Concrete piers for steel buildings 1

[Hasna]

Hi
I’m designing concrete piers for steel columns in high seismic zone. What’s the LFRS should I use ?

 

[Harbringer]

Are you designing a cantilever column? This post has almost no information.

 

[dhengr]

Hasna:
I think this thread and your final design is going to need an independent pier review before permitting.

 

[Hasna]

It’s a concrete pier connecting the steel column to the concrete foundation. The steel column will be fixed to the concrete column with steel anchors and steel base plate.

 

[JLNJ]

The EOR chooses the LFRS based on the structure type, uses the steel Manual for the steel parts, the concrete manual for the concrete parts, and the appropriate building code to determine the loads.

But first hire someone who knows what they are doing to be the EOR.

 

[Hasna]

For the steel structure above the concrete piers, it’s ordinary braced frames. The concrete piers are almost embedded in the ground, can they be considered as part of the foundation and not part of the LFRS ?

 

[dik]

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Harbringer]

For the steel structure above the concrete piers, it’s ordinary braced frames. The concrete piers are almost embedded in the ground, can they be considered as part of the foundation and not part of the LFRS ?

If you are designing the piers only, the brace frame designer should give you reactions. Design for those reactions. You would not recalculate a new seismic coefficient.

If you are also designing the anchorage you also apply the overstrength value for the braced frames to those reactions for your anchorage into the pier.

 

[Hasna]

Hi
Thank you for your answer, that’s right. My problem is for the pier concrete design and detailing, I need to know the LFRS of the concrete.
I will use the reactions given by the steel designer

 

[Blackstar123]

The concrete piers are almost embedded in the ground, can they be considered as part of the foundation and not part of the LFRS ?

I get from your post that you are only responsible for designing RCC pedestals and foundations for a steel superstructure and you’re confused about the need to recalculate seismic forces on the RCC pedestals?
If so, you do not need to do that because as you've implied your rcc columns are underground.
Interaction of sub structure with surrounding ground can be ignored for shallow foundations based on the assumption that foundations do not move differentially with the ground in lateral directions and thus do not deform significantly. Unlike foundations, superstructures do experience inertia forces in earthquake events and therefore require some sort of lateral force resisting system to safely transfer the forces to the ground. The situation might have been different if you had a LRFS with a RCC super structure supporting a steel structure. In which case you would have been right to be concerned about the combine response of both type of structure as a whole.

 

[Hasna]

Hi
Thank you for your inputs. No I will not recalculate the seismic loads.
If I understand your answer the piers are part of the foundations, I will designed them as concrete columns (compression + bending)and the chapter 18 will not apply.

 

[Blackstar123]

Chapter 18 of which code?

Since we've established that rcc columns will be transfering reactions from steel structure to the footings (isolated?), then yes, you need to design the column for resulting compression or tension forces due to vertical reacrions and, bending moment due to horizontal reactions, assuming you've a most basic moment resisting foundation system.

But I hope you know that this is only a small part of the overall design. You also need to make sure that foundations are stable against sliding and overturning, perform all the relevant anchorage checks etc etc.

If you're a junior engineer you should take this problem to a senior engineer in your office who could offer you better guidance.

You should also go through the following link for a great learning experience.

https://www.structuremag.org/?p=342

 

[Hasna]

Chapter 18 of ACI 318,
The foundations are checked against overturning, sliding, bearing, punching shear etc…