Vertical Bracings takes a considerable amount of the gravity load
Vertical Bracings takes a considerable amount of the gravity load
(OP)
when modeled a multistory steel building In SAP2000 or any FEM Program the vertical bracing takes about 35% of the gravity load. I don't Like that; I like designing the columns for the whole gravity load. I want to know what do people do? Do they really design the brace for its share of the gravity load and the columns for the rest?. If not how they model it






RE: Vertical Bracings takes a considerable amount of the gravity load
RE: Vertical Bracings takes a considerable amount of the gravity load
RE: Vertical Bracings takes a considerable amount of the gravity load
RE: Vertical Bracings takes a considerable amount of the gravity load
RE: Vertical Bracings takes a considerable amount of the gravity load
RE: Vertical Bracings takes a considerable amount of the gravity load
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Vertical Bracings takes a considerable amount of the gravity load
But I have two other Questions:
do people really do this when modeling a multi-story building?
The other question why not design the columns and bracing to their share of forces especially this is what really should happen in reality; that is the brace will take part of the gravity loads
RE: Vertical Bracings takes a considerable amount of the gravity load
And to answer your question...structural engineers use a wide variety of modelling techniques, including the good old pad and pencil, especially in the preliminary design phase. When it gets to the final design, many of us design the floor system first, run the loads down for the columns, design the columns, and only then refine/design the lateral system. Sometimes there have to be changes to the gravity system to accommodate loads imposed by the lateral forces, but we don't revise the bracing/lateral system to take gravity loads.
RE: Vertical Bracings takes a considerable amount of the gravity load
most of the columns sections are HEB500. the Vertical bracing are 2channels UPN240 Forming box section
thanks for your quick response
RE: Vertical Bracings takes a considerable amount of the gravity load
RE: Vertical Bracings takes a considerable amount of the gravity load
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Vertical Bracings takes a considerable amount of the gravity load
RE: Vertical Bracings takes a considerable amount of the gravity load
If you imagine a pair of columns with a floor beam below and above, with an X-brace - if the upper two joints move relative to the lower two joints then the X-braces MUST take axial load. There is no getting around this. The question is whether you must consider this axial load in your design (i.e. define a competent load path per hokie66 above).
I think the concern I would have is that if you have a multi-story building with fairly rigid braces which soak up some axial load, perhaps significant axial load, then under a seismic event or wind load condition your brace would have additional axial force where you would have somewhat less capacity due to earlier buckling.
RE: Vertical Bracings takes a considerable amount of the gravity load
You can always adjust your columns (by hand) to carry the full load w/o the bracing. How difficult is that?
RE: Vertical Bracings takes a considerable amount of the gravity load
RE: Vertical Bracings takes a considerable amount of the gravity load
As far as having to create two different computer models in order to do the above, boo hoo, it takes 10 extra minutes, try doing it with moment distribution...
RE: Vertical Bracings takes a considerable amount of the gravity load
RE: Vertical Bracings takes a considerable amount of the gravity load
RE: Vertical Bracings takes a considerable amount of the gravity load
That's the only explanation I have for the fact that both designs work in practise.
It would bother me to put in columns that were light because some of the D & L loads were in the bracing.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Vertical Bracings takes a considerable amount of the gravity load
RE: Vertical Bracings takes a considerable amount of the gravity load
One school of thought is to model everything together (i.e. the ETABs way) and solve it all at once. The other school of thought is to separate out the gravity solution from the lateral solution (i.e. the RISAFloor or RAM method).
Both methods will have their own pros and cons. But, I'd say this thread does a good example of explaining why this 2nd method is relatively popular. If you've got braces that would take significant load out of columns (or beams in a Chevron type frame) under pure gravity loading, then many engineers would prefer the 2nd method over the ETABs way.
RE: Vertical Bracings takes a considerable amount of the gravity load
RE: Vertical Bracings takes a considerable amount of the gravity load
RE: Vertical Bracings takes a considerable amount of the gravity load
I would think that the only gravity loads you'd want the braces to be able to withstand would be from column shortening.
RE: Vertical Bracings takes a considerable amount of the gravity load
If you solve your gravity only load cases (or gravity + notional loads) in your lateral model then the braces will be designed for their share of the gravity loads. That
RE: Vertical Bracings takes a considerable amount of the gravity load
RE: Vertical Bracings takes a considerable amount of the gravity load
the analysis isnt wrong... load follows stiffness. so your bldg wont fail. your emgineering judgement is right. i'd have the same concerns as you. you just need to go model crazy. just upsize the columns by a reasonable amount and be done with it
RE: Vertical Bracings takes a considerable amount of the gravity load
RE: Vertical Bracings takes a considerable amount of the gravity load
RE: Vertical Bracings takes a considerable amount of the gravity load
"We shape our buildings, thereafter they shape us." -WSC
RE: Vertical Bracings takes a considerable amount of the gravity load
RE: Vertical Bracings takes a considerable amount of the gravity load
This one seems to have brought lot of attention and I think points out some limits to consider in the total reliance on computer model results. Most programs in a conventional analysis mode complete the formation of the structural system in a zero gravity environment and then turns on the gravity switch once all the stiffness present (not the way a building is constructed in the real word). This approach will tend to be less accurate for the amount of gravity load present in braces, particularly for upper stories. In fact in high-rise structures it's not unusual to see computer model columns on upper floors go into net tension due to column shortening and load redistribution in the bracing system. Doing a sequential loading model or using a parallel model with reduced brace axial stiffness can be helpful to bracket the solution for the column/brace forces, as others have suggested.
Also if you chose to go with an design that rely on a large portion of the gravity load being carried in the bracing I think you'd have to question the potential ductility and performance of that system. Where does the gravity load go when the brace buckles?
regards,
Michel
RE: Vertical Bracings takes a considerable amount of the gravity load
I would design the X-bracing as a rope tension only and the V and single diagonal braces to take their share of the gravity load under gravity+lateral combinations.