Residential House | Low Rise House Design | part 2
I would like to dedicate this topic to: People who are preparing to build their own house🏠, young engineers who make a living, earn extra money by designing residential houses. This is the most common type of construction project. On the street, you will definitely see more residential houses being built than high-rise buildings. For homeowners, this article explains how to save costs when building a house. For design engineers, please share your experience in making a living by designing residential houses, how to be most productive. Especially full of practical examples as well as full calculation tools 🎁 included.
Bamboo piles
As mentioned in Part 1, Bamboo piles are used as a method to reinforce weak ground: mud, filled ponds, etc. This is a folk method, meaning based on experience. In the South, Cu Tram piles are commonly used, with the same principle. Large low-rise projects can use sand cushion reinforcement.
After being reinforced with bamboo piles, the ground is compacted. At that time, it can be considered good soil such as mainland, hilly land, etc. Then place strip foundation or raft foundation options on this foundation.
To use bamboo piles, there must be groundwater within the pile range. The reason? Without water, termites will quickly rot the pile. At that time, the pile is considered useless🥲
Bamboo pile structure according to experience is to use male bamboo. The driving density is 25 piles/m2, the maximum distance is 200mm per pile. Then spread a thin layer of sand cushion on the top of the pile, before pouring the foundation.
Note that bamboo piles do not need to be spread over the entire land, but only under the base area of the foundation according to the design. Returning to the example in Part 1, only spread bamboo piles under the areas of the foundation in one direction💰. Even under the longitudinal foundation beam of the house, bamboo piles are not needed.
To quantify the stiffness and strength of the bamboo pile foundation, the method presented in Part 1 can be used. A more orthodox way for large projects: use a compression table test on the bamboo pile foundation. Of course, the cost is quite large, but the results are enough legal basis for State agencies. Moreover, the numbers included in the foundation calculation are accurate, not empirical.
Jacking piles
In case the ground is too weak, bamboo piles cannot be made, driven piles are a popular solution for houses. Usually buy precast piles, square cross-section 200x200mm. The length of each pile section depends on the construction conditions. Houses built in narrow alleys commonly use 2m long pile sections, anchored. Due to the anchoring conditions, the pressing load is not large, at best Pmax = 40T. Experience in calculating the load capacity of 1 pile is half, which is 20T. The design advises the homeowner to choose the type of pile on the market with the amount of material just enough for the load capacity, should not be wasted. Because many houses have to drive piles up to 30m deep, 15 pile sections to reach the Pmax pressure.
Calculating the design of pile foundations is not complicated. Structural Engineers can calculate manually according to the rigid foundation diagram. The familiar formula:
Problems to be solved when calculating the positions: foundation beams, pile caps at the border. Due to the construction of the wedge, these foundations must be offset because they cannot extend to the neighbor’s land. If calculated by hand, the results are often not economically reasonable, wasting materials💰 Because the assumption of a rigid foundation diagram is not reasonable in these cases.
A simple, but accurate practice for engineers is to use a Soft Foundation diagram.
Similar to Part 1, the foundation diagram should be the same as the house body frame for convenience and to describe the actual work more closely. The diagram as shown above, is of the attached Example.
Pile diagram with elastic spring at point. This setting is too familiar to engineers. You can search Google for many. I will not present it again.
Just a small note, you can model the horizontal working of the pile. By spring in horizontal direction. The stiffness of horizontal spring according to my experience, is 1/10 of vertical spring, in the same unit system.
Back to Example, the calculation result of foundation reinforcement is shown in the figure below (unit area of reinforcement mm2). The foundation tie reinforcement is not large even though the foundations are eccentric in the horizontal direction of the house (main force bearing direction).
As mentioned, low-rise house design should be in the mindset of saving every steel bar💰 I would like to illustrate how to cut steel bars to save for BF-07 foundation beams as marked in red on the foundation plan. The upper layer longitudinal steel bars will be cut off bars 1a, 1b at positions requiring load-bearing and are no longer needed. The foundation beam steel bars extend all the way to the base of the column, going into the pile cap. Therefore, they are combined to make pile cap steel bars. The remaining pile cap area only needs to be arranged with ø14 steel bars (bars 5, 6).
When constructing jacking pile, it is necessary to pay attention to the conditions for stopping conditions to avoid driving too deep and causing waste. The house that is built in the middle also needs to be careful to calculate the soil pushing when driving piles, which can damage the neighbor’s house. If the number of piles is large and the neighbor’s house has been built for a long time, it is easy to be damaged during the pile driving process, so consider using small diameter bored piles.
Small diameter bored piles
This is a type of pile foundation that is used quite a lot. It is suitable when the number of floors is large (usually over 6 floors) and the fear of pile driving will damage the neighbor’s house. Of course, the cost will be more expensive. It is necessary to compare the economics with other types of foundations before deciding to use.
Regarding design calculations, there is nothing difficult for engineers. The calculation is similar to that of pile driving. The instructions according to TCVN 10304:2014 are enough. Note that each pile is carefully calculated💰, because the cost is not cheap. Of course, there must be a geological survey, this is something that the homeowner should not regret spending money on.
Regarding construction, it is not as formal as a high-rise building. It is difficult to do enough ultrasonic tests to check the quality of pile concrete. Static compression tests cannot be conducted. The risk is high if the concrete is poured poorly. Therefore, the homeowner should choose a reputable contractor.
Raft foundation
This is the least common type in low-rise buildings. It is also in the group of shallow foundations like strip foundations. But it is much more expensive.
Usually applied when the house has many floors, has a basement and weak ground, combined with bamboo piles. Houses with basements should combine the structure of the raft foundation and the basement floor as one. Because the basement floor must be made of concrete anyway.
The calculation of the raft foundation is as presented in this Topic.
Slab on grade
This is the ground floor, when there is no basement. Engineers often “prepare” this structure based on experience. When “preparing” without understanding the nature, it can lead to unfortunate consequences: sinking of the floor, cracked tiles, and wet floors. Please note, do it with care so as not to upset the homeowner.
It’s difficult for private houses. Almost no homeowner does it. Therefore, there is no quantitative data to accurately calculate the foundation. The design engineer can only design based on experience as presented in Part 1. Ask the neighbors about the soil in this area, what kind of foundation should you choose if you built a house in the past.
The designer should advise the homeowner to hire a geological survey drill to be sure if they live in a deserted area. It’s very cheap, just Google it. It costs a few million VND per drill, but it can save a lot of money on building the foundation.
Of course, large low-rise projects must do it. This is an indispensable part of the dossier according to the provisions of the Law⛔️
Basement
Making a basement is an expensive item. Expensive because of digging deeper. There must be measures to protect the excavation from collapsing and damaging the neighbor’s house, such as using Larssen piles. Due to the limited area, the basement concrete wall is often poured as close as possible. Therefore, Larssen piles must be buried underground and cannot be pulled up.
It is better to use rows of bored piles close together (secant piles) to form a basement wall. It is possible to add tens of centimeters of basement wall thickness to the edge of the ground, compared to the expensive cost.
Underground tanks
Every house must have a water tank, septic tank. Usually the tank wall is made of bricks, the bottom and lid are poured with concrete. When designing the foundation, leave room for the water tank, septic tank.
Design engineers often follow experience, each type of tank size according to the drainage requirements corresponding to the scale of the house. Take advantage of available sample designs.
For low-rise projects with tanks, large treatment plants, engineers can refer to how to design water tanks.
That completes the most difficult part of the house. If you find any part of me “crazy” and have a better, faster way to do it, please comment and share with everyone🤝
