R-3 Repository :: Browsing by Author "Krahl, Nat W."

Browsing by Author "Krahl, Nat W."

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A comparative study of flexibility in selected housing systems
(1971) Nara, Ricki Ryuichi; Krahl, Nat W.
This study is an evaluation of housing construction systems seen in the light of progressive technology and changing human environment. In the past, repetitive use of standardized complements and rationalized functions without broad flexibility for variation in plan, for change, or for user participation in the construction process itself, has resulted in a sterile and monotonous physical environment. The dominant construction systems must be derived from practical consideration of user requirements, modular planning, structural system and subsystems. The objective of this thesis is to contribute a solution to conflicts between technological innovation and human needs through the comparative study of several contemporary construction systems in the United States and Japan. To accomplish this objective, this thesis selects particular housing systems, sets up criteria for evaluation, and evaluates the housing systems.
A long-span cable network structure: an investigation of the relationship between space - structure - form in architecture
(1968) Mikos, Dimitrios; Todd, Anderson; Krahl, Nat W.
This study is based on an investigation into the relationship between space, structure and form in architecture. It is divided basically into three stages. In the first stage, I attempt to review and to clarify general architectural and structural ideas from the point of view of the composition of three basically different functions each of which would require, by itself, a structure of bridging long span. This stage of my research includes "problem No. 1" and "problem No. 2". In the second stage, architectural and structural problems are discussed which spring from the specific functions of a Natatorium, an Auditorium or a Gymnasium of the given scale. This stage includes "problem No. 3" of my research. The third, or last stage, includes a survey of various long-span possibilities, and the selection and development of a prototypical example that most directly, simply, and clearly houses the various functions. This resulting long-span cable network structure serves as a demonstration of the relationship between space, structure, and form.
A parametric analysis of a PRT system for southwest Houston
(1975) Smith, Rodney L.; Krahl, Nat W.
This thesis presents an analysis of several aspects of an emerging form of urban transportation - Personal Rapid Transit (PRT). The potential of PRT for some portion of the urban transportation demand is first discussed to determine if there is the need for the development and implementation of PRT. The advanced technology required for a city-wide PRT system is discussed in an effort to assess the availability of the required technology within a realistic time period. The urban design implications and the phasing of an extensive PRT system are discussed in order to determine the desirability of such a system. Indications are that there is a need for PRT in a comprehensive urban transportation network, that the necessary technology can be made available in a realistic time frame, and that PRT would probably have desirable effects on the development of an urban area. The rest of the thesis investigated the patronage estimates for several PRT system characteristics. Many proponents of PRT claim that a PRT system could effectively compete with the auto for trips made within a low density urban area. To test this, trips internal to the southwest quadrant of Houston were analyzed using computer simulations for test PRT and road systems. A parametric analysis was performed to determine the sensitivity of PRT patronage to several of the transportation system characteristics operating velocities, out-of-pocket costs, system configurations, etc. in an effort to determine PRT design characteristics and resulting patronage levels. Analysis of the test PRT systems indicated that PRT patronage was most sensitive to the connection speed to transit (a 1 mph bus-type connection appears to be a necessity), transit wait time (a total wait time for transit of five minutes effectively eliminates PRT as a viable alternative to the auto), and a perceived "hassle factor" associated with utilizing transit. Average auto velocity and perceived auto operating cost also had significant effects for specific ranges of these two variables. Variations in total trip length, PRT fare, and PRT vehicle velocity were found to have only slight effects on PRT ridership. The configuration of the PRT system appears to significantly affect PRT ridership levels. Station placement was identified as a critical factor: a total walk distance of over 1, feet for a trip utilizing PRT effectively reduces PRT patronage levels to zero. The circuitousness of several PRT paths, when competing with the more direct paths of the auto network, is partially responsible for reduced PRT patronage levels. PRT paths closely paralleling freeways were projected to carry reduced loads due to the higher velocities and lower costs associated with auto trips by freeways. Simulations indicate that PRT would not carry a significant portion of the total trips internal to southwest Houston. Longer trips, however, destined for activity centers appreciably beyond the study limits assessed with high auto parking charges were found to generally be more accessible to southwest Houston by PRT than by auto. PRT, therefore, would seem to function much more effectively as a feeder system to major activity centers than as a local transportation system for a low density urban area.
A study of a stiffened suspension roof structure
(1977) Clift, Charles Dunaway; Krahl, Nat W.
A stiffened suspension roof structure is studied under certain prototypical conditions. The geometry and nature of the structure is of a character that has not yet been built. The nonlinear response of the structure to load is analyzed by a three-dimensional elastic frame computer program. This program is written for use on the IBM37 computer at Rice University. A prototypical structure is designed and analyzed to meet satisfactorily the requirements applicable to a real project. The preliminary design is based on the assumption that the cable network takes the shape of an elliptic paraboloid under a uniformly distributed dead load. The gridwork is designed to provide a desired stiffness between hanger nodes in the cable network. The resulting member forces and displacements calculated by the analysis can be used to refine and optimize the design. Linear and nonlinear results of displacements are presented for comparison. The weights of the structural steel of the roof system are compared to existing structures of comparable scale and function. The design and weights of the supporting structure, columns, back-stays, foundations etc., are not included. The results of the analysis demonstrate that the design of the prototypical structure provides a good solution to the requirements for functioning as a long span roof system. The weight of structural steel for the roof of the prototypical structure is less than half as much as that which is required for a space grid type system. A nonlinear analysis for similar structures is recommended as significant changes compared to linear results were found, especially for unsymmetrical loadings.
A study of parameters in the diagonal tension cracking of concrete beams
(1966) Ferrell, Charles Stuart; Krahl, Nat W.
This is a study seeking to gain additional information concerning the diagonal tension cracking of reinforced concrete beams without web reinforcement. The parameters which affect diagonal tension cracking have been defined by previous studies; however, much work needs be done in order to predict accurately the magnitude and influence on behavior of these parameters. The study was conducted in two phases; an analytical investigation of the amount of shear transferred across the face of a diagonal tension crack by the longitudinal steel and of the influence which the crack starting point has on tensile cracking, and an experimental investigation of the ratio of shearing stress to flexural stress at the root of a diagonal tension crack. The results of the analytical investigation confirm the importance of including the "dowel force" in any analysis of the behavior of reinforced concrete beams without web reinforcement when subjected to combined shear and bending moment. New insight was also gained as to the effect of crack starting point on crack development, and hence on beam behavior. This study included both beams of normal weight and lightweight structural concrete, and the results of the computations made in this study show about the same amount of accuracy for beams of both types of concrete. The experimental investigation showed that the ratio of shearing to flexural stress at the root of a crack was greater than previously assumed and that this ratio seems to be inversely proportional to a function of the uncracked depth of the beam.
An analytical study of the behavior of prestressed composite beams
(1966) Reagan, Ronald Steven; Krahl, Nat W.
This thesis presents an analytical study of the behavior of prestressed composite beams. The particular configuration of prestressed composite beam considered is a simply-supported steel beam, prestressed by a high-strength steel tendon with a constant eccentricity, and attached to a concrete slab by shear connectors designed to insure complete interaction. A numerical method for analyzing the statically indeterminate beam was developed and used as a basis for a computer program which found the strains and deflections at a discrete number of equally spaced points along the length of the beam for increasing values of load up to the failure load. These strains and deflections satisfied the stress-strain relations for the materials, the static equilibrium of the beam, and the compatibility of deformation of the tendon. Hognestad's stress-strain relation was assumed for the concrete slab, an elasto-plastic stress-strain relation was assumed for the steel beam, and an actual stress-strain curve was used for the tendon. The numerical procedure combines the method of tangents for solving simultaneous nonlinear equations with a method of successive approximations. A simplification of this program was devised to analyze conventional composite beams. Detailed studies were made of some prestressed composite beams suitable for highway bridges and some suitable for buildings. The effect on behavior of variation of prestress force and tendon size were investigated and compared with the effect of variation of cover plate size for conventional composite beams. These studies show that prestressing a composite beam is an effective means of increasing the load capacity of the beam at all of the following stages of behavior: load causing allowable steel stress, load causing yielding of steel beam, and ultimate load. Prestressing does not significantly increase the load causing the allowable concrete stress nor does it significantly reduce live load deflection. The behavior of a prestressed composite beam is shown to be not very sensitive to variation of slab thickness. The validity of the method of analysis was verified by the comparison of analytical and experimental results for several prestressed and conventional composite beams which were tested in the laboratory by other investigators.
An experimental and analytical study of prestressed composite beams
(1964) Stras, James Campbell; Krahl, Nat W.
This project was an experimental and analytical study of pre-stressed composite beams. The type of beam studied consisted of a prestressed steel beam with a concrete slab attached to its top flange by means of shear connectors. The steel beam itself consisted of a rolled steel beam prestressed by rneans of a high strength steel cable attached to the bottom flange. Three beams were built and tested to failure, and the experimental results were compared with calculations based on existing theory. In addition, an analytical method for predicting the ultimate load of a prestressed composite beam was developed and predictions based on this method were compared to the experimental results.
An investigation into the failure of neat cement cylinders under independently controlled confining pressure, pore pressure, and axial force
(1961) Victory, Sidney P; Krahl, Nat W.; Sims, James R.; Rogers, John J. W.
The purpose of this investigation was to study the effect of independently controlled confining pressure and pore pressure on the ultimate axial compressive strength of neat cement cylinders. In trying to achieve this purpose the ultimate strength, deformation, and volume change of the material have been studied. The test program consisted of a variation in confining pressure and pore pressure from 0 to 20, 000 psi. For any one test the confining pressure and pore pressure were held constant and the axial load was increased until failure of the specimen occurred. The results indicated that Mohr's Theory by no means begins to explain the complex nature of the failure of the material. An envelope of failure was defined by the circles of stress; however, the failure angles indicated by Mohr's graphical solution did not compare well with those obtained directly from the specimens. An empirical relation of the form: fm= fc + a p 0.81was presented. This relation was found to represent data previously presented as well as the data in this investigation.
Deformations and moments in circular plates of linearly variable thickness under self-weight
(1968) Norwood, Edwin B; Krahl, Nat W.
This thesis reports an analytical solution for circular plates of linearly variable thickness, with a central circular hole, under self-weight. The plate is oriented in a horizontal position. The thickness decreases linearly with the distance from the center of the plate. Two cases are considered: Case I has the inner edge simply supported and the outer edge free; Case II has the inner edge rigidly supported and the outer edge free. The differential equation for both cases is solved in closed form for slope. Equations for radial and tangential moments are obtained directly from the slope equation. Deflections are calculated from slopes by numerical integration using Simpson's 1/3 rule. Numerical results for practical problems may be obtained either by use of a computer program which is presented or by use of tables of dimensionless coefficients. Use of computer program and tables is explained by an example.
Experimental analysis of the bending of circular plates using reflected light
(1960) McInnis, Bayliss C; Sims, James R.; Krahl, Nat W.; Paslay, Paul R.
Experimental analysis of thin flat plates to determine moments and deflections using the method of reflected light and the particular technique developed by Ligtenberg is investigated. Circular plates of two thicknesses and two methods of loading are used to evaluate the method. The experimental results obtained for moments and deflections are compared with analytical solutions in each case using the usual thin plate theory. It was concluded that the Ligtenberg method can be used to advantage by engineers in analyzing plates which because of their shape support conditions, or method of loading are not easily analyzed using purely analytical methods.
Fatigue tests of axially loaded K-joints
(1980) Nezames, Theodore Hercules; Merwin, John E.; Bourland, Hardy M.; Krahl, Nat W.
This thesis deals with the procedures, equipment and results of testing large scale, tubular, axially loaded, simple K-joints to failure in fatigue. Each K-joint had a 2" diameter chord, a 18" diameter 3° branch and a 12 3/4" 9° branch. Measures were taken to ensure axial forces with little bending in both branches. This was accomplished using a specially designed load frame with capability for full adjustment of alignment. The design is presented in an appendix. Sophisticated load control was used to apply a variable force of constant peak amplitude in which the mean was zero. Various methods of crack initiation detection are discussed ranging from acoustic emissions to visual inspection. Hot spot strains were measured using rosette type strain gages with locations determined from a finite element program. Data such as strains, load versus displacement loops, and crack lengths were taken frequently throughout the test. Substantial amounts of accumulated data, which include strains and load displacement loops, are placed in the appendicies. Test results are presented in graphical form and tables using information calculated from the raw data. Complete crack progression data were kept throughout each test and presented on drawings of projections of the weld toe onto a plane, and in graphs of crack length versus cycles for the major cracks.
Parametric studies of a stiffened suspension structure
(1978) Baldridge, Joseph Filmore; Krahl, Nat W.
The behaviors of various stiffened suspension structures were studied under various loading conditions. The properties and dimensions of the structural members were changed in order to study how these variations affected the behavior of the structures. Each structure was a synclastic suspended two-way orthogonal cable network with stiffening gridwork. The responses of the structures under load were analyzed by a three-dimensional elastic frame computer program with nonlinear capabilities, which was developed at Rice University. The structures were analyzed and designed to meet present structural requirements. For ease of presentation and applicability to other structures, the results of the analyses have been presented with respect to two dimensionless parameters. The two parameters were general measurements of the stiffhess of the structure and the finenesses of the stiffening gridwork and cable network. The structural responses studied were cable tensions, structure deflections, and stiffening gridwork member forces. The design and weights of the supporting structure, foundations, columns, etc., were not considered. The structures were analyzed for six loading conditions. The stiffening gridwork for each was composed of steel trusses and open-web bar joists. For the gridwork members in one structure nine flexural stiffnesses were computed based upon nine span-to-depth ratios. With the stiffening gridwork having each of the above flexural stiffnesses, the structure was analyzed under each loading condition. In the various structures, the center-to-center spacing of the cables and gridwork members varied in a range from fifty feet to one hundred feet. A structure with a span-to-depth ratio of fifty was scaled according to the laws of dimensional analysis so as to be one and one-half times its original size and was analyzed using the same loading conditions. Within the limits of the results obtained by the computer analyses, the structures studied appeared to be feasible and to meet AISC allowables. However, since the analyses were performed assuming the loads to have been concentrated at the joints, the analysis did not account for local bending produced by the application of loads between joints. Assuming the loads to have been uniformly distributed, the gridwork members to have been continuous, and the node points to have been unyielding pinned supports, the distribution of moment produced under these assumptions was superimposed onto the moment distribution obtained from the computer analyses. These combined moments were used as approximations of the true moments which might have occurred in the gridwork. Under the limitations of this study, one attractive solution for a structure of this type would have a fairly fine gridwork (5 to 8 bays) with a span-to-depth ratio of about sixty.
Response of shear wall-frame buildings to skewed wind load
(1978) Ashcraft, Douglas G; Krahl, Nat W.
Results of an analysis performed on two 4-story shear wall and frame buildings are studied to determine the response of the buildings to wind load from any one of several directions. The two buildings, one of square plan and the other of rectangular plan, are designed to represent typical shear wall-frame buildings. The interaction between the wall and frame under lateral load is discussed to lay the groundwork for the interpretation of results found from the analysis. By modifying analytical techniques derived by others, a model is obtained suitable for analysis by computer with loads representing those which would exist under wind from any one of several directions. The analysis performed is static, elastic, and linear with internal member forces and deflections as the output data. Data from the model analysis are then used to determine member forces and stresses of the actual buildings as well as to study their deformation characteristics. Results show that maximum stresses due to lateral load in several different portions of the shear cores of both buildings are due to wind from different directions depending on which section of wall is considered. In the square building, the maximum stress caused by wind from an angle of 45 degrees to the major axes of the building is 25 percent greater than the maximum stress caused by wind perpendicular to a building face, but the 45-degree stress governs the design of only 14 percent of the shear core. Design of other sections of the square shear core are controlled by stresses caused by wind from other directions. Similar results are seen in the shear core of the rectangular building. Differences in the manner in which two different perpendicular frames interact with the shear core in the rectangular building is found to cause the deformation of the building under skewed wind load to exhibit a non-planar characteristic of small magnitude. The gravity load sizes of the beams and columns in the frames of both buildings are adequate to resist all skewed wind loads. Recommendations for areas of additional study are made.
Some computer aids to architectural practice
(1972) Dev, Gita; Krahl, Nat W.
This study builds a case for an inner-office computerized information system that would increase the efficiency of the design process in the context of an architectural firm. The problem is looked at from two levels: (1) The overall operation of the office. (2) The detailed working of one division of the office -- in this case, the architectural programming and planning division which deals with the initial stages of the design process. At the division level, the development of and specifications for three computer programs are discussed. All three have a hierarchical tree-like data structure and are used for storage of the inventory of spaces in a project, building efficiency analysis and a plotting routine, respectively. At the overall level, the general approach towards developing an information system for shared data is outlined and one module of the proposed system, the space-type file, is described in further detail. Also demonstrated is the use of Linear Programming as a tool in solving architectural problems involving decision making. An example of a space allocation problem for a community college is formulated in Linear Programming terms.
Strength characteristics of mortars containing dispersed fibrous reinforcement
(1961) Weidler, Jay Benoir; Smith, R. B. L.; Krahl, Nat W.; Wilhort, J. C., Jr.
The purpose of this investigation was to gather and interpret data concerning the effect of adding fibrous reinforcement to mortar during the mixing phase. The fibers used in the investigation may be generally., described as short, steel wire segments, having length to diameter ratios from forty to one-hundred and sixty, and possessing either good or poor adhesion with the matrix. The composite material was subjected to both direct and indirect tension, unconfined compression, and flexure. Increases in strength were noted for the composite material over the plain mortar in all cases but a few of the compression tests. The maximum gain in strength was slightly over one-hundred percent found for a six percent inclusion of one-inch galvanized fibers under an indirect tensile load. The results of the tension tests indicated that the increase in tensile strength is directly proportional to the percentage of fibrous material in the mix. The slope of this line is a function of the type and length of fiber, the state of stress in the matrix, and the induced orientation of the fibers.
The application of shallow shells in structural floor systems
(1967) McRae, John Malcolm; Krahl, Nat W.
The purpose of this thesis is to demonstrate the application of shallow concrete shells for structural floor systems. The shell is first placed in its historical context. The writer then proposes the application of shells for floors through rationale based on three points: (1) historical precedent; (2) structural capability; (3) aesthetic qualities. The application having been proposed, the writer then proceeds to an analysis of the problem - from the simplest stages to the more complex applications. The specific criteria which are the basis of the application of shallow shells for floors follow the analysis. The last part is a demonstration of several prototype systems of shells for floors.
The failure of concrete under triaxial stresses
(1960) Erkmen, Erkin; Sims, James R.; Krahl, Nat W.
This study was done to investigate the behavior of concrete under three-dimensional stresses with two objects in mind. The first object was to determine the effect of the lateral stresses on the longitudinal strength of a concrete specimen with empty voids. The other was to determine the effect of the interstitial pressure on the strength of the mnterial. To achieve these two objects cylindrical concrete specimens were subjected to triazial compressive stresses with two lateral stresses constant and equal and the specimens brought to failure under axial compression. Some specimens were tested with empty voids and others, with a liquid under pressure in the pores. To create the first of these conditions a rubber tubing was used on the specimen to protect it from the liquid pressure. In the second case the liquid was free to penetrate the specimen voids. The relative difference, between the two, in the strength of the material gives the effect of liquid under pressure in the pores. The influence of lateral stresses on the axial strength of a concrete member has a direct application in reinforced concrete columns. Also, this influence would help us to understand the behavior of concrete under various stresses and be useful in formulating a theory of failure. The case of liquid under pressure in the voids has a direct application to structures that are subject to hydraulic pressure. This is called the effect of pore-water pressure, or uplift, and it is important to understand its effect on stability for the design of any hydraulic structure. It is suggested by the latest theories that this effect can be determined only by experimental means. There is at present no definite idea about either the exact potential area of surface or the part of it on which the pore pressure acts.
The influence of local stresses on the behavior of concrete beams
(1964) Durrani, M. Khalid; Krahl, Nat W.
This thesis reports an analytical investigation into the importance of local stresses in influencing the behavior of concrete beams under combined moment and shear. A method of analysis which has recently become available was used to make calculations predicting the behavior of certain beams which were tested earlier by others and reported in the literature. Results of these calculations were compared with test results and also with computations made earlier which neglected the effect of local stresses. In all of the beams considered, the local stresses were created by the application of loads and reactions through bearing on the top and bottom surfaces of the beams. All of these beams were rectangular beams with longitudinal reinforcement but without web reinforcement. This investigation indicates that local stresses created by bearing tend to stabilize the propagation of tensile cracks as they approach the load points, and to increase predictions of beam ultimate load capacity, but usually the magnitude of the influence is small. One particular beam and crack path were studied for which consideration of local stresses gave rise to a significantly different prediction of failure from that predicted when local stresses were neglected, but in all other cases the predictions of behavior at failure were very similar, both with and without local stresses. As expected from a consideration of Saint-Venant's principle, the quantitative effect of local stresses is negligible at considerable distances from the loads and reactions. In overall effect, the influence of local stresses on beam behavior seems less important than certain other factors, for example, the amount of shearing force transferred across a tensile crack by dowel action of the longitudinal reinforcement.
Two examples of computer applications in the architectural field
(1970) Hernandez-Jaime, Enrique; Krahl, Nat W.
Computers not only extend our capability for handling large amounts of data and perform sophisticated processes, they also extend our capabilities for making better decisions and exploring widely and deeply different alternatives that otherwise could only be mentioned in passing. Computers have had a great impact in all kinds of human activities, yet for the most part architects were, and some are still reluctant to their application in this field. It is argued that computers can not solve architectural problems, owing to the great subjectiveness involved, or that they tend to obliterate creativity. It is intended here to demonstrate, in the first case, that the computer is not used to "solve" the problem but to help to analyze and make better decisions in a more rational and systematic way and, in the second case that this help will broaden architect's creativity by helping him respond realistically to the problems and by increasing his means to test the validity of his solutions. It is assumed that it is still, and will always be, up to the architect to be creative using computers, and not the reverse. It is only through a systematic approach that problems will be stated properly and that solutions can be expected to approach or reach optimality. Intuition, experience and creativity must be present throughout the whole process of architecture, complementing rather than supplementing rational decisions. Computers do not provide merely numbers, but most importantly the means for evaluating the consequences of making decisions, before they are implemented. Part I of this work is a brief introduction to the ideas leading to the author's arguments, and exposes in detail two cases studied, one for data processing and one for problem solving. Documents complementing this discussion are presented for reference as appendices. Part II deals with some references about experiences already obtained in operations research work, continuing with part III which deals with the relevance of computers in the architectural process, and the importance of interfacing devices (Input/Output). Part IV is a brief summary and statement of conclusions and finally part V presents appendices and a bibliography.
Vaulted brick construction in Guadalajara
(1966) Krahl, Nat W.; Ransom, Harry S.; Houston, Rice University, Dept. of Architecture: New York, distributed by Wittenborn

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