A COST COMPARISON BETWEEN A REINFORCED CONCRETE AND A STRUCTURAL STEEL BUILDING FRAME M for the boom of I. s. MICHIGAN STATE COLLEGE George Seymour I949 . SUPPLEMENTARY IN BACK OF BOOK 1 ‘ “ : MATERIAL I inlul A COST COMPARISON BENIEEN A REINFORCED CONCRETE AND A STRUCTURAL SI‘EEIL BUILDING FRAME A Thesis Submitted to The Faculty of ‘MICEIGAN STATE COLLEGE of AGRICULTURE AND APPLIED SCIENCE by George Mar Candidate for the degree of Bachelor of Science March 1949 INTRODUCTION The comparison of cost is truly vital and necessary to all people both in business and out if they are to receive full value for their dollar. Neither a person's wage nor margin of profit is so great as to enable that person to ignore price variation when buying. It is with this in.mind that I decided to make a cost comp parison.of two types of building frames now in common.use: reinforc- ed concrete and structural steel. Generally speaking, the two frames are used in different types of buildings. The reinforced concrete frame is used where a fire- proof building is required, as in schools, apartment houses, depart- ment stores—-in general, fireproof buildings under twenty stories in height. Steel frame buildings generally are used for mill build- ings, factories, or very tall structures. There is, however, much Overlapping of the two where both.might be addaptable In this paper I wish to investigate the cost of the two types of frames where either type might be used-u-that is, in a building that does not have to be fireproofed and is small enough.to be sup- ported by a reinfbrced conrete frame. ‘With this in.mind I selected a small apartment building to be designed for both a reinforced concrete and a structural steel frame. It is true that for this size building, wood frame construction would be possible, and in many cases, practical. However, from.a.maintainence and life of structure standpoint, wood frame construction was ruled out. Also, while fireproof construction was not necessare, the added protection of a concrete or steel structure was a large factor in eliminating wood construction from.consideration. 11,398.55 ARCHETECTURAL CONSIDERATIONS While the general layout of the building would ordinarily concern the archetect rather than the engineer, it is closely enough tied to the structural design to deserve brief mention here. The initial problem was providing the required floor area with the least outside perimeter while still satisfying the necessary light and ventilation requirements for each apartment as a whole and certain rooms in particular: namely the kitchen and bathroom. For this reason a cross plan was adepted as the plan of the building which allowed two apartments per wing with a central location of stairwell. With this plan, corner ventilation is secured for each-apartment and ade— quate window is available for lighting. A.ceiiing height of eight feet and a story to story height of nine feet was decided upon. In the steel frame building, the eight foot ceiling height was modified smmewhat in the north-south wings where it was necessary to use ten inch beams with a four inch slab, leaving seven feet ten inches clear. This was still further reduced two inches for floor and ceiling finishes leaving seven feet seven inches clear. In this respect, the steel frame building would furnish less desireable apartments than the concrete frame building. It is true that the story to story height would have been increased for the steel frame building. That, however, would have eliminated one neces- sity in this cost comparison of the two buildings: that of'making both designs identical. Again it is true that for both types the story to story height could have been increased. This, however, would increase the cost of the concrete frame structure with no pro— portionate advantage. The nine feet story to story height also necessitated a more or less radical departure from the conventional bean and slab type of reinforced concrete building. Instead of the typical narrow, deep beam, a shallow, wide, and relatively heavily reinforced bass was used. A beam of this type requires more concrete and steel than the conventional beam but this is overshadowed by its many advantages. I believe the greatest of these advantages is the reduction in story height which in this case would amount to about a foot. Archetectur— ally and economically this is desireable. Secondly, the need of a drOp ceiling to conceal the beams is obviated, a considerable saving. The wide shallow beam, protruding no more than four inches below the rest of the ceilingbecomes a harmonious archetectural feature of the room and need not be concealed. The third distinct advantage is the decrease in clear span lingth for the floor slabs with a subsequent decrease in slab steel resulting from reduced critical moment. This offsets in a large way the necessary increase in beam steel. Lastly, the wide, shallow beam is easier and cheaper to form than the deep, narrow beam. In times of high labor and material cost, both of which exist today, this is a substantial saving. For these reasons, the slab-band method of framing was chosen for the reinforced conrete building. Exterior walls of the building are to be of cavity wall construc- tion—brick exterior with a two inch air space and four inch cinder bhock plastered on the inside. All interior partitions are to be of three inch gypsum block plastered both sides. Ceilings are of accoustical tile applied, in the concrete frame building, directly to the concrete with mastic, and in the steel building, to furring strips attached to the steel floor beams. Floors are to be wood block laid in mastic directly on the cementslab. DESIGN OF CONCRETE STRUCTURE The reinforced concrete frame building was designed in accord- ance with the.mmerican Concrete Institute Specifications, and the building code requirements of the City of Midland, Mich. where the building would be erected. Two departures from.the AOI code were made. The first was decreasing the recommended area of the columns from.one hundred twenty squart inches to one hundred square inches. This change was made because, in most cases, the collumns were consid- erably'understaessed. However, the recommended minimum thickness of ten inches was held. The second departure was in using one inch fire protection for beams instead of one and one half inches. Hewever, inasmuch as the beams, except spandrel beams, are at least eighteen inches wide, such beams could easily be classed as slabs and the one inch protection is adequate for a four hour rating. SLAB DEEHHS Both.one way and two way slabs were used in the building, depend- ing upon the spans. One way slab Mark 81 Loading design live load 30 #/sq ft slab weight 50 #lsq ft roofing and ceiling finish 10 #lsq ft Total load‘w 90 #lsq ft specifications fc - 1850 psi n 10 fs = 20,000 psi moment effective span = 11 - 3 + 0.7 = 8.7 ft 2 1/14 x 90 x 8.7"g =- 486 ft-lb Positive = 1/14'W1 2 negative = 1/10 W1 2 1/10 X'90 z 8.72 = 683 ft-lb K-u/bdz-683x12a-12x52-7se 0K steel’area positive As . M g- £st =- 470 x 12 + 20,000 x .87 :x :5 = .112 1112 min allowed .0025 x 12 x 4 =- 0.12 in2 negative As - 683 I 12 + 20,000 x .87 x 3 - 0.157 bond 7'2 90 x 8,7 e 2 a 378 # u = 150 psi sum of perhmeters ='V + ujd a 378 e 150 x .87 x 3 = .97 in2 steel use 1/ 2" deformed bars at 10" shear v =‘V + de a 378 f 12 I .87 x 3 = 12 psi OK Two way slab mark 82 moment effective length = 14' short span pos mom = .054 m3 - .054 x 90 x 142 - 1130 1b-ft neg mom =- .071 W12 - .071 x 90 x 143 - 1380 1b-ft long span pos mom == .049 W12 - .049 x 90 x 142 - 652 lb—ft .087 W12 - .037 x 90 x 14.2 - 868 lb-ft negzmam steel area short span negAa-Mg'srst-iseoxizozopmx.8713-.321 ing posAs= 1130x12§20,000x87x3 -.263 1:13 long span negAs= 861’5’xl.‘3‘|-20,0001c.£1t71:39-.‘BOIin2 pos As 652 x 12 0- 20,000 x ,87 x s - .152 11:2 _use 1/2" bars at 7* for short span l/Z" bars at 10" for long span shear V'= 90 I 7 = 630 # v=V¥bjd=630+12x.8713=20psi 0K BEAM DESIGN The maximun.end moments in beams was determined by the Cross method of moment distribution. The proceedure, outlined briefly, was this. By the use of end coeffecients, a beam.size was determined which was then checked by moment distribution. The first choice of beams turned out to be small and the size of the beams had to be increased and rechecked. A sample of the moment distribution compu- tations are included on the next page. Maximun shear at the supports of the beams was obtained by ad- ding to the shear due to the total load, the difference of the two end.moments divided by the length. Shear in the beams was not crit- ical because of the width of the beams. Stirrups, therefore, were needed only in the spandrel beams of the first or second floors. m oaHA.ss:Hoo n 0 mu 0.0 + 0.5.. 0.00.. 0.00.. mm + 0.H+......~.0.. 0.00.. p.07 «.Hm... H.n a H.» n 0.0 n s.0 + s.0 + 0.0 + a. + e.H + 0.H.+ ».H + 0 0.0 + 0.H + 0.0 + 0 0.H a 0.0 u m.H + 0 0.0 a 0.0H. 0.0H. 0.0H+ 0.0 + 0.0 + 0.0 + 0.0 + 0.0Hn 0.0Hu 0.0Hu 0.m0n 0.00+ 0.H0u 0.H0+ H0. H». on. 4.0 s0. 8. 0.0.. h. .Hé 0R H0. H0. 0.0H+ 0.0H+ 0.snn 0.00. 0.0 n H.H n e.0mu 0.H0+ «.mHn 5.0Hu 0.0: 0.0 a 0.0 a 0.H n 0 .H: H.H . 0.H n H.s + 0.0 + 0.0 + 0.0 + 0.0 + 0.H a 0.0 + 0.H + 0.H + 0.0 n/ «.H n 0.0 u 0.0 u 0.0.+ 0.0H. 0.0H+ 0.0 u 0.0 n 0.0 u 0.0 n 0.0Ha 0.0Hu 0.0Hn 0.H0. 0.H0+ 0.00: 0.00. H0. H0. 00. :10 em. 00.. 0.0.. an. 4.0 00. H0. H0. 0.0H+ n.0Hu s.H0+ H.0 + 0.00: 0.mm+ 0.00: Muwl... mum.“ 0.H + 0..H + 0..H + .0. + 0.H + 0.0 + n.m + 0.e + 0.H I 0.HHn 0.m + 0.0 u .0._.0...H+ I10 0H... 0.0 + 0.0 + 0.0 + 0.00.. 0.0? 0.00.. 0.8+ 0.0T 0.0+ $0 30 OqHOn on. 80 m”. OgHoa $0 3. m H00 .< H00 000m .0000 0 H00 .0 H00 0000 _a00m 0 H00 .0 H00 .mflflwh Havana .aufin CHOKOH 00000.0 008 .sfiflah sepoeh .puan Center moment average end.moment (34.9 + 57.3) o 2 = center moment—simply supported beam Corrected center moment Shear shear due to leading Shear sue to end.moments max shear BEAM DESIGN Specifications fc = 1350 fs = 20,000 n = 10 u - 150 Beam mark CD2 use d = 7” b a 48” neg mom a 56.8 kbft pos mom a 44.2 k-ft neg steel R=M+bd2n56800x12+48x72=299 Compression steel req'd p' - 1% for compression steel Ala a .01 x 48 x 7 a 3.36 in2 p = 1.75% for tension stees As - .0175 x 48 x 7 = 5.87 1212 pos steel A R a 44,200 x 12 + 48 x 72 = 226 OK 41.1 88.5 47.4 lb-ft 16.1 k. 0.9 17.0 k v a 90 8 - 3/4” bars 14 - 3/4" bars As-Mefsjd=44,200x12+20,000x.87x7$4.36 10 - 3/4" bars shear V'= 17 k v =‘V o bjd a 17,000 + 48 x .87 x 7 a 58.2 psi 0K Bond ‘ sum of perhmeters =‘V + ujd = 17,000 0 150 x .87 x 7 = 18.7psi 0K COLUMN DFSIGN column mark B2 001 load 71.1 k use 4- 3/4" bars allowable load P - .8 Ag (.225 f'c + fspg) = .8 x 100 (.225 x 3000 + 20,000 x .0176) 382.4]: ties use 1/4" bars spaced at 48d = 12” FOOTING DESIGN footing mark A3 loading 46.1 k est footing weight 3 k total weight 49.1 k size of footing allowable soilpressure 4 k/ft2 area required. 49.1 e 4 = 12.3 ft2 use 3.5 x 3.5' footing not pressure 46.1 + 12.3 . 3.78 k/ftz 10 depth of footing governed by moment M’= 1.33 x 3.5 x .67 x 3.78 a 10.8 k-ft d = (M +»Rbl% = (10,800 x 12 o 236 x 42)% a 3.6" governed by shear assume d of 12" V'- .33 x 3.5 x 2 x 3.78 + .33 x 2.8 x 2 x 3.78 = 15.85 k b - 136" v = 75 psi d =‘V t va - 15,850 0 75 x .87 x 136 - 1.8" use a d of 12" + 3" cover. This is a larger d than called for by the computations but to allow for construction variables, a d of at least 12" is desireable. steel As =- M 0 £st 0 10,800 x 12 a- 20,000 x .87 x 12 - .62 in‘?’ bond ‘ V'= 1.33 x 3.5 x 3.78 = 17.5 k u - 155 psi sum of perimeters - 17,500 0 155 x .87 x 12 - 12.4 in2 use 6 - 3/4" deformed bars DESIGN OF STEEL FRAMEWORK The steel frame building was designed in accordance with the American.Institute of Steel Construction specifications. The beams and girders were designed as simply supported members with no and res- traint. In a building of this size, the restraint develOped by the concrete floor is sufficient to counteract wind loading. This accounts for the lack of sway bracing. All beams, girders and columns are rolled sections which is common in a building of this size. FLOOR DESIGN The floor system as in the concrete frame building, is composed of a combination of one and two way slabs. However the span length is smaller in the steel building resulting in lighter reinforcing. The span length was decreased in order to obviate the need for large heavy beams to conserve head room. Floor thickness was not decreased because any lessening of the sound insulating value of a four inch concrete floor was undesireable.p Computations for the floor system are the sameas for the reinforced concrete frame building and will not be repeated here. ZEEMM DESIGN beam.mark 561 loading 17 = 875 iii/ft span. 1 a 22' Min depth = 1/24 x 22 1.12 a 11” use 10" NF decrease allowable unit stress 10/11 x 20,000 = 18,200 psi 12 3 - 1/8 x 925 x 222 a 52,900 lb-ft moment == 1/8 W1 section modulus =‘M i f = 52,900 x 12 + 18,200 a 34.9 use 10" NF 33 COLUMN DESIGN col‘mank 02 .loading 156k try 10‘. wr 29 P a (17,000 - .485(96/l.34)2) 8.53 = 124 k too small try 10" WE 59 P = (17,000 - .485 (96/l.98)2) 11.48 == 182 k 0K BASE PLATE DESIGN’ col mark C2 base plate loading 156 k + l a 157 k fc - .8 k/inz area required a 157 o .8 = 197 in2 size of plate col size 8" x 10" length = .8 width lW'- 197 in2 or .8 w2 = 197 in2 w = 15.7 or 16" 1 - 13” max overhang = 3” 1 thickness or plate . ( .15 x .75 x 53)2 == 1.05" use 13" x 16” x 1' base plate for all cols 13 PIER DESIGN All piers 14” x 17" p=1.0% allowable load P = 0.80 (.225 f'c Ag + pg Ag fs) = 167 k steel area As= .01 x 14 x 17 = 2.38 ing use 4 - 7/8" deformed bars ties use 1/4" bars at 48d = 12" FOOTING DESIGN Footing 02 loading = 157 + 3 = 160 k size of footing allowable soil pressure = 4 k/inz area required = 160 e 4 - 40 ftz use 6.5 x 6.5' depth governed by moment 'M a 3.7 x 2.6 x 6.5 x 1.3 = 81.2 k—ft d = (M,+ Rb).g3 =(81,200 x 12 + 236 x 78)%'= 7.5" governed by shear assume d = 12" v = 2x1.5 x6.5 x 5.7 + 2 xl.'7 x3.5 3:15.? = 116.241: 14 d =‘V + vjb = 116.2 + 75 x .87 x 154 = 11.7" use d of 15” + 3" = 15" COST ESTIMAEION Estimating the cost or the two types of framing systems is probably the least accurate portion of this thesis. It is possible to predict with reasonable accuracy the performance of steel and concrete in a building, but the placing of these materials in the structure involves many variables, two of the greatest being the weather and the willing— ness and/or ability of human beings to work efficiently. Consequently, any attempt to predict anything with as many variables as construction work is apt to go awry. In.making up this estimate, great reliance was placed on the "Builders Estimating Reference Book," by ferry Walker, and somewhat less reliance on my own eXperience on construction jobs. Even with two such authorities as guides, hawever, estimating is rather hazardous. ' No account was taken of any functions that would be duplicated in the two building. For instance, no estimate was made of the excavating costs inasmuch as they would be identical for both structures. In all the cost estimating, an attempt was mafie to break the Opera- tion or quantity into its smallest units. This, it is felt, leads to more accurate estimating. On the following page, the floor forms will be estimated in this manner. Thereafter, only total quantities and prices will be noted. 15 REINFORCED CONCRETE FRAME ESTIMATION Floor forms lumber required per 100 ft2 of forms 1" sheathing + 20% waste 2 x 6 joists at 24" 3 x 8 stringers 4 x 4 shores beam sides braces total labor cost cost per 1000 board feet carp 52 hrs at $2.07 lab 28 hrs at 1.35 total form area needed = 17,100 sq ft lumber needed = 171 x 546 = 59,200 bf total labor cost = 104 x 59.2 = Materials cost forms to be used three times total lumber needed = 17,100 + S = 5,700 sq ft cost per 100 sq st of forms 1" lumber (.120 + .021 + .010) x 130 = 2" lumber .063 I 105 = 3” lumber (.046 + .086) x 140 total per 100 sq ft total for the bldg = $45.29 I 57 a 120 bf 10 ” 346 bf $66.20 27.80 $104.00 $5150.00 $19.62 6163 19.05 $45.29 $2580.00 16 total cost-olabor and materials $8730.00 $2580.00 + $6150.00 = col foms labor cost $780.00 material cost 373.00 total $1153.00 footing forms labor cost $36.40 material cost 16.05 total $52.45 reinforcing steel material cost 10.75 ton $3920.00 labor cost 878.50 total $4798.50 concrete material cost 331 yds $3730.00 labor cost 1515.00 total $5045.00 Grand total, reinforced concrete frame $19,778.95 STRUCTURAL STEEL ERNIE ESTIMATE Estimating the cost of the steel, that is, the fabrication.and erection, was a job that I was not qualified to handled. Therefore, I obtained from the Jarvis Engineering 00., Lansing, Mich. an estimate for the Job. To that I added the cost of the concrete floors, piers and footings to get the final estimate. 17 Steel frame, fabricating and erecting 70.5 ton at $285.00 $20,500.00 Floor foams Material cost $1,645.00 labor cost 5,180.00 total $6,8250.00 Footing forms Material cost $13.50 labor cost 27.30 total $40.80 Pier forms material cost $62.00 labor cost 128.60 total $190.60 Reinforcing steel material cost $1,680.00 labor cost 437.60 total $2,117.60 Concrete material cost 186 yds $2,090.00 labor cost 818.60 total $2,908.60 Grand total, structural steel frame $32,602.37 18 CONCLUSION As noted from the previous figures, the cost of the steel frame runs substantially highter than the cost of the reinforced concrete frame. The total difference is $12,823.42. On the basis that the entire build- ing will cost about $150,000.00, this represents about an 8% difference between the two types of framing. The dominant factor in the greater cost of the steel frame build- ing is the cost of fabricating and erecting the steel. I believe the reason for this 1188 in the type of building for which the cost analysis was made. The analysis clearly shows that for a small structure with light loading, it is more economical to erect a reinforced concrete frame than one of structural steel. 19 BI ELIOGRAPHI Apartment Houses Abel and Severud John Wiley and Sons Building estimators Reference Book Frank R. walker Frank R. Walker co. Design of Modern Steel Structures Linton F... Grinter MacMillian Co. Minimum Pmperty Requirements Federal Housing Administration Reinforced Concrete Design H. Sutherland and R.C. Reese John Wiley and Sons Simplified Design of Reinforced Concrete Harry Parker John Wiley and Sons Simplified Design of Structural Steel Harry Parker .7 _ w John Wiley and Sons Strength of Houses 0.8. 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