Downloaded from ascelibrary.org by New York University on 05/11/15. Copyright ASCE. For personal use only; all rights reserved. RAMMED EARTH H O U S E CONSTRUCTION By Surjya K. Maiti1 and Jnanendra N. Mandal 2 INTRODUCTION In the third world countries, a vast population lives in rural areas, and many of them live in houses m a d e u p of locally available materials like earth, timber, bamboo, etc. In m a n y instances, r a m m e d earth is used to construct the walls, and bamboo a n d timbers are used to construct the roof and other supporting structures. Walls are uniform in thickness, which vary usually from 457.2-609.6 m m , and their dimensions d e p e n d on the space required. For example, a typical two room house with two stories may have the following dimensions (Fig. 1): height H = 7,620 mm; wall thickness, t = 547.2 m m ; length, b = 6,096 m m ; and breadth, h = 3,048 mm. The roof is covered by a material such as straw, grass, palm leaves, tiles, asbestos, etc. Its weight occasionally increases because of snow, rain, etc., and is transmitted to the ground through the walls. The walls are therefore subjected to w i n d load a n d other dead and live loads. The wind thrust is a variable factor; its maximum depends on the geographical location of the house. Rough estimates of the thrust are obtainable from the building codes, e.g. Ref. 1. Generally, the roof edges AB, BC, etc., are m a d e to terminate with a wide gap with the walls a n d lie at a low level above the ground to protect any erosion by rainfall. The wall thickness remains almost constant throughout the life of the building. Interestingly, technological principles have b e e n applied mostly to improve, rationalize, and economize urban housing construction, but n o similar effort exists for rural r a m m e d earth houses. Some of the points that are the most important and d e m a n d investigations are: 1. H o w safe are these constructions? 2. Can the volume of materials generally employed be reduced without sacrificing safety? 3. H o w high the walls can be made? 4. H o w can the cost of a house be reduced? In spite of the vast population involved and the economic implications of any means to reduce the cost, n o attempt has yet been m a d e to exAsst. Prof, of Mech. Engrg. Dept., I.I.T. Bombay-400 076, India. Asst. Prof, of Civ. Engrg. Dept., I.I.T. Bombay-400 076, India. Note.—Discussion open until April 1, 1986. To extend the closing date one month, a written request must be filed with the ASCE Manager of lournals. The manuscript for this paper was submitted for review and possible publication on March 13, 1985. This paper is part of the Journal of Geotechnical Engineering, Vol. I l l , No. 11, November, 1985. ©ASCE, ISSN 0733-9410/85/0011-1323/$01.00. Paper No. 20116. 2 1323 J. Geotech. Engrg. 1985.111:1323-1328. Downloaded from ascelibrary.org by New York University on 05/11/15. Copyright ASCE. For personal use only; all rights reserved. FIG. 1.—Typical Two-Room Two-Story Rammed Earth House amine these problems. These issues are addressed in this technical note. Some related issues have been considered earlier in Ref. 2. ANALYSIS The analysis is done considering elastic bending of the wall under the action of the wind thrust and direct compression due to all other loads that act mostly in the downward direction. The wind thrust is assumed to be a constant over the whole height. It is also assumed that earth cannot take any tension and is in a dry state. Our main object is to present results for the wall of a house of the type shown in Fig. 1. We include some results for a lone wall (Fig. 2), which may be of some academic interest. STRESS CALCULATIONS FOR WALL The wall can be treated as a cantilever beam (Fig. 2). The maximum bending moment, M max , is given Mmax = pbh2 y — (1) and the corresponding bending stress (H\2 6Mmax U =3v ^~^r \i) • (2) in which p = the wind thrust; b = the wall length; and H = its height. 1324 J. Geotech. Engrg. 1985.111:1323-1328. Downloaded from ascelibrary.org by New York University on 05/11/15. Copyright ASCE. For personal use only; all rights reserved. FIG. 2.—Rammed Earth Wall The compressive force due to self-weight of the wall is ff» = p H (3) -. in which p = the specific weight of dry earth. The total tensile stress, CT, , and compressive stress, <TC , are given by ff (4) < = 3P ( f ) - P H <rc = 3p (jj + pH (5) The limiting wall height, H max , is taken to correspond to no tensile stress in the wall and is therefore obtained by putting crf = 0. That is ff, = 0 = 3p (-T 5 2 ) ~ Ptfmax or Hmax = — (6) STRESS CALCULATIONS FOR HOUSE WALL (FIG. 3) It is considered that bending results from the wind thrust acting over an area greater than (b x H) because there is an extra obstruction to the wind from the roof. The separation between the rooftop and walltop is usually a constant and is of the order of 1,524-3,048 mm. The maximum bending moment bp(H + H')2 Mmax = - ^ - ^ '- (7) in which H' = the extra height to account for the roof. The correspond1325 J. Geotech. Engrg. 1985.111:1323-1328. Downloaded from ascelibrary.org by New York University on 05/11/15. Copyright ASCE. For personal use only; all rights reserved. FIG. 3.—Schematic Representation of Rammed Earth House Wall ing bending stress _M max /z „ iyi = 1 2 ^ in which Ixx = the moment of inertia of the whole wall about the centroidal axis, xx. Compressive stresses, vw, are developed due to selfweight plus the extra weight, W , which is transmitted to the ground through the walls. Therefore W X <ja = P .Y.V (9) H + — in which A = the total wall area. The total tensile stress, cr,, and compressive stress, crc, in the walls are given by ff = ' "wT"l pH + T j ••••(10) Mmaxh I W'\ - — ^ ~ ]- p H ! (11) 2IXX V A/ The limiting wall height, H max , which corresponds to no tensile stress in the wall, is calculated by putting a, = 0. This gives Mmsxh I W\ CT -i7r-r-»+TJ=o (l2) 1326 J. Geotech. Engrg. 1985.111:1323-1328. From which H milx is obtained as H max = a0 + Vflo - 4fe0 (13) Downloaded from ascelibrary.org by New York University on 05/11/15. Copyright ASCE. For personal use only; all rights reserved. in which a0 = 2Ixxp/bhp - H'; and b0 = (H'f - 4IxxW'/bhpA. SUMMARY For the coastal regions of eastern India wind thrust, p, is 0.001962 MPa, approximately (1). The safe height of a lone wall (Fig. 2) of thickness t = 457.2 mm corresponding to this thrust is obtained from Eq. 5 as 592 mm using specific weight of earth p = 1,700 kg/m 3 . For the wall of a house of the type shown in Fig. 3 with height, H = 7,620 mm; wall thickness, t = 457.2 mm; length, b = 6,096 mm; breadth, h = 3,048 mm; moment of inertia, Ixx = 10.56 x 1012 mm4); and roof height, H' = 3,048 mm, the maximum bending stress is obtained from Eq. 7 as 0.0982 MPa. The compressive stress due to all the loads acting downward is 0.1588 MPa, which is about 1.60 times the bending stress. In this calculation, the roof weight and other dead and live loads has been taken as 25% of the wall weight. A reduction in wall thickness by a third (keeping all other dimensions unchanged) increases the bending stress to 0.1276 MPa, thereby the compressive stress (0.1588 MPa) becomes 1.25 times the bending stress. This therefore means that the wall is still safe; there is no tensile stress in the wall. Such a reduction has tremendous implications because it can lead to a substantial savings in both material and construction costs and a greater availability of space. The tension side of the wall is thus subjected to a stress of very low order of magnitude but that on compression side is quite high, 0.2864 MPa in the case of the reduced wall thickness. Since the compressive strength of hard clay is greater than 0.4 MPa (3), the compressive stress in the rammed earth wall, which is in a dry condition, can be taken to be easily sustainable. Thus, the wall is free of any danger of a compression failure. CONCLUSIONS An elementary analysis has been presented for the calculation of stresses in the wall of a rammed earth house. The wall thickness (greater than 457.2 mm) is quite safe. Increasing the thickness beyond 457.2 mm does not lead to any extra advantage. The wall thickness can be reduced by a third (from 457.2 mm) easily without endangering safety. Such a change will result in a reduction of both construction and material costs and an increase in the space available. The safe height of a lone wall depends on its thickness and the maximum wind thrust. It can be determined using relation 6. The safe height of wall of a house depends on the wind thrust, total weight carried by the wall, and the moment of inertia and area of the wall. It can be calculated using relation 13. APPENDIX I.—REFERENCES 1. Indian Standard Code of Practice for Structural Safety of Buildings: Loading Standards, IS:875, 1964. 1327 J. Geotech. Engrg. 1985.111:1323-1328. Downloaded from ascelibrary.org by New York University on 05/11/15. Copyright ASCE. For personal use only; all rights reserved. 2. Maiti, S. K., and Mandal, J. N., "An Analysis of Timber Earth Composite Beam Used in Rural Housing," submitted for publication, Journal of Geotechnical Engineering, ASCE. 3. Ramiah, B. K., and Chickanagappa, L. S., Soil Mechanics and Foundation Engineering, Oxford and IBH Publishing Co., 1981, p. 15. APPENDIX II.—-NOTATION The following symbols are used in this paper: A ao,h b H H' ^max h *xx Mmax V t W P 0"!, CTC CT( 0"K = = = = = = = = = = = = = = = = = wall area; constants defined in text; length of wall; height of wall; roof height; maximum height; breadth of wall; moment of inertia about x-x; maximum bending moment; wind thrust; thickness of wall; sum total of dead and live load; specific weight; bending stress; compressive stress; tensile stress; and compressive stress due to direct loading. FLEXURAL BEHAVIOR OF REINFORCED SOIL BEAMS By A. P. Chaudhari1 and A. N. R. Char2 INTRODUCTION Zones of tensile stresses develop in earth structures such as embankments, dams, and multilayer pavements due to flexure. Investigations by Ajaz and Parry (1,2), Ajaz (3), and Leonards and Narain (6) show formerly, Postgrad. Student, Dept. of Civ. Engrg., Indian Inst, of Tech., Kharagpur, India. 2 Prof., Dept. of Civ. Engrg., Indian Inst, of Tech., Kharagpur, India. Note.—Discussion open until April 1, 1986. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on March 6, 1985. This paper is part of the Journal of Geotechnical Engineering, Vol. I l l , No. 11, November, 1985. ©ASCE, ISSN 0733-9410/85/0011-1328/$01.00. Paper No. 20116. 1328 J. Geotech. Engrg. 1985.111:1323-1328.