RAM Frame - Masses

Why do I get a warning about mass that's not associated with any diaphragm?

When you have line loads or any members with self weight masses considered (Under Ram Manager Criteria - Self-Weight) that occur outside of the boundary of the diaphragm slab edge you will get the following type of warning:

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Some mass has been detected on one or more stories that is not associated with any diaphragms.
It will be ignored in Analysis unless it is combined with one or more diaphragms.
See the Loads - Masses command.

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Under Loads - Masses you can evaluate the total diaphragm masses and you can combine these values with some diaphragm, override with User Specified Values or choose to ignore them by doing nothing.

One fairly common modeling mistake is to copy the line loads from another level type, but then modify the framing and slab edge without removing line loads that then fall out in space.

A similar error message also occurs when the Gravity load totals used for Notional loads are uncombined. See RAM Frame - Notional Loads for details.

Does the program automatically include storage or snow live loads in the masses?

No, the program calculated masses are derived form the user input Mass Dead Load only (and self-weight settings). Any portion of live, storage, partition or snow loads that needs to contribute to the total mass should be manually added into the Mass DL component of the loads, or the Loads - Masses can be overridden in Ram Frame. Refer to RAM Frame - Seismic Loads [FAQ] for more on the relationship of masses and seismic loads. Refer to RAM SS Gravity Loads [FAQ] for details on self-weight masses.

How can I have only the mass of walls perpendicular to the seismic load considered in the seismic analysis?

For tilt-up buildings or other low-rise structures with lots of mass in the walls, the walls perpendicular to seismic force lean on the diaphragm, but the walls parallel to the seismic load should directly resist the forces from ground acceleration. This cannot easily be achieved in Ram Frame.  Our approach is to take the mass of all the walls (typically from the half-story height as set in Ram Manager - Criteria - Self-weight) and lump it into the diaphragm total. Then a diaphragm force is applied and the lateral walls each resist a portion of the total seismic force based on relative stiffness. 

In lieu of using the wall self-mass in the analysis, mass-only line loads could be applied in the model. Like wall self-mass, the line loads still contribute to the diaphragm total mass that determines the total force. If line loads are applied to the walls in the N-S direction only, for example, then that model would have the right diaphragm mass for seismic loads in the E-W direction. This would require two models, one with seismic loads in the X direction only, and with line loads on the N-S walls only, another with seismic loads in the Y direction and line loads on the E-W walls. To get the correct seismic force into the E-W walls additional nodal loads would also have to be applied to each of those walls. The problem is, even nodal loads will be redistributed when a rigid diaphragm is used. Furthermore, the effects of seismic eccentricity or orthogonal effects need to be considered. Due to this complexity, most users lump all the wall mass into the diaphragm total. 

The same issue also affects dynamic response spectrum analysis since the masses are lumped. 

Using a semi-rigid diaphragm with distributed masses may be a reasonable alternative for some cases. 

See Also

RAM Frame - Seismic Loads [FAQ]

Red Status Lights for Lateral Load Cases in RAM Frame

RAM Frame Semirigid Diaphragms

RAM SS Gravity Loads [FAQ]

RAM Frame - Notional Loads