In the construction design, the calculation of the deflection inspection becomes an essential requirement to ensure the economy for the following situations:
-Large rhythm length
-Large load, very common for civil floors (Landscape on the first floor, roofing floors supporting heavy mechanical equipment …)
When calculating the deflection, the design engineer should pay attention:
-Load combination according to the 2nd Limit state (no loading coefficient)
-The appearance of cracks in concrete when bearing, leading to reduced cross section hardness and increasing deflection
-The long -term work of reinforced concrete structures should be considered for elements from variables and shrinkage as well as long -term effects of the load types. According to Vietnam Standard (TCVN), the total deflection F is calculated as follows:
P = f1 – f2 + f3
in there:
+ f1: The deflection due to the short -term effect of the entire load
+ f2: The deflection due to the short -term effect of the long -term load
+ f3: The deflection due to long -term effects of long -term load
With the working floor structure in two directions, the deflection calculation is only convenient in practice when using a finite element method (PTHH), including the above factors when deformed. Here I would like to present how to apply Safe 8.X and 12.X to calculate the deflection from some practical experiences design consulting.
To calculate the common deflection in the following cases:
-Dead: Only include the weight (Self willplier = 1)
-SDEAD: The weight of the floor finishing layers (Superimpose). In example SDead = 1500kg/m2
-Live: Activated load effect on the floor. According to TCVN 2737: 1995, the load also has a long-term component, usually accounts for 20% -30% of the value of the total load. For convenience, the examples below use the coefficient of 0.3, live = 500kgf/m2
(Safe files below please download from the attached file section with this topic)
-For example, the effect of cracks: Using Normal and Crack Deflections Analysis with reinforced steel distribution, in the example of F16A300 2 directions. Select the cubic for the Interpolation Options, the program will calculate 3 times.
-For example, long -term effects: Use Long Term Deflection Multiplier coefficient, the specific formula is taken according to the design standard of reinforced concrete structure ACI 318. In the temporary example, take L = 2, but in Safe still declared with 1 and multiply this coefficient outside.
-Hammocking combinations:
+ f1 = 1*DEAD+1*SDEAD+1*LIVE
+ f2 = 1*DEAD+1*SDEAD+0.3*LIVE
+ A.
-Results, taken with max deflection (unit M):
F1 = 0.051
Class = 0.044
Indeed = a*category = 0.088
P = F1-F+Aj = 0.095
-Structural design model used with geometry, materials and loads
-For example, cracking effects: cracking analysis options: quick tenion rebar specification Ø16A300 2 directions. Method of calculating hardness after cracking modulus of rupture: program default.
-For example, long -term effects: Using two characteristics, CREEP Coefficients (CR) for variables and Shrinkage Strain (SH) for shrinkage.
Can calculate design according to many standards, in the example calculated according to Eurocode 2 with the following conditions: long -term time, temperature and environmental humidity under Vietnamese conditions.
Calculated: CR = 1.7 and SH = 0.
Hammocking complexes: Define in Load Cases
+ F1, F2 as above, with Analysis Type is Nonlineear (Cracked)
+ F3 as F2, with Analysis type is Nonlineear (Longterm crack); Cr = 1.7 and SH = 0,0003
-Results, taken with max deflection (unit M):
F1 = 0.067
Class = 0.055
FZ = 0.081
f = f1-F2+f3 = 0.093, quite suitable for Safe 8
The new Safe V12 adds analysis in the construction stage, specifically in the Define Load Cases/ Initial Conditions/ Continue from End of Nonlineear Case. Understand that it allows analysis of the current load case with parameters starting from another case that is considered to be the pre -load period (whoever used plaxis will get used to this type of analysis). This function is very convenient and suitable for the actual work of the structure, especially for the case of deflection in construction design.
There are additional loads as below with SH for short -term and LT for the long term:
+ Sh1: 1*DEAD – Nonlinear (Crac ked) – Zero Initial Condition
+ Sh2: 1*SDEAD – Nonlinear (Cracked) – Continue from State at End of Nonlinear Case Sh1
+ Sh3-1: 1*LIVE – Nonlinear (Cracked) – Continue from State at End of Nonlinear Case Sh2
+ Sh3-2: 0.3*LIVE – Nonlinear (Cracked) – Continue from State at End of Nonlinear Case Sh2
+ Lt1: 1*DEAD – Nonlinear (Longterm Cracked) – Zero Initial Condition
+ Lt2: 1*SDEAD – Nonlinear (Longterm Cracked) – Continue from State at End of Nonlinear Case Lt1
+ Lt3: 0.3*LIVE – Nonlinear (Longterm Cracked) – Continue from State at End of Nonlinear Case Lt2
-Thus, the complexes under the TCVN will be: f1 = SH3-1, f2 = SH3-2, f3 = lt3
-Results, taken with max deflection (unit m):
f1 = 0.0317
f2 = 0.0276
f3 = 0.0395
f = f1-f2+f3 = 0.0436, much smaller than the above examples
I personally find this result more appropriate, as the analysis in the phase of simulation more accurately the actual work of the structure: Ensuring the change of hardness caused by cracks developed in the previous stage is mentioned when the extra load is added in the later stage.
Refer to the source:http://www.eng-tips.com/viewthread.cfm?qid=246507&page=1
After calculating the total deflection level F as above, the important thing is to check and the usual conditions are f
-Lunar floor, l = 2 * The level of stretching and stretching
-The floor on the rectangular column mesh works in two directions with the length of each line is L1, L2; L = min (l1, l2)
-With any column mesh, L is the distance between the two adjacent points on the floor with a curvature when deformed in zero, not necessarily a knee point (column, wall). And the deflection f is determined to be the biggest distance from the 2 -point connection to the back floor deformed (see the figure, with D symbol for f)
-Similarly, the stretching level is determined by the range of points with the same curvature point inside the floor to any point along the edge of the floor.
Reference source: Dr. Bijan o Aalami – Adapt Corporation
🎁File attached
