【正文】 =29o(silty finetomedium sand) ? Check ? OK? ? Solution 23:09 51 CONDITIONS FOR FROST HEAVE容易出現(xiàn)凍狀的土層 ? There is a nearby source of water ? The soil is frostsusceptible. ? Clean sand and gravel? ? Clays? ? Silt and fine sand? √ Frost line 凍深線 23:09 52 第九章 23:09:21 53 SPREAD FOOTINGS STRUCTURAL DESIGN DESIGN LOADS ? LRFD method ? Factored loads 23:09 54 Load factor, ? American Concrete Institute (ACI) Code ? U=+ () ? U=(++) ? U=+ ? U=++ ? U=++ ? …… ? U=(D+T) () ANSI/ASCE and AISE Code ? U= () ? U=(D+F+T)+(L+H)+(Lr or S or R) ? U=+(Lr or S or R)+( or ) ? …… ? U=+( or ) () 23:09 55 SUMMARY ? The plan dimensions and minimum embedment depth of a spread footing are governed by geotechnical concerns, and are determined using the unfactored loads. ? The thickness and reinforcement of a spread footing are governed by structural concerns. Structural design is governed by the ACI code, which means these analyses are based on the factored load. ? The structural design of spread footings must consider both shear and flexural failure modes. A shear failure consists of the column or wall punching through the footing, while a flexural failure occurs when the footing has insufficient cantilever strength. 23:09 56 ? Since we do not wish to use stirrups (shear reinforcement), we conduct the shear analysis first and select an effective depth, d, so the footing that provides enough shear resistance in the concrete to resist the shear force induced by the applied load. This analysis ignores the shear strength of the flexural steel. ? 5. Once the shear analysis is pleted, we conduct a flexural analysis to determine the amount of steel required to provide the needed flexural strength. Since d is large, the required steel area will be small, and it is often governed by rmin . ? 6. For square footings, use the same flexural steel in both directions. Thus, the footing is reinforced twice. 23:09 57 ? 7. For continuous footings, the lateral steel, if needed, is based on a flexural analysis. Use nominal longitudinal steel to resist nonuniformities in the load and to acmodate inconsistencies in the soil bearing pressure. ? 8. Design rectangular footings similar to square footings, but group a greater portion of the short steel near the center. ? 9. Practical minimum dimensions will often govern the design of lightly loaded footings. 23:09 58 23:09:21 59 CH10 MATS(筏板基礎(chǔ) ) DESIGN METHODS ? Rigid methods ? the conventional method, the conventional method of static equilibrium ? Assumption: ? The mat is much more rigid than the underlying soils ? No flexural deflection ? Bearing pressure distribution is linear ? 倒 梁 法 DRAW BACKS OF THE RIGID METHOD ? Widthtothickness is larger for mat foundations. ? Not consider redistribution of bearing pressure. ? Cannot produce reliable estimates of shears, moments and deformations in the mat NONRIGID METHODS CONSIDERING SOILSTRUCTURE INTERACTION ? Winkler method ? Coupled method ? Pseudocoupled method ? MultipleParameter method ? Finite element method Treat soils as “springs” Treat soils as “soil” WINKLER METHOD ? Linear springs ? No interaction between springs. PSEUDOCOUPLED METHOD ? Similar to winkle model, but ? Have different ks values depending on their location ? ks_outermost=2ks_innermost SUMMARY ? 1. Mat foundations are essentially large spread footings that usually enpass the entire footprint of a structure. They are often an appropriate choice for structures that are too heavy for spread footings. ? 2. The analysis and design of mats must include an evaluation of the flexural stresses and must provide sufficient flexural strength to resist these stresses. ? 3. The oldest and simplest method of analyzing mats is the rigid method. It assumes that the mat is much more rigid than the underlying soil. which means the magnitude and distribution of bearing pressure is easy to determine. This means the shears, moment, and deformations in the mat are easily determined. However, this method is not an accurate representation because the assumption of rigidity is not correct. ? 4. Nonrigid analyses are superior because they consider the flexural deflections in the mat and the corresponding redistribution of the soil bearing pressure. ? 5. Nonrigid methods must include a definition of soilstructure interaction. This is usually done using a bed of springs analogy, with each spring having a linear forcedisplacement function as defined by the coefficient of subgrade reaction, ks. ? 6. The simplest and oldest nonrigid method is the Winkler method, which uses independent springs, all of which have the same ks. This method is an improvement over rigid analyses, but still does not accurately model soilstructure interaction, primarily because it does not consider coupling effects. ? 7. The coupled method is an extension of the Winkler method that considers coupling between the springs. ? 8. The pseudocoupled method uses independent springs, but adjusts the ks values to implicitly account for coupling effects. ? 9. The multiple parameter and finite element methods are more advanced ways of describing soilstructure interaction. ? 10. The ks is difficult to determine. Fortunately, the mat design is often not overly sensitive to global changes in ks. Parametric studies are often appropriate. ? 11. If the Winkler method is used to describe soilstructure interact