- #Flexural stifness reduction factor wallap manual#
- #Flexural stifness reduction factor wallap software#
Table 4: Final allowable bearing capacity for allowable settlement = 25 mm and a given embedment depth. Table 3 is an extension of Tables 1 and 2 and shows how constant ratio is achieved for all nodes. Node 1 has a tributary area which is 25% of Node 81.
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Figure 4 shows the tributary area for different nodes. This explains Table 2, as it shows the ratio for Node 1 is different than other nodes. It can also be noted that all other nodes have same tributary area. If we look at the above example, Node 1 has a much smaller tributary area than the rest of the nodes. The base pressure is calculated at each support node by dividing the support reaction with the corresponding node tributary area. The program first calculates the tributary area for each node of the footing and then multiplies the modulus of subgrade reaction by the corresponding tributary area for each node to get the linear spring constant at each node.įor a concrete foundation analysis, those springs have to be defined as compression-only, as concrete is assumed not to carry any tensile force.
#Flexural stifness reduction factor wallap software#
The above described tributary area calculation is the key procedure used internally by the commercial software to calculate the linear spring constant. So, it could be assumed that the ratio of pressure intensity and settlement is constant.įigure 3: Node tributary area. It is also obvious that the pressure intensity at the center is maximum and reduces as the elements (or node coordinates) move away from the center. Figure 1b shows the soil pressure contour. The displacement diagram shows a dishing effect as discussed earlier. For this exercise, the software default value for the modulus of subgrade reaction was used. It was modeled and analyzed in STAAD Foundation as “Mat”, which is a flexible foundation the soil was defined using coefficient of subgrade reaction. Figure 1a shows a simple slab-on-grade foundation. Displacement reduces as it moves away from the center. A point at the center of the footing will experience the highest displacement. So the displacement diagram of a footing with a load at center will have a dishing effect. In other words, the ratio between pressure and settlement at all locations of a footing will remain constant. Where p = contact pressure intensity and s = soil settlementĪs Terzaghi mentioned, proper estimation of contact pressure for a flexible foundation could be very cumbersome, so it is assumed that Ks remains constant for the entire footing. Mathematically, the coefficient of subgrade reaction is expressed as: Numerically, kip/in 3 is correct but does not properly represent the physical significance of the measured value and could be mistaken as a density unit or a volumetric measurement. Some express this term in kip/in 3 (or kN/m 3) which can be misleading. For the English unit system, it is often expressed in kip/in 2/in in the SI system it is expressed as kN/m 2/m. This term is measured and expressed as load intensity per unit of displacement.
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Is there any relationship between bearing capacity and modulus of subgrade reaction? Modulus of Subgrade Reaction (Ks) As more and more engineers will use software to design foundations, it is essential for engineers to have a fundamental understanding of this soil parameter.
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Many engineers are not familiar with this term and often try to compare it with bearing capacity. However, these computer programs often ask for an input called “modulus of subgrade reaction”.
#Flexural stifness reduction factor wallap manual#
Manual computation of flexible analysis could be challenging and, in almost all cases, software programs such as STAAD, SAFE, GT STRUDL etc. But for large and multi column foundations, most engineers prefer flexible analysis. That particular assumption works well in practice for small and single column footings. However, that simplicity assumes the footing will behave as a rigid body. Because of simplicity and ease of use, this method is still the fundamental soil parameter for foundation design. The obvious reason is that basic examples given in most text books almost always use bearing capacity to calculate the plan dimension of a footing. Probably the most widely used value in a soil report is soil bearing capacity.