The document summarizes a presentation given at the Structures Congress 2017 that compared the structural integrity of flat slabs designed according to ACI 318 and New York City building codes using alternative load path analysis. The presentation described the research inspiration, case studies designed for each code, an applied element method used for analysis, modeling techniques, results showing load redistribution after column removal was similar between codes at around 26%, and conclusions that bottom reinforcement capacity controlled for both codes but NYC code had higher integrity steel requirements.

1. WELCOME TO THE
STRUCTURES CONGRESS
2017
Use of Numerical Techniques in the New Alternative Load Path Analysis Guidelines
Session ID: 54
Moderator: Ahmed Khalil, Ph.D., A.M.ASCE , Joseph Main, Ph.D., M.ASCE
Track: Blast and Disproportionate Collapse
Date: Friday, April 7, 2017
Time: 10:00 AM - 11:30 AM

2. Structural Integrity of Flat Slabs:
Building Code Comparison using
Alternative Load Path Analysis
Jennifer Lan, P.E., S.E, Ramon Gilsanz, P.E., S.E, F.SEI, Ayman El Fouly, P.E.
LEED AP, GILSANZ MURRAY STEFICEK LLP
APPLIED SCIENCE INTERNATIONAL, LLC

3. • Research Inspiration
• Design of ACI and NYC case studies
• Applied Element Method Background
• AEM Verification for Flat Slab
• AEM Model for Full Structure
• Results
• Conclusion

4. Code Comparison
• ACI 318
• New York City Building Code

5. Mode of Failure
Literature shows the rotation is between 15-30 degrees.
Corley and Hawkins 1968, Peng, Orton, et al, 2015.

6. • In the event of a column
failure, adjacent columns
pick up more load.
• In a slab with a regular
column grid and square
panels, the increase in
load is approximately
25%-35%, depending on
the analysis method.
Column
Failure
Adjacent Columns
Mode of Failure

7. SAFE Model
Before: 82 kips
After: 105 kips (28% Increase)

8. SAFE Model
Punching Shear Ratio
under Service Loading
(157 psf) =0.68
Punching Shear Failure
Load = 230 psf

9. Horizontal Ties
ACI 318 13.3.8.5
• At least two of the column
strip bottom bars in each
direction shall pass within
the column core and shall be
anchored at the exterior
supports.
New York City §28.2-1917.2.3
• At each column, provide
bottom reinforcement that
can develop a tension equal
to the maximum of:
• 3x the self weight of
the structure entering
the column at that
level
• 1.5x the load entering
the column at that
level using the load
combination 1.2D+1.6L
or 1.4D

10. Application of Integrity Steel
In the following slides we will present an
example to show the impact of the two different
provisions

11. Example
• Two way slab with 22’x22’ bays
• 8”, 6000 psi slab
• Typical Residential Loading
– Dead Load 100 psf
– Superimposed Dead Load 17 psf
– Live Load 40 psf
• Typical bottom mat with #4 bars, top bars are #5
or larger

12. Example

13. Integrity Bars
New York City §28.2-1917.2.3
• 3x the self weight of the structure entering the
column at that level
• 3 x 100 psf x 22’ x 22’ = 145 kips
• 1.5x the load entering the column at that level
using the load combination 1.2D+1.6L or 1.4D
• 1.5 x (1.2 x 117 psf +1.6 x 40 psf) x 22’ x 22’ = 148 kips (CONTROLS)
• 1.5 x (1.4 x 117 psf) x 22’ x22’ = 119 kips

14. Integrity Bars
New York City §28.2-1917.2.3
• As = 148 kips / 60 ksi = 2.4 in^2
• Approximately 12 #4s, evenly distributed
around the slab/column interface
• 3 #4s at each column in two perpendicular
directions

15. Integrity Bars
ACI 318 13.3.8.5
• 2 #4 bottom bars pass through the core of the
column in each direction
• Capacity of Integrity Steel:
– 2 Bars x 4 sides x 0.2 in ^2 x 60 ksi = 96 kips
– In comparison, NYC Requires 148 kips of capacity

16. Demand on Integrity Steel after Column Failure
– Assume a 30 degree rotation in the slab
– Assume the adjacent columns pick up 36% more load
DL= 100psf x22’ x22’ x1.36 /sin30 = 130 kips
SDL= 17psf x22’ x22’ x1.36 /sin30 = 22 kips
LL= 40psf x22’ x22’ x1.36 /sin30 = 52 kips
Integrity Bars

17. Capacity
• NYC – 148 kips
• ACI – 96 kips
Demand
• DL = 130 kips
• SDL = 22 kips
• LL = 52 kips
Integrity Bars

18. Taking into account of variation in the slab rotation
(20-30 degrees) and the distribution of loads (25%-
35%), we get the following ranges:
DL= 121-191 kips
SDL= 20-32 kips
LL= 48-76 kips
Integrity Bars

19. Capacity
• NYC – 148 kips
• ACI – 96 kips
Demand
• DL = 121-191 kips
• SDL = 20-32 kips
• LL = 48-76 kips
Integrity Bars

20. The continuum is discretized into
elements connected together with
nonlinear springs that represent the
material behavior
AEM is a stiffness-based method
The springs represent axial
deformations as well as shear
deformations
Applied Element Method
Overall
Stiffness
Matrix
12 x 12 stiffness matrix
Applied Element Method Background

21. AEM Model
Concrete Model Steel Model

22. Verification for punching shear failure modeling
Univ. of MissouriEffects of In-Plane Restraint on Progression of
Collapse in Flat-Plate Structures
Zhonghua Peng, S.M.ASCE1; Sarah L. Orton, M.ASCE2; Jinrong Liu3; and Ying Tian, M.ASCE4

23. Verification for punching shear failure modeling
Univ. of MissouriEffects of In-Plane Restraint on Progression of
Collapse in Flat-Plate Structures
Zhonghua Peng, S.M.ASCE1; Sarah L. Orton, M.ASCE2; Jinrong Liu3; and Ying Tian, M.ASCE4

24. Options for modeling reinforcement using ELS
Explicit modeling in the
form applied elements
Implicit modeling in
form of equivalent
springs at the location
of every reinforcement
bar

25. Results of punching shear failure modeling

26. Results of punching shear failure modeling

27. Results of punching shear failure modeling

28. Results of punching shear failure modeling

29. Mode of failure comparison

30. Res
Comparison of AEM results to FEM results and
Experimental results

31. Options for modeling reinforcement using ELS
Explicit modeling in the
form applied elements
Implicit modeling in
form of equivalent
springs at the location
of every reinforcement
bar

32. Comparison of two options for modeling in AEM
to experimental results

34. Top steel distribution is identical for both ACI and NYC designs
Additional top bars 15’ above center columns and 12’ with hook above edge
columns

35. Bottom steel distribution has all bars continuous in the column strip for both designs
For the field strip, in the ACI design half the bars stop within 3 ft of the column centerline whil
they are continuous for the column strip
NYC designACI design

36. Additional Bottom RFT for integrity is different in the ACI and NYC designs
ACI NYC

37. The same model is used for ACI and NYC except for the change in reinforcement

38. Removed Column Load Before Column Removal Load after Column Removal
104 kips
151 kips
104 kips
151 kips
108 kips
147 kips
108 kips
147 kips
1
1
22
Redistribution of force to neighboring columns
Redistribution was almost 26% at maximum load for both ACI and NYC codes
Load redistribution ratio depends on whether linear or nonlinear analysis is used

39. NYC Case 20 degree

40. Top bar failure
Bottom bar failure
NYC Case

41. ACI Case 22 degree

42. Top bar failure
Bottom bar failure

43. CONCLUSION
-60
-50
-40
-30
-20
-10
0
0.00 0.20 0.40 0.60 0.80 1.00 1.20
Deflection(in.)
Time (sec.)
NYC
ACI