Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1935 A study of the braced rafter roof George E. F. Pickard Iowa State College Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Bioresource and Agricultural Engineering Commons, and the Construction Engineering Commons Recommended Citation Pickard, George E. F., "A study of the braced rafter roof" (1935). Retrospective Theses and Dissertations. 16397. https://lib.dr.iastate.edu/rtd/16397 This Thesis is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. A STOTt OP THE BRACED RAPTEH ROOF y>7 G^eorgo E. F. Plolcard A Thesis Sxibmltted to the Graduate Faculty for the degree of MASTER OF SGIESOE Major Subject Agricultural Engineering Approred: Zn charge of Major Work J> B. PaTlflson Read of Major Department R. g. BuQhanflW Dean of Graduate College Iowa State College 1936 TABLE OF CONTENTS Page I. Introduotlon 7 A. Justification for the Study 7 * B. Ohjocta of the Study 9 IX, RlfltorloaX 10 A. The Project 10 B. Heviev of lilterature 11 III. The Inreetigatlon 13 A. Preliminary Considerations and Investigations 13 1. Analysis of the problem 13 a. Service requirements of the roof 13 b. Structural requirements 14 o. Economic requirements 14 d. Aesthetic requirements 1& Z, Selection of a standaz*d bam shell 15 a. Dimensions 15 b. Eoonomlo use of materials 16 o. The shape of the roof to provide stability 17 3, Wind-load distribution 21 a. The old methods of calculating wind loads 22 -3- Page b. A discerning method of oaloulatlng wind loads 25 o. Vlnd-load dlstrlbutloa 25 4« lavefltlgatlon of reaotlons and bending momenta 26 a. Objects of the study 26 b. Stress analysis 27 o. Conclusions 30 B. Experimental 31 1. Tests of sheathing 31 a. Object of the study 31 b. Analysis of problem 32 o. Apparatus and method of prooedure 33 2. Tests of Joints 35 a. Introduotlon 35 b. Method of testing 36 o. Discussion of results 50 d. Goncluslons 53 3. Preliminary designs of rafters 59 a. Introduction 69 b. Method of testing <60 o» Dlseusslon of results 69 d. Conclusions 67 -4- Page 4. Testa of rafters 67 a. Introduotion 67 b. Selection of soale for models 68 o. Determination of loads on rafters 68 d. The test specimens 70 e. Apparatus and method of procedure 75 f. Results of the rafter tests 80 g. Discussion of results 90 h. Conolusiona 95 17. Summary 97 y. Oonolualons 98 ?I. Idterature Cited 99 TII. Aolcnovledgaents 102 -5- FIOUHES AHD TABLES Page Figure 1, Determination of Shape of Roof to Provide Stability 20 " 2. Roof Strees Analyste* Wind at 90^ to Side Wall 28 3. Roof Stress Analysis, Wind in ^d of Barn 29 4. Southwarlc-Emery Testing Machine 37 5. Joint Test Speoimens 37 6. Specimen of Wf^rped Glued Joint 39 7. Failure of Glusft Joints 39 8. Uodem Oonneotors 41 9. Assembly of Ring Joints 42 10. Bolted Joint in Testing Uachine 43 11. Failure of Hailed aad Bolted Joints 43 12. Imbedding Toothed Rings 46 13. Cutting Grooves for Split Rings 47 14. Oroove-outtlng Tools 47 15. Load-slip Curves for Joints 51 16- Strength Comparisons of Wnrped and Flat Glued Joints 52 17. Strength Comparisons of Full Size and Bodel Joints 52 18. Failure of Split-Ring Joints 54 -6- Pag< Plffur« 19. F^llure of Toothed-ring Joints 55 N 20. Preliminary Designs of Rafters 62 Tabl0 1. Testa of &aall Rafter Hodels; Wind at 90*^, (Windward Side) 64 • II. Tests of Small Rafter Hodels; Vind in End of Barn 65 II III. Sizes of Materials Used in Rafter Models 69 R IV. Wind Loadings; Wind at 90® to Side Wall 72 M V. Wind Loadings; fflnd in £nd of Barn 73 Figure 21. Oonstruotioa of Rafters Tested 74 H 22. Rafter Testing Apparatus 76 N 23. Rafter Loading Diagrams 77 » 24. Recorded Deflections of Rafters; Wind at 90® to Side Wall 61 « 26. Recorded Deflections of Rafters; Wind in End of Bam 82 H 26. V^-Deflection Curves for Rafters Tested 83 R 27. Failures of Rafter Ho. 1 85 II 28. Pailitres of Rafters 88 Table VI. Comparatire Strength of Rafters 92 VII. Oost of Materials for Rafters 92 -7- J' IBTTROOTOTIOH Justification for the Study Statistical study of losses due to wind damage of farm buildings has shoim Tsry definitely the need for more wind resistant construction. Data on constructional losses have been gathered throu^ study of the e^erience of the Icwa Mutual Tornado Insurance Association. For the three year pe riod 1930 to 1932, constructional damage to bams represented 55^ of the total losses in fawn buildings.(5) Of this damage to beras, QO% was described as complete demolition. A considerable percentage of this loss may be attributed to failure of some roof member* throwing stress into other parts of the structure which resulted in the final collapse of the building. Five and one-half percent of windstorm loss was paid for roofs blown off. This is almost always due to the lack of proper anchorage and wind bracing of the roof. The gambrel shape roof as used so widely for bams has three features which contribute much to Its popularity. First is its utility. It provides a large mow space in proportion to the roof area and the shape is well suited to the use of all hay handling equipannt* Secondly» the gambrel shape lends -8- Itaelf well to efficient use of building materlala and the labor inrolred in construction is not excessive. The third oomniendAble feature of the gaabrel shape is its pleasing appearance. In its lines it harmoniaes reasonably well with the rest of the farm buildings. During the past few years the gothio roof has enjoyed wide popularity. This is partly due to the novelty of its shape but it is very definitely deserving of favor on account of its pleasing appearance and the clear mow space it provides. Many gothic roofs have been unsatisfactory Deoause they have sagged out of shape. This can usually be traced to faulty con struction rather than to any weakness in the design. However, the usual types of ganbrel roof constractlon are not affected so critically by slighting of nateriale or faulty woitaanehip. The braced rafter type of construotion is very desirable for gambrel shaped roofs. Its ease of erection, reasonable economy of materials and fulfilment of the service requirements have made it the most popular type of gambrel roof construction, The rafters are self-supporting and light In weight making them easy to raise. After the first two rafters at the end of the structure have been raised into place# braced and tied together* a man can easily ollflii to a position near the ridge and, by means of ll^t boards, tie the succeeding rafters in place as they are raised into position. Use of standard length tlnbers -9- ellalnatee ezoeeslTe outting aad waste. The servloe require ments of a roof are rery well satisfied by the braeed rafter oonstruetlon. Objeotlonable purlin support oolimns are avoided and none of the braces project far into the mow spaoe. Adequate hel^t is provided and there are no obstructions to the use of hay handling equipment. The Objects of the Study The ptarposes of this study are to investigate the strength requirements of the braced rafter roof and to develop a mor* wind-resistant type of oonstractlon. This improved rafter should oomoare favorably in cost with present recommended de sign, Observations on wind damage to buildings Indicate that this improvement may be realized through; (1) Improved method of Joining members; (2) more secure anchorage of the rafter to the mil; and (3) more efficient and economical use of bracing material. -10- HISTORICAL The Projeot This study is a part of the general projeot of the Agri- oultural E3tp«riiie&t Station, Investigation of the Vlnd ai^ iPlro Losses to Para Buildings la lova**. Ths objeot of this division of the project Is to make a study of wind losses and building design In Iowa In order to seours mox^ wind resistant oonstruotlon. This worlc has been pursued In five fields of aotlvlty. Briefly, these are 1, Field observation of wind damage. 2. Statistical study of losses. ^ 5. Meteorological studies. 4. Aerodynamics. 5. Laboratory study. The first three have been discontinued for the time being at least. In the absence of conclusive and entirely dependable ex perimental work on the subject of wind pressures on farm buildings, the aerodynaraio study has become a matter of adapt ing the results of other Investigations to the problen in -11- hand. The wort: of suoh authorltlea as Dr. Prydea (6) of the Bureau of Standards has been of partlouls-r valu#. The laboratory study has centered around the testing of rafters and trusses. Loads sre applied to models of the structures to approxliaate wind loading conditions. Thus the strength, rigidity, and points of weakness of the various designs are determined. Studies hare been aade on Glyde truss construction (2) and on laminated bent rafters, (5) How this study of the braoed rafter roof takes Its plaoe as an integral part of the general Investigation. Review of Llteratuj^ • In his bulletin on Research In Farm Structures, Henry Glese (9) states the following: •The losses due to fire and wind are truly economic* While the Individual may be partly proteoted by Insurance, the loss must be paid ultimately by the agricultural Indus try * "Studies shotild be made as to the reasons for these losses and as to Improved construction methods for reducing future losses." In the bulletin Modem Oonnectors for Timber Construction, •12- prepa2*9d Jointl^r by the Hatloaal Ooamlttee oa Wood Utllisatioit and Forest Products X^boratox^, the authors (10) point out the need for a stronger Joint in wood construction.