8 replies to this topic

[Happy]

So, judging from the NCEES exam outline as well as both the 2008 NCEES sample exam and the 2011 NCEES sample exams - we are pretty much guaranteed a question on Dead, Live and Wind loads. However, I do not see any info on this topic in either Goswami or the CERM.

The 2008 question asks for which load combination causes maximum uplift on a footing, and the answer is wind + dead load. Live load is discounted (I assume) because it transfers a force down onto the footing. Dead load does too, but it is counted (again I assume) because it dead load is always counted.
The 2011 question is similar, but this time the answer is wind + live + dead. Live load is counted this time because it is cantilevered to the footing and doesn't transfer its force straight through it.

These questions seem pretty easy - and I don't want to throw away an easy point. Anyone know of a free website where a little more info on load combinations can be learned. Perhaps a few more example questions? Alternatively, if anyone knows of any 'tricks' these problems typically might have and would care to share that'd be great too.

[Walker D]

It is definitely in the CERM but I ran into the same problem you have run into. It is not quite as easy as looking in the index. I don't have the CERM in front of me but I believe it is section 46 Structural Analysis I of the 11th edition. It should give you the run down on different types of loads.

I don't recall if it goes into great detail about when to use different load combinations. Hope this helps.

[kashifbuttar]

Happy were you able to find out any more info on your question of what combination to use in different situations. I am little confused on this as well. and so far haven't found a correct way of looking at it. So if you have any information that you can share it will be great. Thanks

[treyjay]

the NCEES example problems I have seen usually involve a frame with w, d & l loads applied. the questions ask for the "worst case" loading causing uplift. if you think of these in a manner similar to overturning questions on retaining walls, you will look for the "tipping point." if the w+d+l loads add to the overturning, they then cause the worst case loading and you have your answer. if the live load adds to the resistance to uplift, then you discount the live load, because it is a variable load and cannot be counted on being there.

[jaa046]

So, judging from the NCEES exam outline as well as both the 2008 NCEES sample exam and the 2011 NCEES sample exams - we are pretty much guaranteed a question on Dead, Live and Wind loads. However, I do not see any info on this topic in either Goswami or the CERM.

The 2008 question asks for which load combination causes maximum uplift on a footing, and the answer is wind + dead load. Live load is discounted (I assume) because it transfers a force down onto the footing. Dead load does too, but it is counted (again I assume) because it dead load is always counted.
The 2011 question is similar, but this time the answer is wind + live + dead. Live load is counted this time because it is cantilevered to the footing and doesn't transfer its force straight through it.

These questions seem pretty easy - and I don't want to throw away an easy point. Anyone know of a free website where a little more info on load combinations can be learned. Perhaps a few more example questions? Alternatively, if anyone knows of any 'tricks' these problems typically might have and would care to share that'd be great too.

I am having exactly same problem. I came across that problem in NCEES 2008 and 2011 practice exam book.....I tried to ask this question under Structural Engineering section of this forum. You can check it out.

If you find information about this in any book please let me know. if you find it in CERM, please send me the page number.

[Dano_PE]

In the 2008 NCEES sample problems, go to the structural section (I think).

There is a good problem there regarding snow loads on pitched roofs (flat roofs are calculated similarly).

As far as wind loads the Ruwan book has example of this on temporary structures.

If you need a live and dead load problem maybe consider doing a factored load problem for a steel or concrete beam.

Hope that helps.

[andyrich]

Note, according to the NCEES specs for the civil breadth exam, you are only required to know dead, live and construction loads under the current specs (as of 2012).

I just posted a blog entry about dead loads. This may be a little more of a depth question, since technically you will not likely need ASCE 7 for the morning exam. Nonetheless it should get you thinking about the subject.

[wow]

the NCEES example problems I have seen usually involve a frame with w, d & l loads applied. the questions ask for the "worst case" loading causing uplift. if you think of these in a manner similar to overturning questions on retaining walls, you will look for the "tipping point." if the w+d+l loads add to the overturning, they then cause the worst case loading and you have your answer. if the live load adds to the resistance to uplift, then you discount the live load, because it is a variable load and cannot be counted on being there.

This is the correct answer.

If you have both dead and live loads resisting the overturning moment force of the wind load then for the worst case in this scenario you discount live loads because there is a possibility of actually having no live loads. Dead loads will always be in place so it stays as a resisting force.

If the dead and live loads are contributing to the overturning force then for the worst case you leave the live load in because there is a possibility of actually having a live load.

This basically boils down to figuring moments. You figure out where the overturning point is and see which direction the loads would make the structure spin. If all 3 would make the structure spin in the same direction, then its D+L+W, if D+L counteract the W generated moment, then it's just D+W.

[ELASTIC-1]

Sounds like pretty slam dunkish types of questions. At my old job this was stuff we had to do on the regular as overturn for temporary sets of construction equipment was a big deal and often times live load would only contribute to counteracting the overturn moment of the wind (not always). We usually wouldn't combine the worst case wind load (only half of it) and live load as we know that there isn't any group of construction workers that would be crazy enough to be up on our formwork making a Conga line in 75mph wind gusts.