Vertical Structural Irregularity Type 5

[smokiibear]

Fellows...attempting to understand vertical irregularity Type 5a and 5b as listed in ASCE 07-05 Table 12.3-2.

I am uncertain how it is ever possible to have a lower story that is weaker than an upper story, unless the upper story by passes the lower story and transfers some of it's load directly to the ground. However, if the lower story resists all of the story shear above, and the shear at it's own level, this condition does not seem possible.

Would someone kindly shed some light on this topic?

Thank you!

 

[PicoStruc]

It happen often.

As an example, if you have some shearwall in the building that stop mid-height (Because not required above), you might have a lateral stiffness irregularity !

Now imagine that the architect want to do some holes in your shear walls !

 

[PicoStruc]

IMPORTANT FACT : It's related to lateral stiffness not shear resistance. You can be stiffer laterally and weaker in shear or vice versa !

 

[smokiibear]

hmm...I understand lateral stiffness...which would be Type 1a or Type 1b. But I am not clear about having shearwalls stop mid-height. Could you elaborate a little?

Maybe a split level building would be an example of Type 5 Vertical irregularity? Still not sure.

 

[PicoStruc]

Podium like structure with SRFS in both podium and tower often have big change in lateral stiffness due to the increase in bracing in the podium.

 

[hawkaz]

An example of this could be hotels with masonry shear walls between units above and big open conference areas at the ground level.

 

[PicoStruc]

What Hawkaz just described is a Podium-Like Structure !

 

[smokiibear]

Okay...pardon my misunderstanging. Nearly all of my structural work is with residential, so I am unfamiliar with other types of design. Still, in the example given by hawkaz, doesn't the ground level still need to tranfer all the shear from upper levels down to the ground. If so, it seems that the ground level is "stronger" than the upper level, but would not be as stiff as upper levels. This appears to me a type 1 irregularity, not a type 5.

What am I missing to close the gap between these two irregularity types?

Thanks!

 

[PicoStruc]

You are right... I mixed numbers 1 & 5. I have bad memory !

Still, Lateral discontinuities in Strengh are linked to the stiffness in a way : Usually, More walls = more stiffness might means more resistance, etc...

All These irregularity must be check case by case for each project !

 

[ToadJones]

Imagine you left the plywood sheathing off the first floor of a two story shear wall but the upper level was sheathed. Or the upper level was sheathed and the lower level wall was full of window openings requiring less-stiff moment frames.

 

[smokiibear]

ToadJones: Thanks for the example. However, I'm unclear how shear would get tranfered to the ground if there are "no shear" elements on the ground floor. This seems like an impossibility to me. Base shear would terminate above the ground.

Pico: Thanks for continuing to explain. I do understand the strength and stiffness are related by deflection. What I don't understand why does the code differentiate between stiffnees (Type 1) and strength (Type 5). If the two can be interchanged, why din't they just make type 1a,b,c,d in lieu of type 1 and 5 a,b?

Thanks.

 

[ToadJones]

Moment frames transfer shear.

 

[smokiibear]

ToadJones: "Moment frames transfer shear." Right. And moment frames are also shear elements, correct? So if there was an irregularity, it would be Type 1a or 1b due to variations in stiffness.

If I am wrong in my understanding, then what constitutes variation in stiffness as compared to variation in strength?

Thanks for continued effort to explain.

 

[Gumpmaster]

One instance would be where your top level has more shear walls than necessary (for architectural reasons). The shear walls would have very low stress. The next story down could have fewer shear walls that are much more highly stressed. The strength of the upper floor would be higher than that of the lower floor. The stiffnesses would also be different. You'd get hit with both irregularities.

 

[PUEngineer]

Another example would be steel moment frames with similar beam and column sizes through the height. If the lowest story is taller and the base is somewhere between fixed and pinned, then the lowest level could have a lower strength. Also, nothing says it can't be BOTH type 1 AND type 5 irregularity. If you'll notice, both of the irregularities bring into play similar sections of the code, with the exception of 5b.

 

[smokiibear]

Gumpmaster...thanks. I can follow that. But if the walls were architectural only, couldn't you simply make the walls non-structural (if wood, then modify the nailing accordinly) to avoid being penalized by either irregularity? Or, do non-structural components also contribute to the stiffness of the structural components? I'm sure they do in practice, but are we required to account for it?

 

[smokiibear]

PUEngineer...you hit the nail on the head, which is what prompted me to ask the question in the first place. Specifically, we have a podium type barn that will be wood shear above support purley by cantilevered posts below. In reading through the Vertical Irregularities, the first thought that popped in my head was, when would you not transfer all the upper level shear through the ground floor level (unless the upper level was transfered to the ground independently of the ground level SFRS. In your example, I would perceive the lower floor is less stiff than the upper floors, but not less strong.

In our project, the posts would tranfer shear from upper and lower level...so I would perceive they are stronger than upper level shear walls, but they will be much more flexible...meaning only type 1 would apply. Would you agree with this...or am I still missing something regarding how to evaulate strength?

 

[PUEngineer]

Obviously the examples depend on the actual numbers. Regardless, the irregularity of type 1 and type 5 can occur simultaneously.

You've only stated you perceive the posts to be stronger, nothing about actually evaluating the strength. You'd have to run the numbers. Shear strength of the shear walls compared to the shear strength at the lowest level based on the controling failure mode of the cantilevered columns. Like flexure at the base, base connection capacity, etc.

 

[smokiibear]

PUEngineer...thanks. What you stated caused me to realize that I was assuming that because the lower level could adequately handle the shear, that it was necessarily stronger. However, using sample numbers, I might have an upper level requiring 1000# of lateral resistance and a lower level requiring a 1800# of lateral resistance. Theoretically, if my upper level could have 3000# capacity and my lower level have 1950# capacity, "strong enough", but definitely weaker. At this point, I'd be looking at a type 5a, and on the verge of a type 5b irregularity.

Agreed?