Shock design under quasi static design method 1

[e104909]

Fellow Engineers,

I have a steel lift of 16m high which should be design under shock loadings..in the specification it should be designed to 5g.

In the code it says "the static equivalent load acting on an "item" is obtained by multiplying the mass of the item by the number of g's specified

The question is..should it be designed such that i will be having a traingular load or like a constant pressure? As the selfweight and glass load will accumulate from top to bottom.

2nd is it says "the system is then designed for the ultimate limit state using the dynamic values of material strengths with partial safety factors for materials of 1.

Does it mean that It could be combined to ultimate loadings with factir of safety of 1?

Anyone familiar with shock design? Im afraid i am under designing thr lift as the area should be design to carry shock loadings and it is just beside area where blast is pOssible.

Regards,
E104909

 

[JAE]

Not sure of your source but I would suggest that, based on your statements, the load should be applied based on where the mass is, in other words, if you have larger mass at the top, more of the distributed load at the top, etc.

On your second question - not sure what they mean by "partial safety factors for materials". I would have expected them to say something like "load safety factors". The word "materials" seems to imply strength reduction factors or something.

 

[SAIL3]

I have designed for a max stress of about 75% of ultimate using real (or as close to possible) loading.
That means the old ASD load factors.
In cases where I have to make an engineering judgement, I prefer to work with the actual loads/stresses and then make an informed judgement on how high to go with the allowable stresses, bearing in mind any stability or other factors pertinent to the particular structure.
My reasoning:
1.Shock loads are not a sustained(or following) load.
2.Based on the above, stability issues are less of a concern.
3.The load is gone by the time the structure has time to react
to it.
Each structure should be evaluated for unusual stability concerns
as this is the major area where one takes liberty with reducing the factor of safety.
Would have no problem with letting the stresses go to yield absent any of the above mentioned concerns..anyting beyond yield is your engineering judgement.

 

[e104909]

JAE,

I am using BS standard on this that is why material strength is divided by a certain factor. 1.5 for concrete and 1.15 for steel.

However, If I will apply the case then it is like the load will be accumulated from top to bottom as mass will distribute from members on top to bottom. Say I have steel structure with beams at every 1.2 meters. Then the self weight of members on beams will go to column then I can get the first lateral load which is 5 x selfweight from beam and column on top most (say level 16). Then for the lateral load on level 15 it would be self weight of beams & column in level 15 x 5 plus weight in 16th level? that seems over kill if I do it but thats the way I understand it. I am also not sure.

SAIL3,
thats true. load is gone before the structure reacts to it like earthquake load. but say there is a bomb nearby and it hits the member where my lift is connected. then surely this will experience such loading which we could simplify as static. but this should be considered as rigorous analysis if we want to. though we know it would be a lot of effort.

As per code requirements it states that it could be analyse assuming 5g the most. but the application of the loading is what i am really confused. it did not state in the above phrase.

If i will interpret that statement then I could simply check my members by element. Say beam, apply selfweight and glass loads then obtain the bending and muliply by 5. then thats it. I just compare to capacity. But this is local checks..Globally I need to know how structure reacts as a whole as my lift is a bit high and area is a bit small. I am afraid that I am under designing my structure.

Regards,
E104909

 

[SAIL3]

looking at your post again, perhaps I was abit to general in my response. In your case, where it concerns the presence of personnel/public, then I would tend to be more conservative in my judgements.
I assume that it is a vert impact load which is different than an air blast(pressure wave) load.
In this case, I would tend to consider stability which you are doing by pulling the member capacity from the tables in the code.
I would double check with the customer to clarify if the 5g is a static equivalent load.
If it is, then just use a 5g load and treat it as a static load.
The difference between seismic and shock is that seismic is repetative(or ongoing) and therefore stability is more of a concern.
Also check area of impact for permanent deformation/rupture/failure.

 

[haynewp]

When I have done static equivalent loads for blast they have not been applied as triangular but a constant magnitude. They are not very economical as a replacement for a dynamic analysis in the case of blast, in my experience.

 

[e104909]

SAIL & haynewp,

yes I agree with the static equivalent load of 5g.

Say I have my 3d structure and I want to apply it in columns.

If the value would be of constant magnitude, then how to get this value? 5 x sum of reactions / total height of column? Is this correct? any idea how?

Blast is a different consideration, wherein the area is marked as direct hit. But because the area where blast may occur is where the beam that the steel lift is supported (on top) is located. So if blast occurs, its somehow vibrate/shock or may transmit pressure wave or air blast. As per code the max load is 5g. I just want to assure that I am not under designing the stability of the structure and most important of all is "the connections".

Regards,
E104909

 

[haynewp]

For an entire lateral load system analysis I would take the entire system mass times 5 and distribute the load horizontally in the same profile as the mass is distributed up the structure and then find the structure's response.

Are you looking at the load being applied horizontally or vertically or both? It sounds like you may have an uplift shock as well. You may help clear it up with a sketch.

 

[e104909]

haynewp,

Thank you very much for your reply and also for the interest asking for a sketch.

I have made a rough calculation of assumptions as attached in this thread that may help me identify my mistake or misunderstanding.

This calculation might also give other engineers insight for the shock loading in case they would face it in future or if they haven't experience this.

We might learn from each other. specially for those people who will scrutinize my calculation.

I hope this would help.

please feel free to comment.

fyi..I asked many engineers in our office..it surprised me that it seems no one knows how to do it and some of them seems not very sure.

Regards,
E104909

 

[haynewp]

If the mass is distributed uniformly like that then I think you are meeting your 5g on the mass. Blast loads also act vertically though.

I don't know about that 5g provision, I have never seen anything like that in the blast criteria I use when looking at the actual blast pressure on a system. There are equivalent static vertical loads that have to do with loads on floors and collapse prevention. There are also equivalent static loads for window frames, but that is handled completely differently than using a factor on g. Does it allow this 5g for all shock loads no matter the size of the charge? What BS standard is it?

 

[SAIL3]

Took a quick look at your loading.
I would use a typical load factor(1.6 ?) for the shock load in LC29 because of the following:
1. Structure directly involved with personnel/public safety.
2. Operation of lift is very sensitive to lateral deflection.
3. Deflection of car rails is critical.
4. Model assumed fixed laterally at floor slabs... this may not
be valid , if bldg structure also deflects laterally.
5. Is the lift glass-enclosed?...if so, would not have expected
that for this type of load.
6. I would have expected that the loading criteria and lateral
deflection of the lift structure could be more strict than
a typical structure.
Just plugging in numbers into a computer program and running with the answers is never a good idea and especially with this type of structure. All aspects of the design must be thought thru and using sound engineering judgement either accepted or rejected.

 

[e104909]

SAIL3,

I would not use 1.6 as a factor because as per code requirements this is ultimate already. Though I might under estimating my loading's if I will not include vertical shocks which I am looking now.

Model is "pinned" at the slab at concourse level and is pinned also at the top. The support condition at concourse level is pinned through 12mm thk. steel plate (cast in with shear studs at ends) embedded in concrete (about 150mm offset from face of concrete to face of column). The support at top is welded to a cast in plate. The support on top is slotted in vertical direction.

As you may observe in my model I just put fixed but support conditions just for me to obtain the lateral reactions and check the plates, bolts and welds. For the frames I used moment connection as beams and columns are all welded all around.

Yes the structure is enclosed by glass.

I understand the last statement and I agree. I would like to thank you for reminding me though.

Garbage in = Garbage out..That is the purpose I am posting on this forum to obtain ideas. and of course I could not let computer to run me. =)

Can you elaborate more for 2, 3, 5 & 6?

For number 2 & 3: car rails will be handled by mechanical team. Though is it supported every 1.2 meters. Yet reactions (last page of my calculation?) are quite small.

For number 5? Please elaborate.

For number 6: what criteria should steel lift fall? Is there any deviation from code? I use L/200 or L/300. Can you justify your expectations in numbers?

Looking forward for your response.



Regards,
E104909