Lime Stabilizes Poor Soils

Lime has been used for many years to stabilize road beds and air fields, but now lime is also being used to stabilize buildings sites. Lime stabilization is used primarily to upgrade poor quality clay soils in order to provide adequate subgrade support- a prime requisite for good concrete slab performance. Lime's use is especially valuable when expansive clays are encountered. Expansive clays have been known to crack concrete slabs or to create rough joints as the slabs heaved and then settled during wet and dry cycles. Lime alleviates this problem first by reducing the soils's expansive properties and second by forming a moisture barrier which helps prevent water from reaching the underlying expansive subsoil. Lime stabilization is essentially a type of chemical stabilization process involving hydrated lime. The technique is applicable to heavy clay and silty clay soils, and to plastic aggregates such as pit-run gravel. The reaction between the lime and the clay is essentially two fold: first, during mixing, the clay particles are brought together into one mass due to base exchange, forming coarser silt sizes. This reduces the plasticity and swell and increases the friability of the soil. There is also a pronounced drying action. Secondly, after compaction, the lime reacts with the clay to form a type of cement which binds the soil particles firmly and greatly increases the strength and stability of the soil. It also renders the soil relatively impervious to water. Lime stabilization of base soil can be accomplished in three ways: (1) conventional stabilization- This involves spreading lime, mixing and compacting it and then curing the resulting lime-soil. The end result is a well-cemented, stable layer, generally 6 inches thick. (2) Soil modification- this is similar to the above, except less lime is used. The soil will still be upgraded but to a lesser degree. (3) Post-treatment- here the virgin soil in place is impregnated with lime to a shallow depth. However, the soil is not mixed nor compacted. Instead, the lime is introduced by drilling, trench irrigation, or pressure injection.

Damping Strategies for Structural Systems

The direction of the evolution of tall building structural systems, based on new structural concepts with newly adopted high-strength materials and construction methods, has been towards augmented efficiency. Consequently, tall building structural systems have become much lighter than earlier ones. This direction of the structural evolution toward lightness, however, often causes serious structural motion problems – primarily due to wind-induced motion. From the viewpoint of structural material’s properties, due to the lag in material stiffness compared with material strength, the serviceability of the structure potentially becomes a governing factor in tall building design when high strength material is used. For instance, today, structural steel is available from 170 to 690 MPa (24 to 100 ksi). However, its modulus of elasticity remains nearly the same without regard to the change in its strength. The change of production process or heat treatment influences its strength but not the modulus of elasticity. Regarding concrete, increase in its strength results in increase in its modulus of elasticity, albeit increasing its brittleness. However, this increase in the modulus of elasticity is relatively small compared with the increase in strength. Thus, the lighter structures produced by high-strength materials can cause motion problems. The control of this structural motion should be considered with regard to static loads as well as dynamic loads. Against the static effect of wind loads, stiffer structures produce less lateral displacement. With regard to the dynamic effect of wind loads, not only the windward response but also the across-wind response of the structure should be considered. Generally, in tall buildings, the lateral vibration in the across-wind direction induced by vortex shedding is more critical than that in the windward direction.Regarding both directions, structures with more damping reduce the magnitude of vibration and dissipate the vibration more quickly. With regard to the vibration in the across-wind direction, a stiffer structure reduces the probability of lock-in condition because as a structure’s fundamental frequency increases, wind velocity that causes the lock-in condition also increases. Since the natural direction of structural evolution towards lightness is not likely to be reversed in the future, more stiffness and damping characteristics should be achieved with a minimum amount of material (Moon, 2005). Achievement of more stiffness in tall buildings is related to the configuration of primary structural systems, which were discussed in previous sections. For example, more recent structural trends such as tubes, diagrids and coresupported outrigger structures in general achieve much higher stiffness than traditional rigid frame structures. Obtaining more damping is also related to the choice of primary structural systems and materials. However, the damping achieved by the primary structure is quite uncertain until the building construction is completed. A more rigorous and reliable increase in damping, to resolve tall building motion problems, could be achieved by installing auxiliary damping devices within the primary structural system. The effect of such damping can be estimated relatively accurately. Thus, when severe wind-induced vibration problems are expected,

Premium for Height

The primary structural skeleton of a tall building can be visualized as a vertical cantilever beam with its base fixed in the ground. The structure has to carry the vertical gravity loads and the lateral wind and earthquake loads. Gravity loads are caused by dead and live loads. Lateral loads tend to snap the building or topple it. The building must therefore have adequate shear and bending resistance and must not lose its vertical load-carrying capability. Fazlur Khan realized for the first time that as buildings became taller, there is a “premium for height” due to lateral loads and the demand on the structural system dramatically increased, and as a result, the total structural material consumption increases drastically (Ali, 2001). If there would be no lateral forces on the building such as wind or earthquake, any high-rise building could be designed just for gravity loads. The floor framing system usually carries almost the same gravity loads at each floor, although the girders along the column lines need to be progressively heavier towards the base of the building to carry increasing lateral forces and to augment the building’s stiffness. The column sizes increase progressively towards the base of the building due to the accumulated increase in the gravity loads transmitted from the floors above.
If we assume the same bay sizes, the material quantities required for floor framing is almost the same regardless of the number of stories. The material needed for floor framing depends upon the span of the framing elements, that is, column-to-column distance and not on the building height. The quantity of materials required for resisting lateral loads, on the other hand, is even more increased and would begin to exceed other structural costs if a rigid-frame system is used for very tall structures. This calls for a structural system that goes well beyond the simple rigid frame concept. Based on his investigations Khan argued that as the height increases beyond 10 stories, the lateral drift starts controlling the design, the stiffness rather than strength becomes the dominant factor, and the premium for height increases rapidly with the number of stories. Following this line of reasoning, Khan recognized that a hierarchy of structural systems could be categorized with respect to relative effectiveness in resisting lateral loads for buildings beyond the 20- to 30-story range (Khan, 1969).
 

Preface

In recent years the subject of computer programming has been recognized as a discipline whose mastery is fundamental and crucial to the success of many engineering projects and which is amenable to scientific treatement and presentation. It has advanced from a craft to an academic discipline. The initial outstanding contributions toward this development were made by E.W. Dijkstra and C.A.R. Hoare. Dijkstra's Notes on Structured Programming [1] opened a new view of programming as a scientific subject and intellectual challenge, and it coined the title for a "revolution" in programming. Hoare's Axiomatic Basis of Computer Programming [2] showed in a lucid manner that programs are amenable to an exacting analysis based on mathematical reasoning. Both these papers argue convincingly that many programmming errors can be prevented by making programmers aware of the methods and techniques which they hitherto applied intuitively and often unconsciously. These papers focused their attention on the aspects of composition and analysis of programs, or more explicitly, on the structure of algorithms represented by program texts. Yet, it is abundantly clear that a systematic and scientific approach to program construction primarily has a bearing in the case of large, complex programs which involve complicated sets of data. Hence, a methodology of programming is also bound to include all aspects of data structuring. Programs, after all, are concrete formulations of abstract algorithms based on particular representations and structures of data. An outstanding contribution to bring order into the bewildering variety of terminology and concepts on data structures was made by Hoare through his Notes on Data Structuring [3]. It made clear that decisions about structuring data cannot be made without knowledge of the algorithms applied to the data and that, vice versa, the structure and choice of algorithms often depend strongly on the structure of the underlying data. In short, the subjects of program composition and data structures are inseparably interwined. Yet, this book starts with a chapter on data structure for two reasons. First, one has an intuitive feeling that data precede algorithms: you must have some objects before you can perform operations on them. Second, and this is the more immediate reason, this book assumes that the reader is familiar with the basic notions of computer programming. Traditionally and sensibly, however, introductory programming courses concentrate on algorithms operating on relatively simple structures of data. Hence, an introductory chapter on data structures seems appropriate. Throughout the book, and particularly in Chap. 1, we follow the theory and terminology expounded by Hoare and realized in the programming language Pascal [4]. The essence of this theory is that data in the first instance represent abstractions of real phenomena and are preferably formulated as abstract structures not necessarily realized in common programming languages. In the process of program construction the data representation is gradually refined -- in step with the refinement of the algorithm -- to comply more and more with the constraints imposed by an available programming system [5]. We therefore postulate a number of basic building principles of data structures, called the fundamental structures. It is most important that they are constructs that are known to be quite easily implementable on actual computers, for only in this case can they be considered the true elements of an actual data representation, as the molecules emerging from the final step of refinements of the data description. They are the record, the array (with fixed size), and the set. Not surprisingly, these basic building principles correspond to mathematical notions that are fundamental as well. A cornerstone of this theory of data structures is the distinction between fundamental and "advanced" structures. The former are the molecules -- themselves built out of atoms -- that are the components of the latter. Variables of a fundamental structure change only their value, but never their structure and never the set of values they can assume. As a consequence, the size of the store they occupy remains constant. "Advanced" structures, however, are characterized by their change of value and structure during the execution of a program. More sophisticated techniques are therefore needed for their implementation. The sequence appears as a hybrid in this classification. It certainly varies its length; but that change in structure is of a trivial nature. Since the sequence plays a truly fundamental role in practically all computer systems, its treatment is included in Chap.

13 structural steel buildings that dazzle

The winning projects and their respective team members were recognized on April 17 during 2013 NASCC: The Steel Conference in St. Louis. Each year, awards for each winning project are presented to the project team members involved in the design and construction of the structural framing system, including the architect, structural engineer, general contractor, detailer, fabricator, erector and owner. A panel of design and construction industry professionals identified National and Merit winners in three categories, based on constructed value: projects less than $15 million; projects $15 million to $75 million; and projects greater than $75 million. In addition, the panel awarded a Presidential Award of Excellence in Engineering to one project for structural engineering accomplishment. The 2013 award-winning projects are (photos and project descriptions courtesy