Ram Concept - Load combinations - Alt. Envelope factor

I am trying to create a service load combination to consider 75% of wind load from two wind directions on my roof slab. I want to know if this set up would cover all the combination listed below.



Thanks in advance!

  • Every permutation of load factors and alternate envelope factors for all of the loadings are included with the load combination defined. As a result, all of the load combinations in your list will be covered.

  • Hi    I am trying to understand the use/purpose of Alternate Envelope Factor and it's relation with pattern factors defined in loading properties. As far as "Alternate Envelope Factor" is concerned, the original question in this post and your response to it makes it quite clear that AEF is used to create group/envelope of load combinations with alternative load factors and it seems that it is only useful with lateral load combination or LC involving lateral loads and I think that same thing can be achieved by changing "combo type" to "Lateral" and then add appropriate "key load factor" & "alternate key load factor" although i think we need 2 load combinations in that case if we want to cover all 8 load combinations posted in the question. When I read manual section 11.7, it seems that it cites example of cantilever condition as one of the potential use of AEF stating that live load reduces the positive moment in the backspan. I think I don't understand this example as I think pattern factors (On & Off) will take care of determining max neg at cant and max pos moment at backspan. I don't understand how AEF will determine max and min moment more critical than determined by pattern factors. Also as an example of alternate load factors, it suggest 0.9 AEF for DL and 0 for LL & Lr for LC 1.2D+1.6L+0.5Lr. Based on your response to the original question in this post, this will generate couple of LCs out of which 0.9D + 1.6L & just 0.9D is not a part of any US code provision so I am not sure what it is based on and what we are achieve with this suggested AEFs. I tried turning the live load pattern off by changing off-pattern factor to 1 for LL and then checked the result of this suggested LC and I was surprised to see max and min moment envelope as I thought max and min would be governed by single load combination 1.2D+1.6L (assuming Lr=0) as live load pattern is off. I couldn't understand how it produced max/min moment envelope with LL pattern off.

  • You could use live load patterns to accomplish the same thing. It seems better to use alternate envelope factors as the default combinations are already set up to cover that condition and you would not need to devote one of the 10 permitted patterns to account for that condition.

    It is common to produce net axial forces in design sections for gravity load cases, especially in models with beams or slab areas with varying thickness/surface elevation. Refer to the web page below for more discussion. When that happens, there may be a net compression force under dead load that decreases reinforcement requirements for axial-flexure interaction compared when the net compression is ignored. Use of 0.9 for an alternate load factor is a good way to ensure that you are not overestimating the dead load and the benefit of the net compression load.

    Table 15 in the RAM Concept Manual has a good summary of how the maximum and minimum context envelope results are determined for all cases.

    RAM Concept T-Beams and Axial Forces [TN] - RAM | STAAD | ADINA Wiki - RAM | STAAD | ADINA - Bentley Communities