Dear all,
When we obtain strut loads from plaxis 2D as kN do we multiply by 2 to account for the loads being from both sides.
As shown in the figure below, during excavation, the strut element will experience active earth pressures from both sides (side 1 and 2),
for this the obtained value will be multiplied by 2 or it is the maximum and final value.
Dear Jamal,
If a strut has a load of 100 kN on the left and 100 kN on the right than the strut force is 100 kN. I don't understand why you would multiply it by 2?
Actually, the strut force is always the same left and right if an anchor is used, because and anchor has no interaction with the surround soil to transfer load to the soil. Hence, the anchor force on both sides is always the same.
With kind regards,
Dennis Waterman
Thank you Mr. Dennis. This is where I was confused, during design, some local codes require to magnify the obtained load by a certain factor of safety. I was wondering as the design load of struts is for example 100 kN on both sides, so it would be 100 kN x design factor of safety then multiplied by 2 to account for both sides. It seems that I misunderstood the value obtained by plaxis (maximum from all phases).
Hi Jamal,
As Mr. Dennis said, the strut force can directly use from the analysis result. Since you are confusing about "some codes mention to multiply the force by some factors", will explain about that based on my understanding.
I'm explain this based on Eurocode (EC). There are 2 types in applying factors to the action, they are 01. Directly applying factors to the action & 02. Applying the factors to the effect of action which means applying factors to the result of the analysis like AF, SF, BM etc.
Let's consider DA1: C1 and C2 based on EC7, the unfavorable load factors for the permanent and variable loads are (1.35,1.5) and (1.0,1.3) respectively. In the soil, soil pressure and GWT are considered as permanent and surcharge is considered as varaible.
If you see the factors, for DA1: C2, the unfavorable factor for the permanent action is 1.0, so we can use the action method to define the loadings and directly use the output from the strut force in the calculation without applying the factors.
But for DA1:C1, the unfavorable factor for the permanent action is 1.35. Since we define the unit weight of the water and soil as the base value, inorder to comply the factors all the load factors are divided by 1.35 to make the factor of the permanent load as 1.0, so in this case the factor for the variable load is 1.11 (1.5/1.35) and analysis is carried out. After the analysis, effect of action is used since we reduced the load factors. In this case all the results like AF,SF,BM,strut force is needed to multiply by 1.35 to get the actual value (effect of action not action).
These are depended on the method and the code reference you are using. If you use "Design approach" and unit weight of the water then no need to follow the effect of action. If not (using base value for unit weight), then need to use effect of action.
Conclusion => The strut force from the analysis can be used directly from the Plaxis analysis for the design (no need to multiply by 2 as it is restricted 2 sides). If you use effect of action, then need to multiply the result by the partial factors which I explained above (not by 2 as it is supported 2 sides).
Thanks.
Thank you so much for your valuable comments Nitharshan.