JP7728111B2 - How to design a building - Google Patents

Description

The present invention relates to foundation structures and methods for designing buildings and foundation structures.

The foundation structure type for mid- to high-rise buildings and super-high-rise buildings is generally selected based on the strength of the ground (supporting ground) that directly supports the base of the building. When the supporting ground is strong, a spread foundation such as a mat foundation is often selected, but when the supporting ground is not strong enough, a pile foundation may be selected. Piles are usually installed so that they reach the solid stratum, so when the solid stratum is deep, the piles become long, increasing the cost of the piles. For this reason, in recent years, instead of traditional pile foundations, foundation structures that combine a minimum number of friction piles with a spread foundation to keep the settlement of the building within the required performance values are sometimes used. This type of foundation structure, which is intermediate between a spread foundation and a pile foundation, is sometimes called a piled raft foundation (Patent Document 1). Because piles do not need to reach the solid stratum and the number of piles can sometimes be reduced, piled raft foundations are considered an effective method for reducing costs and construction time.

JP 2010-59655 A

Pile raft foundations are a rational foundation structure, but the design generally becomes more complex as it is necessary to consider the interaction between the piles, foundation slab, and soil under both long-term and short-term loads.

The purpose of this invention is to provide a basic structure and a design method that allows for simplified design.

The present invention relates to a design method for a building having a foundation structure and a superstructure supported by the foundation structure, the foundation structure having a rectangular foundation slab having long and short sides when viewed from above, and a plurality of pile foundations connected to the underside of the foundation slab. The design method for a building includes the steps of: determining the length of the short side of the foundation slab and the total height of the foundation slab so that H/S is 4 or greater, where S is the length of the short side of the foundation slab and H is the total height of the foundation slab and the superstructure; selecting a supporting ground for the foundation slab that satisfies a predetermined standard value for the amount of settlement of the building; providing the plurality of pile foundations only on the periphery of the foundation slab so that the number of pile foundations along the long side is greater than the number of pile foundations along the short side; designing the plurality of pile foundations to support only short-term loads; and supporting the foundation slab and the plurality of pile foundations on ground with an N-value of 50 or greater .

The present invention provides a basic structure and a design method that allows for simplified design.

This is a comparison diagram showing the concepts of spread foundations, pile foundations, and piled raft foundations. This is a conceptual diagram showing the concept of load sharing in a piled raft foundation. This is a conceptual diagram showing the relationship between the height of a building and the N value of the supporting ground. FIG. 1 is a conceptual diagram showing the arrangement of pile foundations.

Embodiments of the foundation structure of the present invention will be described below with reference to the drawings. Figure 1 is a comparative diagram showing the concepts of spread foundations, pile foundations, and piled raft foundations. Figure 1(a) shows a spread foundation, in which a foundation slab 1 (mat foundation) is placed directly on a strong stratum B. Figure 1(b) shows a pile foundation, in which tip-bearing piles 2 reach the strong stratum B, and the load is supported primarily by an upward reaction force acting on the tip of the pile. Figure 1(c) shows a piled raft foundation, in which pile foundations 3 rest above the strong stratum B. Because piled raft foundations share the load between the spread foundation and the pile foundations, the spread foundation is a foundation slab 1 (mat foundation). In other words, a piled raft foundation is a foundation structure 8 having a foundation slab 1 and multiple pile foundations 3 connected to the underside of the foundation slab 1.

Figure 2 conceptually illustrates the concept of load sharing in the piled raft foundation of this embodiment. The pile foundation 3 is a friction pile or a tip-bearing pile. Figure 2(a) shows long-term loads, such as dead loads (such as the building's own weight). The long-term load is borne solely by the foundation slab 1. In reality, part of the dead load is also supported by the pile foundation 3, but in design, the pile foundation 3 bears zero load. In other words, for long-term loads, the pile foundation 3 is considered non-existent when designed. Therefore, the amount of settlement of the building must satisfy a predetermined standard value without considering the piles. For this reason, it is desirable for the supporting ground of the foundation slab 1 (the ground that contacts the underside of the foundation slab 1 and directly supports it) to have a certain degree of hardness. Conversely, the foundation structure 8 of this embodiment is suitable for supporting ground that is hard enough that the pile foundation 3 is not necessary as a countermeasure against subsidence. As a guideline, for mid- to high-rise buildings with 20 floors or less (the number of floors in the superstructure 5, described below), the N-value of the supporting ground for the foundation slab 1 should preferably be 30 or more (Figure 3(a)), and for high-rise buildings with 21 floors or more, the N-value of the supporting ground for the foundation slab 1 should preferably be 50 or more (Figure 3(b)). The N-value is the hardness of the ground measured in accordance with JIS A1219:2013 "Standard Penetration Test Method."

Figure 2(b) shows short-term loads. The following explanation uses earthquake loads as an example, but wind loads and other loads can also be treated in the same way. For simplicity, we consider long-term loads and horizontal seismic forces as short-term loads. Short-term loads are shared between the foundation slab 1 and the pile foundation 3. For example, when a leftward earthquake load is applied as shown in Figure 2(b), the left end of the foundation slab 1 is pushed downward by the rotational moment caused by the earthquake force, while the right end of the foundation slab 1 is lifted upward. Therefore, the upward reaction force R of the supporting ground is distributed in a trapezoidal shape, larger on the left side and smaller on the right side. The ground directly below the left end of the foundation slab 1 is subjected to large compressive forces, which can easily lead to a lack of bearing capacity. The pile foundation 3 on the left side resists increased compressive forces in the ground. In other words, the friction force F1 generated between the side of the pile foundation 3 and the ground and the reaction force F2 received from the ground at the bottom of the pile foundation 3 bear part of the compressive force of the ground. Meanwhile, the pile foundation 3 at the right end of the foundation slab 1 is subjected to a force in the direction of pulling out. The right-side pile foundation 3 is subjected to a downward friction force F3 that resists the pull-out. The friction forces F1 and F3 and reaction force F2 acting on the piles bear part of the leftward rotation moment M caused by the seismic force.

The magnitude of the seismic force acting on the edge of the foundation slab 1 is proportional to the aspect ratio of the building. Figure 4(a) is a side view of the building, Figure 4(b) is a cross-sectional view along line A-A in Figure 4(a), and Figures 4(c) to 4(e) are cross-sectional views along line B-B in Figure 4(a). The building 4 comprises the aforementioned foundation structure 8 (foundation slab 1 and multiple pile foundations 3) and the superstructure 5 supported by the foundation structure 8. Here, the building 4, i.e., the foundation slab 1 and superstructure 5, are assumed to be rectangular with long sides 6 and short sides 7 when viewed from above (i.e., when viewed vertically from above to vertically below). The shape of the building 4 does not need to be strictly rectangular; for example, if there is a cutout in a part of the building, the cutout can be ignored. The aspect ratio is defined as the ratio H/S of the height H of the building 4 (the sum of the heights of the foundation slab 1 and superstructure 5) to the length S of the short side 7 of the foundation slab 1 (which usually corresponds to the length of the short side of the superstructure 5). The aspect ratio increases as the building 4 increases and the length S of the short side 7 of the foundation slab 1 decreases. In other words, a building 4 with a large aspect ratio is a slender building when viewed from the long side direction Y. The larger the aspect ratio, the greater the compressive force of the ground during an earthquake, making it more likely that the bearing capacity of the ground will be insufficient. Furthermore, the larger the aspect ratio, the greater the pull-out force of the pile foundation 3. Because the bearing capacity of the ground and the pull-out of the pile foundation 3 become more severe for buildings 4 with a large aspect ratio, it is reasonable to consider the frictional force (resistance force) of the pile foundation 3 when dealing with earthquake loads. There are no particular limitations on the aspect ratio, but the effects of the present invention are particularly great when the aspect ratio is 4 or greater.

In this invention, multiple pile foundations 3 are installed only on the periphery of the foundation slab 1, i.e., near the long and short sides 6 and 7, and not in the center of the foundation slab 1. Because the piles are installed directly below the columns (not shown) of the superstructure 5, the pile foundations 3 are installed only directly below the outermost columns of the superstructure 5. This is because, as is clear from the above explanation, the peripheral pile foundations 3 most efficiently bear seismic forces. In particular, because the bearing capacity of the ground and the pullout of the pile foundations 3 become severe against seismic forces in the short side direction X, it is preferable to install multiple pile foundations 3 only along the long side 6 of the foundation slab 1, as shown in Figure 4(c). However, if the difference in length between the long side 6 and the short side 7 is not too large, multiple pile foundations 3 can also be installed along the long and short sides 6 and 7 of the foundation slab 1, as shown in Figure 4(d). Furthermore, in some cases, diagonal seismic forces may be the most severe in design. In such cases, it may be possible to install multiple pile foundations 3 only at the four corners of the foundation slab 1, as shown in Figure 4(e). When designing, it is desirable to place the pile foundations 3 preferentially at the four corners of the foundation slab 1, then along the long sides 6 of the foundation slab 1 depending on the seismic force and aspect ratio, and if necessary, along the short sides 7 of the foundation slab 1.

In conventional piled raft foundations, the effect of the pile foundation 3 is considered for both long-term and short-term loads, and the idea of considering the pile foundation 3 only in terms of short-term loads has not been considered. In conventional piled raft foundations, piles are placed across the entire foundation slab 1, requiring complex consideration of the proportion of the building 4 load to be borne by the direct foundation and the pile foundation, which lengthens the design period. In contrast, in this embodiment, the pile foundation 3 is designed to bear only short-term loads. Because the pile foundation 3 is not considered in long-term loads, the design of the foundation structure 8 is simplified. Furthermore, because the design is simplified, it is possible to avoid complicating building confirmation applications. Buildings of a certain size require building confirmation applications, but complex designs require the preparation of a performance evaluation report by a performance evaluation organization, which can result in longer building confirmation application processes. In this embodiment, the design is simplified, at least for long-term loads, increasing the likelihood that the preparation of a performance evaluation report will not be necessary, and the design work related to building confirmation applications is also more efficient.

In this embodiment, the load borne by the central piles of the foundation slab 1 is relatively small compared to the piles on the periphery, making it possible to eliminate the central piles. This reduces the number of pile foundations 3, leading to lower costs for the foundation structure 8. Furthermore, reducing the number of pile foundations 3 also makes it possible to improve the efficiency of design work.

1 Foundation slab 3 Pile foundation 4 Building 5 Superstructure 8 Foundation structure

Claims (4)

A design method for a building having a foundation structure and a superstructure supported by the foundation structure, wherein the foundation structure has a rectangular foundation slab having long sides and short sides when viewed from above, and a plurality of pile foundations connected to the underside of the foundation slab,
When the length of the short side of the foundation slab is S and the total height of the foundation slab and the superstructure is H, determining the length of the short side of the foundation slab and the total height so that H/S is 4 or more;
selecting a supporting ground for the foundation slab that satisfies a predetermined standard value for the settlement of the building;
Providing the plurality of pile foundations only on the periphery of the foundation slab so that the number of the pile foundations along the long side is greater than the number of the pile foundations along the short side;
Designing the plurality of pile foundations to bear only short-term loads;
The foundation slab and the plurality of pile foundations are supported on ground with an N value of 50 or more;
A method for designing a building having the above. The design method described in claim 1, wherein the plurality of pile foundations are provided at at least four corners of the foundation slab. The design method described in claim 2, wherein the plurality of pile foundations are provided only along the long sides of the foundation slab. The design method described in claim 2, wherein the plurality of pile foundations are provided along the long and short sides of the foundation slab.