Click for next page ( 10


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 9
ll CONCLUSIONS AND RECOMMENDATIONS The recommendations which follow are, in the opinion of the Com- mittee, based on the best information and engineering judgment available concerning the problems posed, for use until such time as additional research and experience data indicate the need for further revision. The Advisory Committee has developed procedures for selection and specification, or, when necessary, design of given slab types. These, it believes, constitute an engineering approach which is as rigorous as current knowledge will permit. It is recognized that experience and the state of engineering knowledge are such that precise answers to many of the problems posed must of necessity be considered beyond attainment in the immediate foreseeable fu- ture. Nevertheless, the approach recommended herein is consid- ered to be sufficiently valid to warrant application now. For the future, the Committee has developed and recommends a program of research which it believes will provide much of the data needed for the ultimate solution of the problems under study. To permit future refinement of the approach developed, the Com- mittee recommends Hat FHA implement and/or encourage others to implement these research recommendations. As data are devel- oped which can provide a valid basis for further revision, the Com- mittee requests that opportunity be provided for appropriate revi- sion of recommendations. Many variables affecting finished-slab performance must be considered when selecting a slab type and when specifying or exe- cuting a slab design. As stated in the Introduction, to aid in the 9

OCR for page 9
10 RESIDENTIAL SLABS ON GROUND selection of a slab type, the Committee has delineated four major types which it believes provide a range adequate for all combina- tions of variables that may occur. As previously indicated, these are Type I: Unreinforced Type II: Lightly reinforced against shrinkage and temperature cracking Type III: Reinforced and stiffened Type IV: Structural (not directly supported on ground). The Committee recommends the procedure below (pare. 1 .1 ) for the selection of slab type and for materials specification or slab design. Use of these procedures of course should be tempered throughout with engineering judgment and experience, and, again, nothing said here should be construed to imply that FHA should not consider innovations in either slab design or analytical proce- dures which may be offered as alternatives, as long as the basic principles stated herein are satisfied. 1 .0 SLAB SELEC TION AND/OR DESIGN PROC EDUR E 1.1 Selection of Slab Type Step 1-Conduct a soil investigation of the building site to determine: a. Types of soil on the site b. Thickness and distribution of each type of soil c. Consistency of clay soils and density of granular soils. Unless competent engineering advice is offered to the contrary- such as may be derived from documented soil histories of the area, or, in the case of large developments, a full site investigation-at least one test boring should be performed per slab site. When this boring reveals unusual conditions, such as the presence of organic soils, soft or loose soils, highly plastic soils, or rock, additional borings are recommended as necessary. These borings can be made with simple tools and should extend at least 15 feet below existing ground or below the bottom of the slab (whichever has a

OCR for page 9
CONCLUSIONS AND RECOMMENDATIONS 11 lower elevation), or to solid rock. A record of soil classification, depth, and moisture content should be kept. Where CL, OL, CH, or OH soils are encountered, the unconfined compressive strength (qu) should also be obtained by laboratory testing, unless qu obvi- ously exceeds 7.5w (Step 2a, p. 15~. Step 2-Determine, from the map of Fig. 1, p. 38, the climatic rating (Cw) for the area within which the building site is located. Step 3-Determine, from Table I, below, the slab type recom- mended for each soil type on the building site, i.e., Type I, II, III, or IV. TABLE I Conditions Slab-Type Recommendations Based on Soil and Climatic Minimum Density2 Climatic Recommended Soil Type1 or PI or qu Ratings Slab Type4 GW, GP GM, Go, SW, SP, SM, SC, ML, MH GM, GO, SW, SP, SM, SC, ML, MH CL, OL, CH, OH All Densities All I Dense or medium dense Loose PI < 15 and qU/W 2 7.5 All I All II5 All II PI>15 and (cw245 II qU/W 2 7.5 ~ Cw < 45 III 7.5 ~ qu/w> 2.5 All HI qU/w < 2.5 All IV Pt All All IV 1As classified under the Unified Soil Classification System (Appendix D, p. 289). 2Unconfined compression strength of undisturbed sample. 3From Fig. 1, p. 38. 4Minimum requirement believed adequate for particular condition. 5Type I slab may be used if soil is densified by compaction to its entire depth before placement of concrete slab.

OCR for page 9
12 RESIDENTIAL SLABS ON GROUND Subsequent procedure for each type appears below. 1.2 Procedure When Type I Is Recommended If a Type I slab is recommended for all soils found on the site, proceed as follows: Step 1-Determine whether heating coils or ducts are to be embedded in the slab. Heating coils should not be embedded in Type I slabs. Heating ducts, however, may be used provided the slab over the duct is reinforced for 19 inches on either side of its long axis (or to the slab edge if closer) with 6 x 6 - 10/10 Welded Wire Fabric (WWF) reinforcement. Step 2-Determine slab dimensions. If the maximum dimen- sion is greater man 32 feet, dinde me slab into squares or rec- tangles using weakened plane joints (4.5, p. 41), each with maximum dimensions not exceeding 32 feet (the maximum dimension meaning the longest side of a rectangular unit); or use a Type II slab. Step 3-Determine We location and weight per lineal foot of partitions. Type I slabs cannot accommodate partition loads over 500 plf. If partition loads are greater than 500 plf, they should be supported on separate foundations and isolated from the slab, as shown in Fig. 3, p. 42. Step 4-Determine extent and size of openings in the slab, and provide 6 x 6 - 10/10 WWF reinforcement for 25 inches around all openings 12 inches or more in width. Step 5-Provide weakened plane joints at the junction of ir- regular shapes to divide the slab into squares or rectangles with maximum dimensions not greater than 32 feet. Step 6-Determine whether the top surface of the slab will con- tain irregularities in its horizontal plane. Where Me vertical dif- ference is 1-1/2 inches or less, as for bathrooms, the uniform 4- inch slab thickness should be maintained by tapering the underside of the higher slab plane, beginning 24 inches away from me verti- cal displacement. For vertical differences greater than 1-1/2 inches, the slab thickness should be maintained and, in addition, the irregularity should be reinforced for 25 inches on either side with 6 x 6 - 6/6 WWF reinforcement.

OCR for page 9
CONCLUSIONS AND RECOMMENDATIONS 13 Step 7-Determine the location and amount of concentrated loads, such as those of chimneys. These loads should be supported on independent footings which rest on suitable unyielding foundation soil, and should be independent of the slab (Fig. 3, p. 42~. These footings should be designed in accordance with ACI Building Code Requirements1 for footings, using the same bearing pressure as for other foundations of the residence. Step 8-Determine the number of weakened plane joints re- quired by Steps 2 and 5 above. Locate these joints, preferably under partitions or like elements, where the resulting crack will be least objectionable. Where reinforcement is to be provided for other reasons, it should not be carried across the joint; instead, provide a properly designed joint in the feature requiring reinforce- ment, or locate the weakened plane joint where it will not cross the reinforcement. Step 9-Determine the degree of compaction needed over the entire slab site (Fig. 22, p. 128~. 1.3 Procedure When Type II Is Recommended If a Type II slab is recommended, determine the depth of the soil needing this type of slab. Where this soil is underlain by soils suitable to Type I slabs to at least 10 feet, or to rock, considera- tion may be given to removing the unsuitable soil and replacing it with a suitable nonplastic fill material properly placed, so that a Type I slab can be used. If this is not feasible or desirable, pro- ceed as follows: Step 1-Determine the maximum dimension of the slab between design joints. Provide minimum reinforcement midway in the slab as follows: O to 45 feet-not less than 6x6 - 10/10 WWF 45 to 60 feet-not less than 6x6 - 8/8 WWF 60 to 75 feet-notless than 6x6 - 6/6WWF. Step 2-Determine the location and weight per lineal foot of partitions. Type II slabs cannot accommodate partition loads over 500 plf without additional reinforcement. If partition loads are 1Appendix C, Reference 1 (or later edition, if any).

OCR for page 9
14 RESIDENTIAL SLABS ON GROUND greater than 500 plf, an additional layer of WWF reinforcement is required for the full length of the partition and extending 25 inches on either side. Step 3-Determine extent and size of openings in the slab. Provide an additional layer of 6x 6 - 6/6 WWF reinforcement for 25 inches around all openings 12 inches or more in width. Step 4-Determine the location and amounts of concentrated loads such as those of chimneys. These loads should be supported on independent footings which rest on suitable unyielding foundation soil and should be independent of the slab (Fig. 3, p. 42~. These footings should be designed in accordance with ACI Building Code Requirements for footings using the same bearing pressure as for other foundations of the residence. Step 5-Determine whether the top surface of the slab will contain irregularities in its horizontal plane. Where the vertical difference is 1-1/2 inches or less, as for bathrooms, the uniform 4-inch slab thickness should be maintained by tapering the under- side of me higher slab plane, beginning 24 inches away from the vertical displacement. For vertical differences greater than 1-1/2 inches, the slab thickness should be maintained and, in addition, the irregularity should be reinforced for 25 inches on either side with an additional layer of 6x6 - 6/6 WWF reinforcement. Step 6-Determine degree of compaction needed over the entire site(Fig.22,p.128~. 1.4 Procedure When Type III Is Recommended Investigate the possibility of removing the soil (to a depth of at least 15 feet) which requires the use of a Type III slab and replac- ing it with a soil of better consistency so that a slab of Type II or Type I can be used. If this is not feasible or desirable, proceed as follows: slab. Step 1-Estimate the total average dead and live load on the a. Estimate the per-square-foot dead load (Wd) of the slab itself from the empirical formula

OCR for page 9
CONC LUSIONS AND RECOMMENDATIONS 1 5 Wd = (2L + 30) psf where L is the long side of the rectangular slab in feet. b. Compute the total superstructure load (ws), allowing for a live load of 30 psf of floor area and 10 psf of roof area, and intro- ducing all superstructure dead loads with their true values. c. Set average total load (w = wd + ws). Step 2-On the slab site, determine the following parameters: a. The lowest unconfined compressive strength (qu) obtained from undisturbed samples within the top 15 feet of soil immediately below the lowest point of the slab. (If qu obviously exceeds 7.5 times the average dead and live load (w) on the slab, its determination is not necessary. However, verify that qu/w ~ 2.5, because if qu/w < 2.5, a Type IV slab will be necessary-Table I, p. 11.) b. The plasticity index (PI), which is taken equal to the effective PI determined as specified in pare. 7.8.1a, p. 65. (If, instead of PI, the PVC meter reading or percentage swell of the soil is used to evaluate sensitivity of soil to moisture change, then the effective value of the PVC meter reading or me percentage swell should be determined as specified in pare. 7.8.1 b and c, p. 67.) Step 3-Determine from Fig. 6, p. 53, the support index (C) in terms of the climatic rating (Cw) obtained from the map of Fig. 1, p. 38, and the PI determined in Step 2b above. Wherever special field conditions prevent variations in soil moisture from being as severe as would otherwise be indicated by the climatic rating (Cw) for that given geographic locality, equate C to the modified support index (Cm) given by the empirical equation Cm = 0.5 (1 + C) of pare. 7.4, pp. 54 and 56. In the case of compressible soils (7.5 > qu/w 2.5), equate C to the reduced support index (Cr) as determined in pare. 7.5, p. 56. In computing the ratio qu/w, use the tentative em- pirical value for w obtained as in Step la above. Step 4-Obtain from Table m, p. 50, the maximum allowable deflection ratio (^ /L) for the contemplated type of superstructure. Step 5-Divide slabs of irregular shape into overlapping rec

OCR for page 9
16 RESIDENTIAL SLABS ON GROUND tangles in such fashion that the resulting exterior boundary provides complete congruence with the slab perimeter-e.g., ;~F ,, 1 3 _ , 2 ' _ L:. ~ F;~ ~2 1 ~ Then, design each of the derived composing rectangles as described below.1 If the support index (C), determined as in Step 3 above, is ~ 0.9, proceed with Steps 6 through 12. If the support index (C), deter- mined as in Step 3 above, is > 0.9, proceed with Steps 6a through 9a, lla, and 12. Step 6-Determine the effective load (w) along the long (L) and short (L ~ dimensions of the slab, i.e., w = w tP (1- C) for L dimension w = w(1- C) for L' dimension where UP = 0.5 or 1.4 - 0.4 (L/L'), whichever is greater. Step 6a-Determine the effective load (w) along the long (L) and short (L ~ dimensions of the slab from the expressions w = 0.1 w UP for L-dimension w = 0.1 w for Ll-dimension where UP = 0.5 or 1.4 - 0.4(L/L' ), whichever is greater. Step 7-Select a layout for the stiffening beams. Beams should be spaced equidistant along each slab side, not to exceed a 15-foot clear spacing, and, preferably, a spacing between 9 and 12 feet. However, corresponding beams of overlapping rectangles of irre- gularly shaped slabs should coincide, even though some variation in spacing may result. In any event, the spacing of beams along any side should be kept as nearly equal as possible. 1See 5.0, pp. 21-22.

OCR for page 9
CONCLUSIONS AND RECOMMENDATIONS 17 Step 7a- For the w-values of Step 6a above, design for slab dimensions and reinforcement in accordance with Steps 7 through 9 (pp. 16-19), obtaining values of d, bs, and As for beams both in the long (L) and short (L') dimensions. Step 8-Select basic dimension. In cases where L/L' exceeds 2, beams along the short dimension can be designed with smaller depth than in the long dimension, provided there are definite cost or construction advantages and design computations have been ad- justed properly. Select a trial ratio (L/d) using the following rule of thumb: For a w along the long dimension (L) 20 For a w along the long dimension (L) 2 25 but < 50 psf, L/d = 17 to 20 For a w along the long dimension (L) ~ 50 but ~ 150 psf, L/d = 14 to 18. Select tentative design values for beam widths B and B' (L and L', respectively). Initial values for B and B' may be taken as 8 inches. Beam widths preferably should not be less than 8 inches nor more than 14 inches. Step 8a-Determine depth (d), width (bs), bottom steel (As), and additional top steel (As) for each beam from the following rela- tionships: bs = bs (as determined in Step 7a) d =2in.+10~1-C)(d-2in.) AS=lotl-C)As As = 10 (1 - C)AS - 0.65 in.2 where the values of d and As in the right-hand side of these expres- sions are obtained from the computations of Step 7a above. Step 9-When designing for steel (for slabs with qu/w 2 7.5) a. Compute the depth ratio (~/d) in the long and in the short di- rection of the slab, i.e., L/d and L'/d, respectively.

OCR for page 9
18 RESIDENTIAL SLABS GN GROUND b. Compute the load index tw (! /b)] in the long and in the short direction of the slab, i.e., w(L /B) and w(L/B ), respectively. c. Determine the required steel ratios (p) for both long and short directions of the slab. Using the value of the load index tw(t'/b)], enter the appropriate graph, i.e., Fig. 14, 15, or 16, depending on the value of 6/L for the slab under design. For this load index and the value of Q/d, determine the required steel ratios (p), first in the long direction in terms of w(L'/B) and L/d, and then in the short direction in terms of w(L/B') and L'/d. If the slab is supported on compressible soil (2.5 < qu/w< 7.5), modify the design for steel reinforcement in accordance with 7.9.3, p. 91. If Me steel ratios (p) as obtained above correspond to a point above the "limiting line" or above the "shear line" (Fig. 14, 15, or 16), the tentative values for d, B. and B' adopted in Step 8 should be revised and the steel ratios determined for the revised values of d, B. and B' If the steel ratio (p) in either the long or the short direction is determined to be less than 0.3%, the mimimum percentage of steel (0.3%) should be used in computing required steel. To optimize the design empirically, follow the guidelines pro- vided under Step 9a and b, pp. 81-82. d. Compute me cross-sectional area of steel required for the bottom of stiffening beams, from AS = pbSd using the p-value which corresponds to the direction of the beam under design. Compute the cross-sectional area of the steel required for the top of each stiffening beam, from As=As-0.65in.2 e. Adjust for unequal beam spacing. If, in the case of irregular slabs, the beams within a particular rectangular slab are not equidistant, adjust the steel computed as above to the unequal spacing of beams. Accomplish this by comput- ing me average beam spacing; then increase the steel in beams spaced at a larger distance by an amount equal to the ratio of the

OCR for page 9
CONCLUSIONS AND RECOMMENDATIONS 19 true spacing over the average spacing. If the steel ratio (p) corre- sponding to a beam is located within 0.0015 of the v = 75 psi-curve (i.e., if the shear condition is almost critical), then, in adjusting the steel for spacing of beams, there must be an increase of both the steel and the width (bs) of all those beams which are at more than average spacing. Otherwise, an increase of the steel alone will be sufficient. I. Reinforcement of the top slab of Type III slabs should com- prise: No. 3 bars each way at 12 inches on center (o.c.) if the max- imum clear spacing between stiffening beams does not exceed 12 feet, and No. 3 bars at 10 inches o.c. each way in all bays when the clear spacing of stiffening beams exceeds 12 feet, irrespective of the direction of spacing. Top slab thickness should be 4 inches. If this thickness is made 5 inches, the reinforcement can be reduced to No. 3 bars at 12 inches o.c. each way for all bays, irrespective of the spacing of stiffening beams. g. The limitations of 7.10.2, p. 92, apply to reinforcing steel, beam stirrups for positioning longitudinal steel, and positioning of steel. Step 9a-If the required bottom steel (As), as computed in Step 8a above, is smaller than 0.65 in.2, then a. There is no need for additional top reinforcement (As) and the computation for As in Step 8 may be eliminated. b. The top slab reinforcement can be reduced in each direction to 0.18AS per foot of slab length. However, reinforcement cannot be less than that provided by bars placed at 12 inches o.c. If WWF is used, reinforcement can be further reduced to 0.15 As. In any case, this reduced top slab reinforcement should not be less than the WWF reinforcement specified for Type II slabs in Step 1 of pare. 1.3, pp 13-14. Step 10-Check dead weight of the designed slab. If it deviates from the estimated dead load (wd) by an amount which alters the total slab and superstructure load (w) by more than 5%, adjust the design for the actual slab dead weight and redesign.

OCR for page 9
20 RESIDENTIAL SLABS ON GROUND Step 11-Structurally design those slab bays between stiffening beams which receive concentrated or unduly heavy loads (pare. 4.7~. Step lla-Apply Steps 2, 4, and 5 for Type II slabs (pare. 1.3, pp. 13-14) to provide for unduly heavy partition loads (Step 2), signify cant concentrated loads (Step 4), or irregularities in slab surface (Step 5) whenever such conditions prevail. Step 12-Provide for uniform compaction over the entire slab site, in accordance with Fig. 22, p. 128. 1.5 Procedure When Type IV Is Recommended If a slab of Type IV is recommended, the ACI Building Code Re- quirements should be used for all Reinforced Concrete design as- sociated with this construction. 2.0 HEATED AND UNHEATED SLABS Only unheated slabs should be constructed without reinforcement, and then only on suitable soils. Since heated slabs will be subjected to stresses which are conducive to excessive and objectionable cracking, a residential slab which is to be heated by the inclusion of coils should be provided with minimum reinforcement as follows: O to 45 feet-not less than 6x6 - 10/10 WWF 45 to 60 feet-not less than 6x6 - 8/8 WWF 60 to 75 feet-not less than 6x6 - 6/6 WWF. Also, when heating ducts are to be embedded in or placed under the slab, the slab over the duct and extending for 19 inches on ei- ther side of the long axis of the duct (or to the slab edge if closer) should be reinforced with not less than 6x6 - 6/6 WWF reinforce- ment. Further, heating coils and ducts should be encased in not less than 2 inches of concrete.

OCR for page 9
CONCLUSIONS AND RECOMMENDATIONS 21 3.0 NONREC TANGULAR SLABS Certain special situations also require that slabs be reinforced. When slabs are of irregular shape, or where soil conditions are less adequate than for a Type I slab, or where other conditions for Type I slabs are not met, the slab should be provided with rein- forcement in accordance with pare. 2.0, above, as a minimum. Where Type I and II slabs have irregular outlines, as in the case of T- or L-shaped plans, these should be divided into squares or rectangles by the use of weakened plane joints. In Type II slabs, the WWF reinforcement should be continuous through the weakened plane j oint. In Type I slabs, however, 2 5- inch- wide WW F strips should be placed at each side but not carried across the weakened plane joint making the division. In fact, regardless of the reason for using WWF reinforcement in Type I slabs, it should never be carried across weakened plane joints. Whenever ducts or partitions pass through or over a weakened plane joint, a properly designed joint should be provided in the duct or partition. 4.0 SLABS ON UNSTABLE SOILS Expansive soils should not be singled out as the only problem area in residential slab construction. While it is recognized that the more readily apparent problems occur with these soils, all soils require and should receive careful consideration and evaluation, including compressible and poorly compacted soils. The criteria of 1.0 above provide for proper slab application on unstable soils consistent with present knowledge. 5.0 SLABS OF UNUSUAL CONFIGURATION It is not possible to evolve simplified slab design methods such as are presented herein, which will permit resolution of all problems which might be encountered in residential construction. For example, the irregular slab configurations shown under Step 5, pp. 15-16, are but a few of those which are possible. It must

OCR for page 9
2 2 RESIDENTL9-L SLABS ON GROUND recognized that, by designing of overlapping slab rectangles, full consideration will not be given the added and often eccentric loads that will be applied by virtue of upward or downward pressure on those portions of rectangles which do not actually overlap. There- fore, even though consideration has been given for general condi- tions in the design method recommended, i.e., modest areas of "no overlaps," when these areas of "no overlap" are large and/or eccentricities are pronounced, special care will be needed lest underdesign result. Also, where a planned carport will constitute an actual exten- sion of the residence, the carport slab should either be considered to be integral with the house slab and the total slab designed, or be designed and constructed as an independent slab with proper construction joints to permit movement between the two slabs and superstructures at all points of juncture. If an unbroken roof line is desired, the former solution would add significantly to cost, whereas the latter would be virtually impossible to achieve. If the carport is to have perimeter walls which can be made, in conjunc- tion with house walls, to impart rigidity to the entire house-plus- carport slab, it may well be possible to achieve a design which would simply necessitate extending same-size beams from house to carport. However, any such solution would need to flow from a careful analysis. Such specialized analyses are beyond the scope of this report. As stated on pp. 9-10, it is the basic principles set forth herein which should be satisfied. The absence of coverage of all conceiv- able problems neither invalidates the approach recommended nor does it imply adequacy when actual conditions obviously strain the credibility of the design methods presented. 6.0 QUALITY CONTROL PRAC TIC ES Design alone cannot ensure satisfactory performance of residential slabs-on-ground. Quality control practices are a necessary adjunct to any overall procedure for ensuring satisfactory slab performance. These practices, as outlined in Part B. should be considered as guides for use in conjunction with the design procedure recom- mended above.