Replacing Portion of Slab-On-Grade of Pre-Fab Metal Frame Building

[Eng_Struct]

Hi everyone,

We're working on a project involving partial slab replacement in a pre-engineered metal building constructed in 1998. The existing 5" thick slab-on-grade (reinforced with wire mesh) includes hairpins from column piers for lateral thrust resistance. Although there is debate on relying on hairpins for lateral thrust through slab-on-grade (wire-mesh capacity, relying on larger tributary widths, saw-cuts causing discontinuity from column to column), the hairpins appear to be old ways of doing things and the building appears to have performed well to date with this detail.

For increased racking loads, we need to replace ~50% of the slab with a thicker, reinforced slab. The new slab will be dowelled into the existing slab using Hilti epoxy anchors, and I plan to expose existing hairpins at the columns and splice them with new rebar projecting into the new slab thus maintaining the existing detail. Unfortunately, the other half of the building (offices, etc.) must remain untouched, so I can’t replace the slab from column to column.

To maintain the tension load path in the slab:

• I'm designing the dowel spacing (10M @ 8" o/c with 8" embedment) to develop the required tension due to lateral thrust across the joint. Note that the existing design appears to rely on slab tributary width equal to the column spacing for tension load development and this is what I also intend on doing

Cutting a 1 ft strip from column to column and adding tie bars (as done in new construction) is not feasible due to limited access.

Question:

Is there a better or more reliable way to ensure lateral load continuity between the new rebar-reinforced slab and the existing wire mesh slab, considering the constraints?
Also, are there any concerns or lessons learned with relying on epoxy dowels for this kind of tension force transfer across an old slab-on-grade?

Any thoughts or suggestions are greatly appreciated!

Thanks in advance!

 

[Ron247]

Note that the existing design appears to rely on slab tributary width equal to the column spacing for tension load development and this is what I also intend on doing.

Are you saying the tip-to-tip distance of the hairpin is equal to the bay spacing?

PEMB DL&LL thrust magnitudes are mostly governed by clear span of frames (shorter the better), frame height (taller the better, yes taller), Roof Snow versus Roof Live (less is better) and bay spacing (less is better). There are combinations of these where hairpins work well because the thrust loads are low, not because hairpins are a great design concept. Buildings that clearspan 60' and less in width are less of a concern to me than 100'+ ones.

Before getting advice, what is the estimated DL thrust and what chance will there be that you will get Snow or Roof Live during the process? Wind and EQ are also a concern. You have to deal with the DL thrust the entire time you are under construction.

 

[Eng_Struct]

Are you saying the tip-to-tip distance of the hairpin is equal to the bay spacing?

The hairpins are 10 ft long on both side of the pier projecting into the existing slab 1X:2Y slope (y direction parallel to the frame). I am using the slab rebar engaged based on the failure zone below. The width of the developed failure zone with developed slab rebar comes out to be equal to the bay spacing (25ft c/c).

Before getting advice, what is the estimated DL thrust and what chance will there be that you will get Snow or Roof Live during the process? Wind and EQ are also a concern. You have to deal with the DL thrust the entire time you are under construction.

I have two span systems with each span being 110ft. The DL load thrust at the outer column is roughly 8.2 kips. The construction is happening during summer so snow or roof live is not a concern. I have checked the footing for sliding and overturning considering unfactored forces and passive resistance and I am getting F.O.S for overturning to be around 1.11 and F.O.S. for sliding around 2 for deadload thrust case when the slab is not present.

I have also checked the wind (unfactored) for temporary condition and getting similiar F.O.S. as above.

I am thinking the structure should be OK for temporary conditions with the above F.O.S. but let me know if you think otherwise.