘ding +0 speCit‘icqtions has +0 be mul- i'iplied bg +he eoeFfi'eizrnL l.‘7=218,ooo Equal’ing €X+eflnal anal 1V1+ermal Morn- ¢n+5:v I z = 2': H1“ 5 var-{as From “720*/o-- +0 1750*49» M: 218,000 x12 = s EI- according +0 +he condifion 0P Jrhe wood. To be 561192 use 720*43-1 2155,000x12,=‘Iz():c%;>‘b1(TE‘L = 05" 85" he'mfl Jrhe Maximum w1d+h nec- essar/Lt under +he worsf conditions 0? weather and ’rreanmemL , II 51L1’1n’36F‘S 5" wide would be e- nough. 2. Flooring :- Mlnlmum ll’nclLlneSS :— Snow-(l; a? the clisl'ance l02+w¢¢n 5+rinrqer‘s Minimum wiolHn 1‘ 10" Using mlh. lonflll’ucllnal $loowlhfl o‘l1 2")“0' planks over‘ +he ‘l‘r‘anSVet‘se Flooring ‘l’he loaJ ls 5Fr‘eacl over 3 Planks or‘ 3 XIU = 30" A SSUMlng l2”8" slr‘ingers anal 0 l0 ‘Fee‘l Clear roaéwafi qu clis‘l’ance belween Slr‘inrqers will be WEE—=93" According +0 lhc Michigan Sled-e Lauus +he +ire Can he loacl ecl a malximum 01? 7001* Per inch wicl‘l’h cl: lrcacl. 50 ton YYlaxlmum shear use ’rhe ‘Followin/Q loading .‘v 700 ”71;... 'm. 3.3" 1‘ 9.. 22 Maximum shear ='7GOX—'7:x3.3:|1551* Allowable uni-l" shear alress is wow/0» 7/: 1;“; 0': Tile—3%‘a =38” Maximum momenl=éfwl1=JfX700*§§-Q M 3 “I52 in. lb.= 720 Xi; x 30 x 0” 042.2104 air-.514" Using 'l'he minimum +hiCI¢ness «Cor- ’l’mns- verse planking 0,0 3") l’he +o+a1 mob ness 0? tloor =- 5" U06 s—tanclarol 6"X6" gLAOkol rolls. 3. Dental Weighlz-fl Wl'. 04’ Douglas Fir -'- 32t/cuflFl’. Guavd rails .-—(2)(1t)(—,%)(-,—g-_)(32) = 256. *’ Flooring, :- (1e)(11)(-,—%) (32) z 2340*: Slringer‘s :-(1l)(T%-_)(-',—£;i~)(lb)(32)= 50| 0* 7,6 12*” To’ral Deacl 0qu = loacl : “in = 475.8 ”41. Ma. The uni harm cleaol The dead l000l momenl 01+ 01 '15 Ma - 1%3—xfi'xlfl =51J8oo ga- Equal-i143} Exler‘nal ¢lnlrewnal momenl'sz- b320-2" making a ‘l’ol'al a? 20.2‘l'8’5’: 105.2." necessarfl. l3 slang- ers wil—ln H34" will he setticlen’r. Ll. Shear:— Wil'h H-ZO loading , maximum shear ‘l’dlaes place when +he 32.000‘taxle '15 an ln— Hnilessl mal 50040 '4, 3300 . —l l 10' J 2. 7132. Th e Z ¢X+ rat at M mg er's add 2(1gi)cil§>(lb)(32) or‘ 912'”: ‘l‘o “the +o+al deacl wl’. Max. V = RZ=3ZOOO+ fix 8000+é“(°llz+ 7,612) v: 37,202 1* Unltl' var-Heal Shear» Unil- horizonl’al shear" =%X 22.4 = 33.é#/0" '13 égllz/u“ 50 fine __ 37.26;: _. “ 13XSXI6 ”224190" Allowable unll’ shear sfrin/Qer‘s are saf’e {Tor shear: 5. Trestle: The maximum unsupported lengl—h a? wood- en Columns l5 30 lime: +l’l2 leasl' cli— mension or‘ 20' For an 6"r5“ P05+' s.=s[I-é—(2'-a)]~=~§—- 37 24.2 ‘ ~'—2_——"‘ : 65[ ~l—(‘i' l, oo,ooo.g 8 x b 3 l 3 lZV'ache‘z—s— b==12”7q" 2'8"x0" columns would be 3u9?ien+ For 01 beighl 04‘ 20 43+. bun‘ “”16 size of +h2 columns would have +0 be +0 increased 10"x10“ {-01, q 24' height Howevek ac- cording +0 specil3ieal'1'ons ben‘ls should HO‘l' l’lOVZ l¢€7$ 4'th Ll Pofi‘l'f) COM/h. 6.1—7‘” 0 6+eel e‘l'v‘ihger W‘l'. 0'? floor -‘- 2340 Wl- 0P fluakcl rail =1 259 %§X'2600=2934 A55ume 3 I-Beams weirflhz— 3666 6000* W‘l': 25—3—1: 300#/lin.l2+. The may, moment occurs when Hee ln‘ve load OP 3.20001t 1's 01+ the midpoin+ of lhe span. lmPae‘l" is 100070. 32000* 32000“ “ a‘ :l I 3Cot/lin, {:‘l’. L 22‘ l P 1 ‘ R. R2 12.: 64000 x.b'+ 54300 x11 =35’,:—300‘4'r Ma = 35300x1l—300x léxn = 364,100 H. lbs. 364)loo x \Q. =M: s lazilaooo 35E. ' I E. ‘1'— 242.73 Fhom Carnegie Steel Hancl book +01b|es 3 $+PinflCP5 l8"deep weigh'nxffl 54.7‘t/H Wl ‘l'Ll :5 188.4, 0" accumulated’:265.2 W’l': 54,7 x22 K3: 3e101‘01» less than I+he 0:55 umecl 01+. Max. shear :V=32000+~2§ix8000+lb4.|x1| V: 36,715“— Shear‘ is “halzen onl/q log “A? web. 6 I5 ’0: 3;?)th “a =I147q‘%" 3-\5" 545,117+”. I-Beams are salisfaclor‘g. III Design of Wooden Truss Bridge 141.5% Mil1l’ar/14 5Pecitiealions l. Given 2-- The truss is 'l‘o be designed “l0 Corr/g a 30 +014 fahlz. concen—l't'alecl inl'o ‘l’uJo l5 ton loaclS 4.5Fee’l' apartlon/gi‘lmlznall/g. span '-— 50 Fir Tfipe :—- Howe +rwss , Aeck. 1-0001: ‘lransmi'l’l’ecl Dial/14 0+- Panel Po'1n+s, The Panels will be l0 teel' lonrg. Roadwa/lfi :~ [0 {leel‘ in clear, Floorbeams .'~ 13 Feel loh/q. Wheels 0? ‘l'QHla. are 64" aPQr‘l'. 2. Floor Beam For maximum benclin/q momern‘ lhe 'Follqu- in/g clifi‘lr-ibuflon 0? locals shoulcl he used l 000* l5000¢ [“ 56“ : ‘ 64" t i 240:. l . ‘ l l , 1 12. i2, 22. x o 4- 2M2 = 0 22: 56 15' 00 12.0 X\500_C1 ‘ 12. x (2. 2.1:: 15,320 52.5 11,680 Max. momeni- 7— “,680 1‘56 =554,000 +50% Impaci’ M = < = Ll x1085xl.25:+5420‘* L,L2 = —-6W +an «2 —é¥l0&5xl.25:-8|30¢ L\U1 =2W 52c: °< = Z XlOB’JX L6 = —l- 3470* L1U1=~W = - “>85“ LQU3: 0 03 La:— 0 L9_ L3=—c, w +ano< =—exnooS‘Xl.25=-al30* U1U3 ‘31-6 w hm o< = +6 H085 Y ('25:+8\3O# Tolal dead load alresses :— Lou, :— +8l00+ c.6140 mtg-£4011" LOL. = *6325’5420 :——H,vq5‘* L, u, :— -2530-3255 =—5,'785** U. U; = + 54.20 + 63515 =-- + ””4543: L.L;L =-‘?,4qO-8,|30 == —|7,620# L4); = +4050 +3470 : +7157“): Ln03=‘|085 +0 __—_\)085# Ug,L3 -.- o Liug : o Lngf—"8I30v-Ci‘7‘90 =-|'7,62.o**‘ U1032+8130+94QO=+I7,620* Live Sl'r‘essvzs :—~ For maximum loading on one lrass Consider one wheel on +Me edge of H12. road. Léz: 64" r: 66'. A] l , , 144 2. at E, = W x 45000 = 20, 825 ”’- which :5 +442. max. Peacllon ‘Fr‘om each axle. Then 'For‘a ‘l‘en Poo-l spam +146 may. loading would be 2—20825“ loads 4.5 Pr. a« pawl or‘ For max. slresses, I load 0? 32,280# al— one, Panel Pl. anal a loacl 0? 4,3707”; all an adiaczn+ panel Pl Case. a :— 32,280“: al U, at 9370 0+ 02. R2: “3,2004”F 2, = 65450“: LOU,: 50,25001hr LoleL,L1339,3oo* 0.1.1: I330” U1L2=830‘t 021.3 = 03L4= ()4 Lszlefia‘oo‘" 0. U2 = 38,250 4* U1U3 3L3L4=25,5004* u3 U4 = L4 L5 : lZfl’SOit U3L3 sud 1.4: l0,200# Case b :--32,280*l‘a+ U. ft 4,370“ all Uo 22:- 6,460“: 2.: 25,820# LOU, 41,300": La L, = L, Ll= 32,240”: U,L2=U2_L3 —.:.U3L.4-_-qufir:l0)33c)1w U1L2=U3L32 U4 L4 2:: 6,46011. UZU3= L3L4= 10,..2044‘ U3 U4 =1.a L5 = 8,060“? Case. C =— 32,280*a+ U7. 4i 9,3‘70#a+ U. 21:- I4,7qofi 9,:- 26,560fi L, 0, =43 0001* LOL. =U,U.L:’33,6oo# U| L. = |7,4qo* L.U,_=27,Q40‘* LL; =L2 L3:— 55’4401’; U1L3=U3 L4=U4L5==23,620* U3L3=U4L4= |4,790* U193 ‘5 L3 L4: 36,"lbo* U3 ua = 1.4 L5 = la,460“‘ Case 0 :- 32,250*a+ 02 ¢ q,370‘*a+ U3 $22: I8,530*l: R.=23,|20* LOU,: L,U?_= 37,000“ LO L,=U,u2: 2.8,Cloo*t U.L,:23,u20* L,LZ=L.,‘_L3= 57,8001F U2L3= I4,660* U203=L3L4 = 46,320“: U3L3 :9,\60 UgL4‘3U4L5 = 2Q,eoo‘* U4 L4 = l8,530* U3 U4 5 L4 L5: 2.3) lGOfi: Max. slr‘ess 3n H16 members is ad 430l— lowsz— (Max. live +max. cleacl 1‘ impacl.) L. U, = 50, 300 + 15,040+ 25,150 =- cl0,~4<1|01t Com ,0. LOL, = 39,300+ ”)745 +l‘h650 :- 70,e '15 sallsl’aclor‘g. Lower char-A members in +eh5lon wlll ler-B'xlc." +;mber3 buill—(AP. 7. Design 0‘? splices. The compression splices require no {Tig— UY‘ln/q. The balls 0? llne Vleces loinecl musl— be care‘PUll’g squared so ‘l'lna‘l' +l1e sl'r‘ess will be ‘lr'ansmll’reo axial- llfi- The size 0‘? «H50 Fiskplales neecl onlgbfsupljiclenl l’o Preserve l’lfle P05- 1l‘10n anal allignmenl ow“ Hm bulls. For 'l'enslon 5Pl1'ees +l/1e maximum +en’ sion will be 104,320” or 52,160* ‘m one plece 8"x16“. Using an o“x10" Spacer and a 4“x \6" llislnpla‘l‘e ou—l'sicle +lne spllce gives +han enough area. From milll'awa lobes on bolls:- (or l-le" bolls +l1e bearing “value is 5000* and +lne Value as a beam is 4300*? no. ol: bolls=§4%—g—%~9 or 12 are required on eaCH 81le Ol3 “H12 loinl’. For slmplicfl'g‘ malee compression anal +en5ion {ishplqles +l1e same. 5.Desig 0F Lal’er‘al {ll Swag Bracing Milllar‘g Speelliaallons use 6! Wincl loacl 0’? 35 lbsper sa.Fl. ona ear-Face equal +0 Hne side area Ol“ +l1e lwo ‘l-r'usses. COHSlClQF' Hoe loacl evehlfl divided be+ween llne apper‘ anal lower panel poinls 0+‘ one lr'ass. Load: on chords : 50x5.33x55 = C1,340 and posls = 8x533x35 = 1,4614 diagonals = 5x13x5.33x35=12,120 loacl on balk chords: 2.2,qeo WinA loacl on Floor 5% s+em ls car” Pl ‘ZCl b3 ‘llqe Mpper panel Pail/1+5. 50 x2. x2x35=~1ooo¢ load on lower panel pl. 2-g‘?cl"0=22<2 flooring is 10': Minimum lon/Qiludinal lzlooring is 2“1(10'.' Using longilaclinal flooring over llne 'lr‘olvisver‘se (load carrying) JFlooring, +l1e loacl is spreacl over 3 plank or 30? Assuming 8-8"wioll'ln slr'in/qers fill a Poacl‘ wag U)‘ in Jrlne clear, "H’le Aislance be, lween sl'r‘inger's will be le127-8x8 or 51.7". Michigan Slale Law allows a maxiYnum 0-? 700* per lHCl’I wiol+l1 oi: ‘lv‘eaA. The max loading is as Follows:— l 70" 1w/|in. iml L Cl}? in. J F W . 2. 22 Ma {imam shear =7oo x.5 x617 :33‘15 4* allowable ’V 1‘, lOO ‘For good grade limber‘s, deplh = .965 ——- ’33“ - 30x100 7‘ Ll?) inches Maximum Momenl L‘é—wll :7ooxg x9511: 5,240 " fi- __ GM __ 6x526}? _ - depHi — sln“ _ 72.0me i .207 Incl’les Th 6V2 Pore 1 ll’le minimum ‘l’lqieleHess OF '3“ will be (As-Eel 'ljor +lne lkans— Verse ‘plOOV‘infl, making a ‘l-o‘l'al lkiclzness a? ‘l'lfle llloorin/a ol3 5"m. Use minimum size Pails. OP 6"Xéa" For Quail-cl 3. Slringers :— The maximum benclin/q momenl Occurs when He rear axle loaol of 32000:“? l5 Gl‘l 'l'l’lZ Miclclle OF ‘l‘l’la 5P0”. Mé'idax): 12%;)ng = 80,000 X17] x12" :beggloOouit- Egaa‘l'llflg ZX'l'ernal 8: in-lernal momen‘l’s 1,052,000: I 52-: 720x3g—xlox Tai- b:- 53“ or‘ 7—a"x 16" s‘lrinfler‘s wil’ln 3"wicll'l'1 surplUs. Deacl we'irqln‘l' 3- Guaro r‘ail‘fi» :- 2(1ol(T‘-’7:)(—,%)(32) =100 Floor :-— -,52:(\'o)(11)(32) :9 l7GO Wl”. o? slriiflers:-%(8)(-l%)(10)(32)= 2280 4200* Uni?ovm cleao load 242,30 1420*4‘1. Max. momenl t 513- wla‘ =—‘- 5 x420x100¥l.7=3q20'* Equal‘mg QXl-ernal BOIZO X l2 I1 El inlernal momenls 720 NC; x b x256 b '2 8.48 inches (7 S'l'V‘lYl'fl er's Sal‘lS‘FaC‘thg Coy- B,M,l Max. she/air~ 132000+.5(4200—280)233,660?“ ' ,_ ~ 2 33,560 __ 2k UHI‘l‘ veil—.cal shear ———-————7 Xavle - 37.5 /a.. Unil' horizonl‘al Shear : LS‘XB’LE): 56-7 lt/cau Allowable shear is 65%." SO 7 sl'ring‘ ers are enough JFor shear. Use 2"x4" bridging in each panel. 41Floor—5eam. Dead Wl’. :- Floor, Sl’rinlgers eguard rail =-' 3920fir Floor—beam allowance: 1000all Faslenin/qs allowance: 630:“: Tolal eleaol loaol : 5550* Lbacllng l?or max. B.M. is as follows leooo IGOOO L. 3.5' _, 6' _£.' [V i“ "V'l SSoTE/ct 10' V 2, 22. e— 5550.16“... x 971—3942.: 15 C175 1* .5 M(max)- — 13,0175 x3. 5—- 555 x3. 5 X-§2~=52,400‘='* 52,400x12 “M: 5%g7goxéxbx72‘52 b=l3J inches U82. a ‘lTlooreloeam l4"x20" V(max) = I6000 +4 x\6000 =22,400** . . a .- 22 00‘ #. “ Uml' horizon+al shear=l.5xao: ‘20 1:‘79" allowable horizon-lad sl'iear : 65*? n Necessar/q $\oop-bcam area:§_§zgégo arear—Cl‘lq sain. allowing, a Clap”, Cl: 20", ‘l'lnC wiol‘l'lfl would loe 24.7" Tl’lQY‘Z llor‘e i+ is necessar/a +0 use ‘l‘wo \2.\x201\ +imber5 as Ji‘loor—loeam. 5. Truss Slresses 5’rresses are due lo dead load, live load and lohgiludinaliload (clue 'lo braking.) Deaal loacl Sl resses are l‘ounol Simil1arl/q +0 l'lnoSe in III,A~4. Live load siresses were Pound in line gallowin/Q manner :- (a) lnljluehce (lid/q- rams were clrawn 1For all ill/1e mem- bers (b) We l‘l‘ZO loading was placed as near HA2 lrass as pos- Sllole and l’lfle Slress ligurecl {iron/1 Hie 1n?luence ,‘M’,o lbs. Tbe load on ‘l’lfle lower C/lflOlr‘d \5 £851: £1,420 nos USQ a Prall Truss wi+h +ension diagon- als *Por‘ bracing. No lal'er-al bracing needed belween uPper 4% chords because 04‘ $loor sgslem. Swa/bj lor-aClH/Q ‘l’ehslon rods:— max ~lrensiom = lGQZO x.2 xt.2-—'—4,oeo* area required : 6%:129 sq. in. Use 71-" round rods wl-l'b area 2.248 sq. uh. Laleral bracing ‘l’CHSlOH rods ‘.-— Yhax. +ension = 4,420 x2 x 2 XI.3= 4,‘loo* area required : 3.322 = .272 sq. in. laooo Use DIG! .l'ounol rods wi-Hn area = .307 Sq. 'WI. La-l-eral bracing compression member max. Comp. -_-Gy,420x2‘x.24\e,ciz.o:c.2:'H527'it area required: swig-2% = 14.3 sq. in. Since \engllx over \eas‘l dimehsun carmol be grea+er Hnen 30, a €"X6" Piece will bavg +0 be Used 05 CO'm— pr‘ession member. Sl’ra? dimensions may. V‘-'-"‘l,l521-‘t allowable Slress is l8000 fl/m" area reguiredrzLD—LZ—«qu 50,. 'm. IBOOO '— Use slraf): C‘T” lbiclé, Ll inches wide4 lat—med up 4 . 71"!“ IV. Conclusion An exhaustive analysis of the differences in the de- sign methods practiced by the military engineers and the state highway engineers was impossible because of insuffic- ient time. However, the main differences have been shown in the computations and will be commented upon here. In the matter of allowable stresses, eepecfihly that of tension in wood, the military engineer in specifying ome unit stress for all conditions and varieties of one kind of wood do not take the lowest allowable stress but nearly the largest stress. This practice cuts down the factor of safety considerably in case some low strength timbers are used. The state highway engineers allow different unit stresses for different conditions and they also are in pos- ition to determine the condition beforehand whereas the mil- itary engineer usually has to guess at the condition of the timber. Trestle: are not built with as large a safety factor by military engineers as by state highway engineers. The militar; engineers allow two or more columns in the bent with their length to least dimension ratio as high as 36. The state highway engineers allow no less than four columns to a bent with their length to least dimension ratio a max- imum of 30. The state high we: engineers do not allow an addition- al live load for impact, on wooden members, but the: mult- U iply the bending momenta b? a coefficients Which depends upon the material to be used. The military engineers allow an impact factor of 50% for all members. Therefore the mil- itary engineers have a larger safety factor in such parts as the trusses of their bridges. Military engineers use approximate charts whenever pos- ible. These charts, however, are not figured too closely as is shown in the computations. For instance, the floor bearer of the truss bridge was selected from a chart based on the maximum bending moment when they should have based their design to take the maximum.shcar. In this case the safety factor was cut almost in half by the military eng— ineers. In figuring lateral bracing and sway bracing, both classes of engineers use much the same wind load. However, in figuring longitudinal bracing, only the state highway engineers consider the horizontal thrust caused by the sudden application of brakes. According to Michigan State statutes the clear width of roadway over a bridge must be nineteen feet for any class of road. This would eliminate the possibility of using military bridges, which are only ten feet usually in width, as anything but temporary structures. To do even this additional stringers, floor bearers and a longitudin- al flooring should be made apart of the floor syst m and if it is a trestle bridge additional columns and bracing in and between bents probably would be necessary. The above analysis of the differences in the design of military and state highway bridges is not exhaustive e- nough to be used as a final judgement of the two types of design but it does show that the difference is enough that to use one type as a substitute for the other, a thorough examination to find the maximum allowable stresses for the different members would be advisable. war '5‘. ” i ROOM U51: ONLY. LIBRARY BOOK BINDERS east: «:9 ANS rTL UNWERSHY a '«« « «« « «««« «l:«« 1293 0319 06