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# Moving load analysis

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Version 3

Q: How we can analyze a slab for moving load in SAFE? I have a podium slab which is to be designed for vehicular load.

A: This item is in our wish list and a completely automated moving load analysis option will be available in the next major release of SAFE. However in the meantime you can use the following method.

In this model for the purpose of demonstration a moving load represented by 4 wheels is moved along the design strip CSA3. Two rows of joints have been created at a suitable interval and at any given instant of time only two pairs of joints placed next to each other are loaded by the vehicle load. It is tacitly assumed that 2 pairs of nodes are enough for representing a vehicle load and vehicle is moving symmetrically on the grid and vehicle wheels are maintaining identical distance from the central grid. If you need more points then you need to keep a track of all the points and how these are to be loaded at any time instant. We have assumed that speed of the vehicle is 1m/sec and hence for a time step of 1 sec we need 25 pairs of points for the vehicle to move from one end to the other completely. At any one instant only 4 points representing the wheel load are used for applying the load of vehicle. Next we have defined 24 different load patterns and each pattern will have load for vehicle for a particular time step. See the image below. Here highlighted dots show the vehicle load for a particular time step. See the instructions in the image below and open the model and simulate same thing on your desktop to get a feeling of this.

Figure 1: Define Nodes where vehicle response is to be computed

Next we have defined a load combination "move_comb" of Envelope Type having load patterns LPAT1 to LPAT24 with a scale factor of 1.0. You can choose a scale factor higher than 1 to include effect of impact. Next see strip forces for the load combination  "move_comb" first for maximum. This gives the max. positive moment for the strip for moving load.

Figure 2: See strip moments for Envelope type combination (+ve moment above)

Next see strip forces for the load combination  "move_comb" first for minimum. This gives the max. negative moment for the strip for moving load. So at any section you need to design the strip for max. -ve and max. +ve both.

Figure 3: See strip moments for Envelope type combination (-ve moment above)

Currently we can do moving load analysis using this simple method.

A word of caution:

You need to mesh the slab at your defined points. This can be done by selecting the points added by you and select the option as given in the image below. This will ensure that slab is meshed at your defined points. If you want to have more refined results then you need to define more points at a closer spacing and apply the loads. You need not do manual meshing in such a case and when you want to investigate some other area for same vehicle load simply select the points defined by you and do replication for the new location and remove older defined points and then see the strip results for the new location.

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