Reducing live load when calculating construction design, especially when designing pile foundations for high-rise buildings, allows achieving economic problems and maximally optimizing structural design. Current design standards all have provisions allowing live load reduction for civil buildings. The nature of accepting live load reduction is due to taking into account the non-simultaneous loading of live loads (furniture, users, etc.) on all floors and floors in actual use compared to the calculation model. The following article presents practical applications from TCVN 2737:1995 to reduce live load in common cases when designing multi-storey building structures.
the Code
- For the occupations: House, apartment (bedroom, dining room, living room, toilet, bathroom, billiard room, kitchen, laundry room), Office (agency, school, hospital, bank, laboratory, scientific research facility), Technical room (boiler room, engine room and fan… including machine weight); called rooms of type 1. The live load reduction factor is ψA1 determined according to:
- For the Occupations of crowded public works: Library reading room, Restaurant, Trade center booth, exhibition, Meeting room, dance room, waiting room, audience room, concert hall, grandstand, sports; Warehouse rooms; Factory areas; Balcony, Loggia; called type 2 rooms. The live load reduction coefficient is ψA2 determined according to:
In which $A(m^2)$ is the load-bearing area, which is the area of transmission to the corresponding structure under consideration.
When determining the vertical force to calculate columns, walls and foundations bearing loads from 2 floors or more, considering that the live load is not evenly distributed to all floors, it is allowed to reduce the live load by multiplying by the live load reduction factors as follows:
- For rooms of type 1, the live load reduction factor is ψn1 determined according to:
$$\psi_{n1}=0,4+\frac{\psi_{A1}-0,4}{\sqrt{n}}$$
- For rooms of type 2, the live load reduction factor is ψn2 determined according to:
$$\psi_{n2}=0,5+\frac{\psi_{A1}-0,5}{\sqrt{n}}$$
In which n is the number of floors transmitting to the considered structure on the considered transmission area. The design engineer needs to pay attention to determining the bending moment in the column, wall to be considered to reduce the load according to the coefficients ψA as above in the beams supporting the column, wall.
Practice
In reality, due to the large number of components and the number of floors, the volume to be calculated for the transmission areas for each component is very large. A practical way that can be applied when modeling a building from structural design software (ETABS, SAP…) is to create an Excel spreadsheet to calculate the values ψA, ψn according to the formulas of the above standards for each column and foundation structure. Export the results “Tributary Area and LLRF” of a considered column from all floors in the ETABS model (with the foundation being the foot of the column of the lowest floor) as follows:
The LLRF column is the Live Load Reduction Factor calculated according to foreign standards integrated into the software. Since we calculate according to TCVN, we temporarily ignore this column and only take the value of the “Tributary Area” column, which is the load transfer area to the column structure being considered according to each floor. ETABS outputs this value in the form of cumulative floors from top to bottom, so to obtain the value A to calculate according to TCVN, we need to subtract the value of the floor being considered from the floor above it. After transferring the values from the above table to the Excel spreadsheet to calculate the live load reduction factor according to TCVN, we get the following results:
It can be seen that for the same column C1, the lower the floors, the value of the live load reduction coefficient ψn gradually decreases due to the number of floors transmitting load to the column increasing and reaching the smallest value at the foundation surface.
🎁Excel spreadsheet convenient for construction design practice here.
