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.
Talking about high-rise buildings, earthquakes and prestressing… seems far-fetched to many people. Due to the nature of the profession, design engineers rely on big houses as their main source of income. But there are not always high-rise buildings to build, especially when the economy is difficult. There must be a secondary source of income, “outside” to maintain. The source of houses for residents is ideal, because the number is much larger. Especially for young people who have just graduated from school and started their career. A cousin built a house for them, an acquaintance who is an architect for a homeowner who needs to do the structure is the first thing to do… Let’s look at the cash flow, the salary is 15,000-20,000 VND/m2 of floor. A house with an area of 50m2 multiplied by 4 floors, the salary is also 4,000,000 VND. The salary is 1-2 days. If the relationship is good, the connection is steady, 15 houses can be built continuously per month, 60,000,000 VND. Not bad✌️
Broadly speaking, this type is low-rise housing. In addition to residential houses, there are large low-rise projects, mostly townhouses and villas. This type is often mass-produced, designed with only a few models and then multiplied to build and sell. Capital recovery is fast and effective.
Most people think_Oh, this tiny house doesn’t need to be calculated, the country workers can do it well from the foundation to the roof. How much iron is needed for the foundation, why do you need a degree? Thinking like that is both right and wrong. This person has experience in building many houses and can freely prescribe medicine, even more accurately than newly graduated engineers calculating. Saying that is to show that experience cannot be underestimated. But ask why this is made of this much iron (steel) and not less. I dare you to explain. Then the house has a strange shape, like a large cantilever supporting the 3-storey column above. So you can’t put down Knowledge. A university degree is meaningful in explaining why. (Masters can explain the why of why, PhDs know the why of why of why) 😉 It’s best to have a lot of experience to explain why in all situations, limiting naive results due to inexperienced people choosing the wrong working diagram of the house.
Strip footing foundation
This is the most common type of foundation structure for low-rise houses built in between. It is effective for soil that is not too weak for common houses with a height of 3-4 floors.
Personally, I find that calculating strip foundations convincingly is more difficult than calculating pile foundations. How to model the foundation so that the calculation results are not unreasonable, how to calculate the foundation beam is a problem. It is necessary to have a step to check and criticize the calculation results because no diagram can simulate 100% of the actual working of the structure.
The first calculation diagram in the engineer’s mind is Soft foundation. The foundation slab on elastic foundation. The stiffness coefficient of the elastic foundation is determined through settlement calculation. To calculate settlement, the deformation modulus of the ground must be determined.
To calculate, the foundation model is usually independent of the house frame model. In the house frame diagram, the column foot connected to the ground is considered an absolute rigid clamp. The load of the construction will cause the reaction force of the soil (hard) on the column foot. According to Newton’s 3rd law, this reaction force is the load on the Foundation model.
The results are sometimes quite funny. The foundation spins like a pinwheel. Even the design of the foundation’s reinforcement will produce the upper layer of steel if you blindly follow the diagram. At this point, experience is needed, do a lot, go see many houses that have been built and stand firm without any cracks. In reality, how can the foundation move because the house above is pressing down and the surrounding houses are blocking it from all sides. The foundation diagram needs to take into account that “pressing”.
Recommended foundation diagram
Refer to the book “Design and calculation of shallow foundations” – Prof. Vu Cong Ngu. The author has put a lot of effort into it, explaining the foundation calculation diagram issues thoroughly. Of course, it is necessary to inherit selectively, because the methods in the book were written in a time when there was no computer software like now. To avoid being too academic, take advantage of the current productivity method of calculating structures by computer. I would like to propose a simple practical method, based on experience, that is well applied to low-rise buildings with acceptable errors and tends to be safe. If in the design situation of other types of constructions, this method gives unreasonable results, please comment🙂
Step 1️⃣: Determine the load on the foundation
Starting from the load-bearing frame diagram of the house in which the column legs are connected at the foundation level. Running out the column leg reaction, we get the vertical reaction component (N). Put it into the spreadsheet. We will arrange in 2 directions as shown.
It is possible to arrange a strip foundation in one direction along the short side of the house. In the diagram above, the strip foundation direction is A-B.
Step 2️⃣: Determine the ground strength
(calculated pressure R according to the standard). To do so, first choose the foundation depth (calculated from the ground surface to the bottom of the foundation). The depth chosen is the minimum possible to save labor and money on excavation. But it needs to be deep enough to leave space for pipes and underground tanks to operate normally.
To determine the strength R, there are 2 situations:
👉In case of geological survey. Everything is clearly calculated. See more on the topic of calculating strip footing presented.
👉In case of no geological survey (common with residential houses). Apply TCVN 9362:2012 Appendix D to determine the conventional calculated pressure Ro.
Note that the values in Table D.1 correspond to foundations with width b1=1m and depth h1=2m. In reality, the foundation is not that deep, so the calculated pressure R is determined by:
$$R=R_o\left(1+k_1\frac{b-b_1}{b_1}\right)\frac{h+h_1}{2h_1}$$
+ If the mainland soil is quite good, it is considered as Dusty Sand, low moisture, medium compact state: Ro= 250kPa, k1=0.05.
+ Bamboo pile foundation, Melaleuca piles, … are considered as Fine Sand, moist and water-saturated, compact state: Ro= 300kPa, k1=0.125.
Step 3️⃣: Determine the foundation size
We will roughly determine the foundation size from the load in step 1. The simple principle is to divide by the intensity in step 2 to get the minimum size to withstand the force. From the load table arranged by row and column, with the expected strip foundation running along the number axis (1, 2, …), it needs to withstand the load equal to the sum of the columns along each axis (sum of columns A, B, …) as shown above.
For details, see the attached example 🎁
The known length of the foundation Bc is the width of the house (short side). Take the foundation area divided by Bc to get the minimum width of the strip foundation $b_{tinh}$. Choose the actual foundation width larger than $b_{tinh}$, especially the eccentric foundation row at the edge due to the reason of construction.
After choosing the foundation size, check the average pressure on the foundation p, equal to the total load of the column foot, add the weight of the foundation and the backfill, divide by the total area of the selected foundation bottom
$$p=\frac{\sum{N^{tt}}}{F_m}+\gamma_{tb}h$$
This value must be less than the ground strength R with a safety factor of at least 1.2 to take into account the bending effect of the column and other factors such as no geological survey, rudimentary construction level, difficult to control quality…
Step 4️⃣: Calculating settlement
The common case is that there is no geological survey, calculating settlement by the elastic deformation method with finite thickness. See more on how to calculate settlement of the slab on grade.
The most important thing is that the Eo deformation module also uses the method of looking up table B.1 TCVN 9362:2012. Still considering the bamboo pile foundation as “Fine sand, moist and saturated with water, compact state”, according to the physical correlation, corresponding to the void coefficient e=0.65 (5th column of table B.1). Looking up Eo= 28,000kPa = 280kG/cm2.
Why is the settlement calculation so simple❓ Because there is no survey, doing it in a complicated method does not help to be more accurate.
Step 5️⃣: Calculate internal force and structure of reinforced concrete foundation
Use a simple rigid foundation diagram to calculate how much steel is needed for a strip foundation. The familiar calculation diagram is the same as for a single foundation: Consider the foundation wing as a cantilever beam clamped at the foundation beam face. The load on the foundation wing is 1.2 times the average pressure at the bottom of the foundation p calculated in step 3 (equal to the maximum pressure on the ground).
Next is the calculation of the foundation beam according to the flexible foundation diagram.
As mentioned above, the diagram of things that “press” the foundation does not allow the foundation to move freely:
– Foundation diagram at the same time as the house frame. Instead of being separate.
– Lock the displacement (Restrain) in the x, y directions at the column foot positions at the house edge.
Common houses are houses built in between, tube houses. The foundation wing at the edge cannot extend to the neighbor’s land. That is called an eccentric strip foundation.
A common strip foundation is an intersecting strip foundation (strip in 2 directions). A more compact version is a 1-way strip foundation. Only make the strip in the main direction, usually the short side of the house. The remaining direction (longitudinal direction of the house) is only used as a foundation tie.
back to the Example:
From the foundation and house frame diagram, the calculation results of the foundation tie reinforcement are as shown above (unit area of reinforcement mm2). Based on that, place the reinforcement for the foundation ties. 2 longitudinal house ties (not strip foundation direction) should cut longitudinal reinforcement for sections that do not need much according to the calculation. The foundation plan below shows how to cut this foundation tie reinforcement (GM).
Well, answering “why” for strip foundations is not simple. It takes a lot of effort to figure out. Why is it like that??
For the benefit, not only for homeowners not to waste money 💰 If you don’t calculate each steel bar as above, with a large low-rise real estate project: just think of medicine, just 1 extra steel bar multiplied by hundreds of houses. It will cost a lot of money, especially when steel prices increase. At that time, will the investor leave it alone?
It can also be applied to all types of large and small projects: Schools, Hotels, … every steel bar is carefully designed.
Brick foundation
This is a popular house foundation structure during the subsidy period. Currently, reinforced concrete is faster to construct, and the foundation construction is also more expensive. Therefore, it is fortunate for engineers that they do not have to calculate the brick and stone structure. It is quite difficult and not good for labor productivity.
The topic of brick and stone structure will be presented later🧱
(To be continued)
