Also a physical object, the load-bearing structure of the house is affected by expansion and contraction due to temperature changes. The designer needs to ensure safety in cases where damage may occur due to temperature: the house is too long, the climate zone has extreme temperature changes. As well as avoiding wasting money 💵 by increasing concrete and steel indiscriminately, just calculate correctly and sufficiently. Here is a suggestion for a convenient practice in this field of construction design
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A common situation is that it is impossible to arrange expansion joints on the ground plan due to:
– Architectural design requirements: Aesthetics, functionality – inconvenient in use
– Complicated waterproofing, joint filling materials do not ensure long-term durability, increasing maintenance costs. Especially basement areas require large areas
– When high-rise buildings have long ground plans, if expansion joints are arranged, they will be separated into different blocks, so that the blocks do not collide with each other when moving horizontally due to wind, earthquakes, the width of the joint is too large. Leading to architectural impossibility
1. Why do we have to make expansion joints?
When the house is too long, design consultants often arrange expansion joints to reduce the length of thermal deformation of each part, avoiding generating too much internal force due to thermal deformation causing damage (cracking, destruction, …). The joint has a certain width for thermal deformation of each part.
Experience in designing monolithic reinforced concrete structures, if expansion joints are arranged with a distance of no more than 50m, the internal forces arising from temperature deformation of the structural blocks are insignificant enough to require reinforcement. According to PCA’s suggestion, this distance is 60m for normal reinforced concrete floors and 150m for prestressed floors (according to PTI). You can refer to table 1.2 ACI report number 224.3 on expansion joint distance
If you do not want to arrange expansion joints, you need to ensure that the structure can withstand the internal forces caused by temperature.
2. How is temperature load understood?
Free objects tend to expand when the temperature increases and contract when the temperature decreases. The structure of a house is not a free object, it is always prevented from expanding. For example, floor beams are prevented by columns, foundations, etc.
Temperature load is the change in temperature, which is the cause of internal force and deformation in the load-bearing structure. Including:
+ Daily temperature variation (short term):
Daily changes in environmental temperature affect uncovered structures (roofs, open floors, etc.), causing temperature differences along the thickness of the floor. This effect is short-term and repetitive. The temperature difference along the thickness depends on the level of exposure to the environment and the thickness of the structure, and can be greater than 10oC.
+ Annual temperature variation (long term):
In reality, the project goes through a long construction process, then the structure works at a time calculated in years when it comes into operation. Therefore, the structure works long-term, evenly throughout the floor thickness, under temperature variations and gradually decreasing with concrete creep. This is a point that design engineers need to grasp.
3. Impact of Temperature Load
Internal forces and deformations due to temperature changes are added to the effects of other loads such as dead loads (self-weight, etc.), live loads (users, furniture, etc.), prestress, etc. when the structure is prevented from deforming freely.
When the temperature increases during the day, due to the resistance to expansion, compressive forces arise in the structure. Conversely, when the temperature decreases at night, tensile forces appear. With reinforced concrete structures, a decrease in temperature variation is a more unfavorable case, because concrete has poor tensile strength. It is necessary to ensure that the structure can withstand the tensile forces arising from these temperature changes, for example, by arranging to increase the reinforcement or increase the size of the concrete.
🗻Photo 2 is a design diagram of a multi-storey frame structure, the bottom floor is blocked by the foundation, which will cause internal forces due to temperature changes. This internal force decreases very quickly when going up to the upper floors. Right on the floor above, the internal force has returned to almost zero. Normally, structural calculation diagrams are elastic diagrams, but in reality, at the positions where the column and wall feet are connected to the foundation, very small cracks appear. These cracks will release stress due to temperature, so the concrete structure rarely has large internal forces as in the Elastic calculation diagram. 🔍This is a point to note when calculating construction design.
Photo 2
4. Calculation practice
The design calculation diagram of the project often uses computer software (ETABS, SAP…), so the temperature load function of these software will be used. 🔍Parameters to note:
+ Thermal expansion coefficient a
TCVN 5574:2018 clearly states the value a = 1.0 × 10-5 °С-1 for normal concrete. Reinforcement can use the same value because it is approximately equal.
+ Temperature variation ΔT (negative value when the temperature decreases)
+ Temperature load combination, according to TCVN 2737:1995, is a long-term temporary load.
Special attention should be paid to the value ΔT when included in the calculation diagram, ensuring:
+ Explainable to state agencies, preferably in compliance with Vietnamese Regulations and Standards. QCVN 02:2009 “Natural data in construction” has sufficient data according to the administrative place name at the District level.
+ Structural design needs to model the actual working when bearing the load of temperature variation. Avoid the situation where the load diagram becomes “thermal shock”, causing very large internal forces in the structure that are not true to reality. Reinforced concrete structures in reality can release internal forces due to temperature load through the formation of very small insignificant cracks, through frame joints, through structural steel bars.
🔲Choose the value of temperature variation ΔT
According to QCVN 02:2009, there are 2 types of average and largest and smallest temperature amplitudes that are statistically calculated into monthly and yearly data for geographical areas.
In reality, the heat transfer mechanism from the climate, through the enclosing wall, to the indoor air and to the load-bearing structure. The temperature in the structure will increase or decrease slowly. Thus, this process does not occur suddenly but slowly over a long period of time, so the average highest and lowest annual air temperature values are used. In QCVN 02:2009, the data is given in Tables 2.3 and 2.4.
The concrete structure has generated heat due to hydration during the concrete curing process since pouring. Therefore, there is an initial temperature. The design engineer needs to understand that ΔT is not the temperature variation of the air, but the temperature change compared to the initial temperature at the time the structure starts working, bearing the load (when removing the formwork) To.
Another problem is when determining what type of To:
– There is no data on the temperature at the time of concrete pouring, which is a future time compared to the design time
– Almost no temperature measurement is conducted during the concrete curing process (setting)
Without data, we have to rely on the Standard, but TCVN does not have a regulation on the value of To. Therefore, it is necessary to understand the principle of temperature change when concrete hardens.
Photo 3
When cement is mixed with water, heat is generated from the exothermic chemical reaction between water and cement and is called heat of hydration. The temperature development and heat distribution in concrete depends on the concrete mix, shape and size of the poured block, type of formwork, and environmental conditions. The temperature development due to the hydration process can cause damage to concrete due to thermal cracking. Thermal cracking in concrete occurs when the temperature difference in the concrete block exceeds the limit, this temperature difference causes thermal stress and can lead to structural cracking. According to TCXDVN 305:2004, the limit for temperature difference between points in a concrete block is 20oC and between any 2 points 1m apart is 50oC/m, large concrete blocks will be damaged by heat. The process of temperature change of concrete is shown in 🗻Photo 3. Immediately after pouring, the concrete has a temperature of Tp which is the temperature of the concrete mixture. In fact, heat can be lost through the faces of the block or pouring layer. The maximum temperature reached is Tp + Tr (Tr is the increase in temperature due to cement hydration) and then gradually decreases to a near-stable temperature value Tf.
(1). Initial stage: from the time of pouring concrete to almost the end of the cement hydration process. During this stage, the temperature of the poured block and the elastic modulus develop rapidly over time;
(2). Intermediate stage: from the end of the initial stage to the time when the temperature of the concrete block reaches a stable value Tf.
(3). Later stage: after the concrete has completely cooled to ambient temperature, the thermal stress is mainly due to changes in ambient temperature.
The exact way to obtain temperature distribution data in the poured mass can be to use a thermal sensor to monitor the temperature in the field or to use mathematical models to simulate or combine these two methods. Currently, there are some software used to simulate the heat distribution due to the heat of hydration of cement such as: ANSYS..
Thus, at the initial working time of the structure (For example, 28 days when removing the floor formwork), the temperature To in the concrete is equal to the air temperature. It is possible to use the average annual air temperature according to QCVN 02:2009 for construction design work, if there is no data on construction time.
5. Temperature load model on the structure
The calculation diagram for the design of a normal house structure is the frame and floor beam system, without the model of the enclosure system. Therefore:
– Assign the changing temperature load to all floor beam elements.
– Experience shows that the temperature load assigned to the floor is the most unfavorable internal force for the columns and walls of the house. For the bottom floor, the temperature load on the column and wall causes a large internal force due to the column foot connection.
Note that Rigid Diaphragm should not be used for the floor because it reduces the impact of the temperature load compared to reality.
– Load combinations with temperature load:
Earthquake load is a special load, so temperature load is not present in earthquake load combinations.
– If you want to reduce the stiffness of reinforced concrete structures due to the appearance of cracks:
Use the stiffness reduction coefficient for the components: For example 0.35 for beams, 0.7 for columns and walls (0.35 if the columns and walls are cracked), m11=m22=m12=f11=f22=f12=0.25 for the floor.
(see more about how to determine the stiffness reduction coefficient according to TCVN in the next topic)
💎Example 1:
The project has been put into use in Hanoi. Short houses (<50m) do not need to arrange expansion joints in terms of intuition.
The temperature parameters for design consultation are taken from QCVN 02:2009:
– Average annual air temperature: 23.6oC; that is To
– Average lowest annual air temperature: T1 = 21.2oC
The temperature load assigned to the column, beam and floor structures is based on the temperature difference ΔT = T1-To = -2.4oC; the decrease in temperature causes internal tensile force in the floor, which is a more dangerous case.
Verify the calculation when there is a temperature load, the steel reinforcement does not increase significantly. Answer with numbers for the intuition from the beginning: there is no need to calculate the temperature when designing a short house structure.
Photo 4
Slab reinforcement – no temperature load
Photo 4a
Slab reinforcement – with temperature load
💎Example 2:
Design and construction of a 10-storey open parking lot with a floor plan of more than 200m without expansion joints. The design consultant chooses a prestressed reinforced concrete floor structure, with a maximum span of 8.4mx10.6m. Construction location: Hanoi.
Because there are no surrounding walls, all floors are subject to temperature changes according to the weather. Temperature parameters are as in Example 1.
Floor cables are calculated according to vertical loads. Reinforcement is usually designed in 2 cases: with and without considering temperature loads.
Floors from 3rd to the roof, there is no significant difference in calculated reinforcement with and without temperature loads.
Photo 5
Slab reinforcement – no temperature load
Photo 5A
Slab reinforcement – with temperature load
The difference appears on the bottom floor (2nd floor).
Photo 6
Slab reinforcement – no temperature load
Photo 6A
Slab reinforcement – with temperature load
Reinforced steel usually increases by about 10% compared to the case of the structural engineer not considered to temperature.
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🎁Summary:
– Expansion joints can be omitted to satisfy aesthetic and economic requirements. Temperature load on the structure must be calculated.
– The above calculation method is simple, takes advantage of available software, and is sufficient for explaining to State agencies because it complies with the standards.
– When designing and constructing long houses, expansion joints should not be made, reinforcement should be increased, and attention should be paid to limiting long-term cracking.
🎉File attached with example
