Definition of Blast accidental loads in Robot
Most likely there are some rules that describe the equivalent static force that you could try to search for. For the dynamic approach (time history analysis) check the attached example.
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Artur Kosakowski
Attached the modified file. Some of modifications made:
1/ switched off DSC to avoid unjustified message related to dampers in modal decomposition method
2/ load in load case 3 changed to 1 kN
3/ time history forcing functions defined using the conservation of momentum principle as described in more detail in my example supplied previosly by Artur:
m*v = F*t
Assumed constant force F during the impact lasting t (precisely it should be integral of F in time domain) and checked 2 lengths t of impact: 0.02sec (case 5), and 0.01sec (case 9)
So for instance for t=0.02 sec: F=18000kg*8.33m/s/0.02sec = 7500000 N = 7500 kN
As it can be seen below no big difference between them - it is difficult to notice that these are pairs of overlapping diagrams:
Assumed modal damping 0.05 for all modes.
Other assumptions
Only the selfweight of the structure was considered in the mass of the model:
a/ no conversion of other loads to masses - converting load case 2 to mass would result in significantly increased inertia of frame thus significantly reducing displacements and completely changing frequency and shape of oscillations. After similar changes it may be necesary to divide bars of the frame to more precisely consider vibrations corresponding to higher modes
b/ truck treated only as the "source" of force and not mass - as if separating from the frame just after collision
c/ ...and of course everything linear and elastic
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Regards,
Pawel Pulak
Technical Account Specialist
Accepted solution
1) So, the force calculated by me 894,3 kN is too small, because it calculated for impact lasting t=0.168 sec ? Why did you take for the Time History analysis two lengths of impact t1=0.02 sec and t2=0,01 sec, does it comes from some Norms ?
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In your original model you have defined the impact of 894.8*63=56340.9kN lasting 0.05 sec and then linearly decreasing to zero during next 0.05 sec.
When I have defined (attached model - see load case 15) the impact 894,3 kN lasting t=0.168 sec (forcing function constant during 0.168 sec) the results comparable with shorter impacts but with bigger force (but the same F*t product) were received - see the screen capture below. In load case 15 smaller and delayed response can be observed because the impact time is already comparable with 1/4 of the periods of basic vibration modes of the frame.
I have taken for the Time History analysis two lengths of impact to illustrate the effect that for short lengths of impact (short in relation to the periods of basic vibration modes of the structure) the response depends on the product F*t and not only the force F. I have not taken it from any norms - pure machanics.
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2) My perpose in Blast analysis to discover the response of structure from impact of truck. I need to ensure that if even one column isn’t resist the impact of truck, the structure itself will be strong enough and not collapse. What is the practical usage of Time History analysis ? How I can get forces from it in proper form(table form or on the screen) for static analysis of the collided column ? If I know force F in advance, for instance, for t=0,01 sec it is F=15000Kn, why I can’t put it STREIGHT in Static analysis and calculate the capacity of the column and general deformation of structure , that benefit I can get from Time History analysis ?
3) Now I found only how to get the forces in graphic view. Please, give me clear explanation how to use the results from Time History analysis ?
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Time history analysis (THA) give results (forces, displacements, moments,etc) in series of time steps.
For instance in my examples it was defined with such paramaters that the results were output in 501 steps from t=0 sec to t=5 sec.
The results of all these steps can be seen in the tables when tables are filtered only to THA case - see below:
In the graphical window the results (deformed shapes, diagrammes of moments,...) can be also displayed for selected time (as if a single picture from the video) selecting THA load case and appropriate component of it:
Direct application of the force of 15000kN as static load case would give completely different (bigger) results than THA because it would correspond to static application of this load without vibrations (constant displacements, forces, moments etc). In case of THA the load is acting very short (0.01 sec in my example) and is resulting in variable in time displacements, forces, moments etc.
When using THA results in design it is the best to find time steps with the extreme forces or moments and to convert them to simple cases - see the green rectangle in the screen capture above. Such cases can be easily combined with other load cases and used in design.
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If this post answers your question please click "Accept as Solution". It will help everyone to find answer more quickly!
Regards,
Pawel Pulak
Technical Account Specialist
Hi @Pawel.Pulak, I apologize for reaching out after such a long time. I hope you don't mind me reviving this topic, but I have some doubts regarding the solution applied here.
- Did you change the static force to 1kN because you assumed that the mass of the truck is not connected to the structure, and its vector force will be applied using the THA over time, correct?
I couldn't find any information regarding what the specific static force or mass should be. The help files mention that it should be a static force or a mass, but they don't provide clarification on what values to use. If we consider a vertical force, shouldn't the truck's self-weight be applied instead of 1kN, as the truck would remain on the bridge?
For example, let's say I have a gas cannon that shoots gas into a vessel, and I know the force is 62kN over 50ms. Should I apply this THA using the static force of 1kN case to obtain accurate results? In this case, I still need to know the speed of the gas shot, correct? Or at the very least, the mass that is being moved, so that I can indirectly calculate the speed. Alternatively, can I directly apply the force of 62kN over the 50ms?
In this specific example, would it be reasonable to assume that the manufacturer should provide at least one more data, such as the speed, mass, or acceleration, to accurately model the situation?
Assuming this is a gas explosion, and its speed is approximately the speed of sound, 343m/s (at 20°C), I have considered two scenarios:
- THA 1: F(T) = 62kN
- Trial and error to reach a static force of 62kN as the resultant.
The final result to THA1 is 21kN to -35kN.
Which approach seem correct to you (assuming the speed calculation is accurate)?
Please let me know your thoughts and if any further information is needed to clarify the situation. Thank you for your assistance.