Long Span Joist Axial Load and End Moments- Vulcraft Load Table

[cgstrucg]

Hello Everyone,

I have a project in NYC where I have a huge roof in which we have a lot of long-span joists. We are preparing a load table for vulcraft and I have 2 questions about the loads.

1. Vulcraft asks for Live load end continuity Moments/Wind and Seismic end moments. We are already providing them uniform load and other point loads on the joist. My question here is - Why do they need these moments? We are already providing them with all uniform and point loads and the moment can easily be calculated right? I think they are still asking for this as the force collector joists will ultimately have more loads and they need to know that value - please correct me if I am wrong.

2. They also need Em values. From ASCE7-05 I found Em is (0.2*Sds*Dead Load) +/- (Overstrength factor*Seismic Load). If I use this for end moments, I have cases where dead load is positive and seismic is negative. Do we still give them the worst conservative value by considering seismic as positive. For eg - if following are your values - Dead Load Moment 10k-ft, Seismic Load Moment -30k-ft, Overstrength factor 3, Sds 0.3. Then in one case you will have 0.2*0.3*10 + 3*(-30) and then there is this case which is overly conservative 0.2*0.3*10 + 3*(30). I don't want to go ahead with conservative as I don't think that's right. But it would be really helpful if someone can advise me on this.

Thanks

 

[SteelPE]

The joist manufacturer is asking for the end moments on the joist (if you plan on connecting the bottom chord of the joist to decrease deflection or achieve frame action). If the joist is simple supported then there is no end moment. If you welding the bottom chords to something (say a column) then you will need to provide the manufacturer the end moment that needs to be resisted. These moments would come out of your analysis program. If you were welding the bottom chord, I would also give the manufacturer an effective moment of inertia as well.

What type of load do you have on this roof? I assume you have uniform dead and snow loads (maybe snow drift). You also probably have point loads from RTU's and maybe uplift from wind. What other types of loads do you have?

 

[cgstrucg]

Thank You for the clarification. I have clarity about moments on joist now.

We have cranes, movable walls, rtu's, solar panels, wind uplift, parapet drift, seismic, uniform dead and live. I am providing all these values. About Em - Should we still provide the more conservative value?

 

[straub46]

I presume you're providing DL, LL, WL, etc. values to them? Sds? If so, I think EM is just E * Omega, and that they run the combos required to get the loads the joists need designed for. Do they ask for an E load as well?

I could be wrong however. Maybe calling and asking would be beneficial.

Go Bucks!