Anchors in uncracked or cracked concrete

[dcceecy]

We are designing exterior steel stud walls for a military hospital.
The lateral resistant system of the 5-story building is special steel moment frame. The floor and roof are light weight concrete on metal deck (diaphgram). The building is under Category IV, seismic design category D, and some parts of exterior wall are subjected to blast load.

I have some questions about the anchor of steel studs to the concrete floor and the under side of metal deck (into concrete through metal deck).

1. Are we expecting concrete floor to crack?
2. Do we need to use those anchors approved for cracked concrete?
3. if we use approved anchors (for cracked concrete), Do we need to use reduced capacity of anchors in cracked conrete? Due to the blast load, the demand of anchor capacity is unreasonably high.

Actually, A/E has approved the shop drawings. we did use approved anchors for cracked concrete. But we did not use the reduced value of anchor in cracked concrete for blast load (the worse case controls the design).
The wall contractor brought up this issue. Any suggestions? if somebody finds that later, it will be big trouble for them.

 

[ishvaaag]

Normally one would use the reduced value of anchor in cracked concrete where the concrete is expected to be cracked, the tensile sides of slabs and beams (and beamcolumns, and walls).

However the blast load is one absolutely eventual, and this means that probabilistic -i.e., average- (not characteristic, i.e., warranted) values for the anchor would typically be tolerable for the hypothesis. Since delivering average a bigger anchor force probabilistically, your hypotheses may be close to the permissible value in that approach, but that would need to be examined.

 

[ishvaaag]

My comment above starting from common approaches to eventual and extraordinary solicitations. However your governing code may have specific direction on the matter.

 

[ishvaaag]

Just to also mention it, the traditional approach for extraordinary events loading -aiming to survival of people and goods- is to compare expected actual average solicitations with actual average strengths, but of course this is a minumum that may be overridden by the governing code.

(Overstrength to inflict a bigger load on something can for such cases be seen as just a device of restitution of the proper level of expected actual loading).

 

[amec2004]

Below is extraction from ASCE Design of Blast Resistant Buildings in Petrochemical Facilities section 5.4.6 on Page 5-7

5.4.6 Anchor Bolts
Blast loaded structures produce high reaction loads at column supports. This usually requires substantial base plates as well as high capacity anchor bolts. Achieving full anchorage of these bolts is of primary importance and will usually require headed bolts or plates at the embedded end of the bolts to prevent pullout. When anchor bolts are securely anchored into concrete, the failure mechanism is a ductile, tensile failure of the bolt steel. Insufficient edge distance or insufficient spacing between bolts results in a lower anchorage capacity and a brittle failure mode.

Post-installed bolts will be required at times for attachment of equipment which may be subjected to large accelerations during a blast. Expansion anchors should be avoided for most blast design applications unless the load levels are low. Typically "wedge" type anchors are qualified for dynamic loads although most of these ratings are for vibratory loads and are based on cyclic tests at low stress levels. These should only be used where ultimate loads are less than the rated capacity with a margin of
safety. Epoxy anchors have shown excellent dynamic capacity and may be considered for critical applications.
Often anchor bolts are designed for the maximum axial and shear reactions at the base of the columns as a static load. This method requires a large number of bolts even using dynamic material properties. In reality, the bolts will yield under tensile loads and to some degree, shear loads. That is why it is important to use ductile materials for bolts to guard against sudden failure under peak stress. It is possible to model the tensile response dynamically and take advantage of the strain energy capacity of the bolts. This allows the bolts to respond to the load-time history rather than just a peak load. A dynamic analysis is warranted only for special situations, such as where the reuse of existing bolts is important. For typical designs, a dynamic analysis is not performed because there may not be a cost benefit over a static bolt design. Because shear deformations are more difficult to model and generally don't control bolt sizing, bolts are designed for the maximum predicted shear load rather than a time history response.

I give you the following suggestions

1. In terms of anchor bolt design for blast resistance building, the critical thing is to achieve a ductile design and eliminate brittle failure modes, especially for connection, this applies to anchor bolt design as concrete anchorage connection. The ductility reflects the capacity for structural components to absorb energy during blast explosion.
The anchor bolt ductility requirements specified in ACI 318-08 Appendix D clause D.3.3 for seismic application applies to anchor bolt ductility in blast load as well. You can go to

http://www.civilbay.com

and download the anchor bolt design spreadsheet or anchor bolt design software and check the anchor bolt ductility design part. The anchor bolt ductility is related to anchor rod material, anchor bolt edge distance, anchor bolt spacing, and anchor bolt embedment depth etc.

2. The use of cracked or uncracked anchor bolt capacity is NOT the main issue. The main design concept is to provide ductile anchor bolt design. That is, the anchor rod capacity is less than all other concrete associated failure modes’ capacities.

You shall use cracked capacity for the design because

1) Concrete is naturally cracked material, plus cracking, as well as permanent deformations resulting from a plastic range response, are an expected result of such an unusual type of load like blasting

2) Anchor rod steel capacity might be less than anchor bolt capacity (uncracked) but higher than anchor bolt capacity (cracked) , so using of anchor bolt capacity (uncracked) will cause unsafe design in terms of anchor bolt ductility

3. The seismic design criteria and reinforcing detailing provisions provided in ACI 318-08, mostly on ductile design concept, are also applicable to the design of blast resistant structures

4. If anchor reinforcement is used for anchor bolt design to replace the concrete breakout strength, rebar development length should not be reduced for excessive reinforcement.

5. For load combination, only use 1.0DL + 1.0LL + 1.0 Blast Load for anchor bolt design

6. If post-install anchor bolt is used, avoid expansion anchor, epoxy anchor is good for ductility

 

[hokie66]

Just curious...in a steel building, why not connect the facade to the steel structure, rather than to the concrete deck?

 

[dcceecy]

it's a ribon window in the first floor wall. The sill has structural steel tube. but the header did not have structural steel.
This wall is offset (to inside of the building) from the perimeter of the building and under 2nd floor slab. so there will be exterior soffit under 2nd floor slab.

 

[haynewp]

Besides the ASCE book referenced by amec, see:

"SHOCK LOAD CAPACITY OF CONCRETE EXPANSION ANCHORING SYSTEMS IN
UNCRACKED CONCRETE"
By H. Salim1, Associate Member, ASCE, R. Dinan2, J. Shull3, and P.T. Townsend4

Whether cracked or not depends on the conventional loads prior to blast. The above paper states that precracked concrete prior to the blast load may not always be representative and may be less often the case. If determined to be cracked prior to seeing blast, I would use cracked approved anchors.

As far as I know, blast engineers typically use equivalent static reactions for components such as metal studs based on all the plastic hinging results from a dynamic analysis to design the anchors, and use a static approach to design these connections including reduction factors.