Tower Design using Etabs: Chapter 2 Pile Caps Design
ABSTRACT “Chapter 1 of Tower Design using Etabs ” produced a full model with the
needed loads and parameters thus this t his series will discuss about transferring the loads from the vertical structural elements ele ments to the foundation. Our aim is to clarify why we are using piles, when they t hey are used and how to use them as a foundation for our structure.
Wri Wr i tte tten by : Nada Zarrak C hecked by by : Ahmed Sameer Zarrak
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Table of Contents: ABSTRAC ABSTRACT T................................ ................................................. .................................. ................................. ................................. ............................ ........... 1 *Piles *Piles Concept: Concept:...................................... ....................................................... ................................. ................................. ............................... .............. 3 -Definiti -Definition on of Pile: ....................... ........................................ ................................. .................................. .................................. ................... ... 3 -When Piles are used?................................ ................................................. .................................. ................................. ...................... ...... 3 -Number -Number of piles:.................................. .................................................. .................................. .................................. ........................... ........... 3 -Load Applied on pile: ....................................................................................... 4 -Capacity of pile due to soil: .............................................................................. 4 *Pile caps and Piles first Assumption: ................................................................... 6 *AutoCAD *AutoCAD Stage:................. Stage:.................................. .................................. .................................. ................................. .............................. .............. 9 -AutoCAD Drawing Steps: ...............................................................................11 *Etabs *Etabs Stage: ................................. ................................................. ................................. ................................. .................................. ......................13 ....13 *Safe2016 *Safe2016 Stage: ................................. ................................................. ................................. ................................. ................................1 ................14 4 *Modal Analysis: Analysis: ............................... ................................................ .................................. ................................. ................................2 ................26 6 -Piles -Piles Reaction Reaction Check: Check: ............................... ................................................ .................................. ................................. .....................26 .....26 -Pile -Pile Caps Design: Design: ............................... ................................................ ................................... .................................. ..........................29 ..........29 1.
Punching Shear Check: .......................................................................29
2.
Pile Caps Reinforcement: ....................................................................30
3.
Pile Caps Typical detail Plan: ..............................................................35
Conclusi Conclusion: on: ................................. .................................................. .................................. ................................. ................................. ...........................36 ..........36
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Piles Concept: Definition of Pile: Piles are vertical elements used mainly to transfer loads of the structure to the t he soil or rock . Piles material could be reinforced concrete, steel or t imber and it depends on the situation of the site or the decision of the structural st ructural engineer. The design of piles are divided into two sections, short pile and long pile. When length of pile (L) over its diameter (D) is less than t han 4, it is considered as a short pile and when its length (L) over its diameter (D) is more than t han 4, it is considered as a long pile. It is the job of the subcontractor to classify the pile design in order to design it as per requirements.
When Piles are used? Piles are used in areas where the bearing capacity of o f the soil is very low when compared to the load applied from the structure. To illustrate, when the applied vertical load (Reaction from the vert ical super structural elements-walls or columns) over the area of the structure is greater than the bearing capacity of the soil
(
≥ s). Thus, deeper foundation is required to reach to a higher
bearing capacity s.
Number of piles: When the structural engineer determines to use the Piles as a foundation foundation type for the super structure, he has to determine the number of piles required. The piles n umber are determined by dividing the load applied over the pile’s capacity. pile’s capacity. 3|Page
=
From Etabs we can get the load applied on each column thus we can deter mine how many number of piles needed under each column. The pile capacity is divided into two capacities. First is due to its material and second due to soil. Since the pile capacity due to material is more than t han the capacity due to soil, we have to take the smallest value to ensure a safer design. To get the pile’s capacity capacit y due to soil we have to check the technical soil report.
Load Applied on pile:
As we know, we have the factored load and the service stat e however, which one we should consider in the design stage! We might think it is proper to take t he factorized loads but in case of pile caps and piles, t he safest among these two is the SERVICE LOAD STATE. The The reason behind it is the difference differe nce between the Safety Factor of loads applied and pile’s capacity. pile’s capacity. The average factor of safety considered for piles is 2.5 however, for the Ultimate Load Combination Combination we have 1.2DL+1.6 LL. Such noticable difference will reduce the amoun a mountt of Safety Safet y Factor and in such cases; the Service Load State should be taken into account.
Capacity of pile due to soil:
The relationship between the pile and the soil are fr iction and end bearing. Both relationships gives the pile a capacity to t o handle the vertical load applied. There are several theories to calculate t he friction and the end bearing forces but as a str uctural engineer, the soil laboratories will provide both forces depending on the t ype of the
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soil, the standard penetration test and many other factors. As a result, r esult, the total capacity of the pile will equal to t he capacity due to friction and end bearing.
Figure 1: Friction and End Bearing Capacity due to Soil, http://civiltraining.in
The table below is an example of a pile capacity with reference to its length, depth of boreholes and pile diameter. For example for a depth of 20 m, a pile length of 11 and a pile diameter of 0.6m, the pile capacity due to skin friction is 2305 KN and 1018 KN from end bearing thus total capacity of the pile is equal to 3323 KN.
Figure 2: Soil Report example for a pile cap capacity due to soil
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Pile caps and Piles first Assumption: In general, the Soil Report confirm about using Piles as a Foundation. As a result, the first step is to draw t he first assumption of the arrangement of the Piles and the Pile Caps depths. Open EtabsDisplayForce/Stress DiagramsSupport/Spring reactions
Figure 3: Etabs2015-Display Reactions-Envelop Service
In order to show the Columns and Walls reaction you have to set the Plan to 3 -d from the tool bar on Etabs.
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Figure 4: Etabs-Column and Walls reaction
An Example on how to determine number of o f Pile needed below each column co lumn or wall is as the following: Reaction from Wall = 1255 KN + 1121 KN = 2376 KN Using Table of Piles Capacity “Figure 2”, Pile diameter of 0.6m and z= 20m has a capacity of 3323 KN which is more than the wall reaction thus one pile is used. Several Parameters are used when drawing Piles and Pile Caps: 1. Min distance between two piles is 2.5Ø, while Ø is Pile Diameter. 2. A cover of 20 or 25 cm is added from the first edge when assigning sizes of Pile Caps. 7|Page
3. The center of mass of Pile should be located at the center of mass of the Column. 4. Tie Beams should be added in order to link the Pile Caps together. 5. Minimum Pile Cap Thick = 50 Ø (Pile Rebar Diameter) + 20cm Rebar Cover + 10 cm Pile embedment
Figure 5: Pile Cap Min Thickness Explanation
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AutoCAD Stage: One of the main part in the Design Process is AutoCAD as it is used to DRAW the first assumption of the required Piles beneath each column and wall. The figure below shows the application of the criteria mentioned previously. A Pile of 0.6m diameter (green) is used to support a 2m Wall (yellow) with w ith a cover of 20cm (red) at t he first face for the size of the Pile Cap (white).
Figure 6: Case 1, Pile and Pile Cap DWG example.
Figure 7: Case 2, Three Piles distribution below a vertical support
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As shown in the figure above, the reaction from the wall resulted in three piles in a diameter of 1.2 m. The min distance from c/c of pi les was (2.5x1.2) =3 m.
Figure 8: Case 3, Piles distribution below Core Wall
Figure 7 shows the economical side of arranging piles. The Core wall is only carrying five podiums thus only four piles needed with a diameter of 0.7m. The piles were located at the inner edge of the core cor e as they satisfy the t he regulation of min 2.5Ø distance from c/c pile and as a result, the Pile Cap size s ize has been reduced. In addition the following steps below will help you to produce a neat and easy drawing to be exported into SAFE2016. SAFE2016.
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AutoCAD Drawing Steps:
1.
Arrange Pile Caps, Piles Diameter and Tie Beams in different Layers as shown in Figure 9 in order to ease drawing in SAFE.
Figure 9: AutoCAD Layers of Piles, Pile Caps and Tie Beams
2. Make sure the Origin Points in your Drawings same as in your imported E2K in order to be located at the t he same position. 3. Make sure the Unit used in AutoCAD is “Millimeters” as in your E2K file.
Figure 10: AutoCAD Drawing Units
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4. Save the Drawing into a DXF File Format in order to be read by SAFE2016
Figure 11: AutoCAD Final Stage DWG with Piles and Pile Caps
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Etabs Stage: The first chapter of Tower design using Etabs modeled and analyzed a residential tower. Our next aim is to design the foundation “Pile Caps” in order to support our structure. The main step of the whole process is to export t he reactions from the vertical elements “Columns and Walls”. 1.
File Export Story as Safe V12.f2k File
Figure 12: Option for Export to Safe.f2k file
Story: Base
Loads to Export: Export Floor Loads and Loads from Above
Select Load Cases: All
Select Load Combinations: Service Load Combos ONLY
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Safe2016 Stage: 1. From AutoCAD save the DWG to a DXF 2. Open Safe File SAFE.F2K File Choose you F2K File. 3. File Import DXF/DWG Architectural Plan
Figure 13: Imported E2K with DXF Plan
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1. Define Material: Define Material Add new Material:
Material Name: Fcu 50 Material Type: Concrete Weight per Unit Volume: 25 kN/m3 Modulus of Elasticity: 4700 √ ′
as
per ACI 318
Specified Concrete Compressive Strength, F’c = 0.8xFcu
Figure 14: SAFE2016, Material Property Data
2. Define Slab Property: Define Slab Properties Add new Property:
Slab Material: assign the Material you defined Analysis Property Data, Type: Slab Analysis Property Data, Thickness: as assumed Choose Thick Plate
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Figure 15: SAFE2016, Slab Property Data
3. Piles capacity in SAFE are represented as Point Springs with
Vertical stiffness K=
, ∆
P is Pile Capacity Δ is allowable displacement for pile= 0.01*D, D=Pile Diameter , mm Horizontal Stiffness = 0.1K Define Point Spring: Define Point Spring Properties
Add
New Property:
Translation Z (Linear) = K Translation X = 0.1K Translation Y = 0.1K Nonlinear Option= Option= None (Linear), used in case of a sensitive building
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Figure 16: SAFE2016, Point Spring Property Data
4. Make Sure an Envelope Service Combination Co mbination Exists.
Figure 17: SAFE 2016 Load Combination Data, Envelope-Service
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P ile Caps using “Draw Slabs/Areas" Command in the left 5. Start Drawing the Pile bar tools.
Draw Slabs/Areas
Slab Thickness:
Blue: 3000mm Yellow: 2000mm Red: 1200mm
Figure 18: SAFE2016, Pile Caps Plan with section properties
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Point Springs as mentioned previously: 6. To draw Piles that are represented by Point Try to show each pile diameter in separate plans p lans to ease assigning For
-
example: Option
Architectural
Plan Options Opt ions Choose Pi60
In order to assign Point Spring you have to draw first a Point Draw
-
Draw Points “Check Figure 19”
-
Figure 19: SAFE2016, Draw Points Command
-
Select the Points for Pi60 Assign Support Data
Point
Springs
Choose Pi60 for an example. -
Follow the same steps for the rest of Piles Diameter
-
Make sure Points option is checked in the “Set Display Option”
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Figure 20: SAFE2016, Piles as Points Spring Plan
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7. Define Tie Beams: Define Beam Properties Add New Property:
Beam Material: Fcu50 Beam Shape Type: Rectangle Beam Web Width at Top: 300 mm Web Width at Bottom: 300mm Depth: 1000mm
Figure 21: SAFE2016, Beam Property Data, TB
-
Draw Beam using “Draw Beams/Lines Command” on the left bar tools.
Draw Beams/Lines
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Figure 22: SAFE2016, Tie Beams Plan Extrude View
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8. Draw Design Strips: Design strips are drawn in order to check and design the
Pile Caps. Both direction direct ion (X&Y) should be added and they are represe nted by Strip Layer Type. It’s the Engineer’s decision in defining the direction type. Usually Layer A is considered the X direction and Layer B is the Y Direction. Strip widths usually taken take n to be 3m.
Draw Design Strips
Figure 23: SAFE2016, Design strips Layer A
Figure 24: SAFE2016, Design strips Layer B
-
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Start Width and End Width: used to define the width of the strip st rip in which it will study the slab area for the t he design process. Drawing design strips is like section cut but it covers more in width. Draw design strips for each pile cap separate ly in order to find the reinforcement required for both directions.
Figure 25: Pile Caps size
-
Figure 25 shows a pile p ile cap of 160x460 size To draw design strip for the pile cap of 160x460 size
Figure 26: Design Strip width example, Layer A
Figure 27: Design Strip width example, Layer B
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-
-
Figure 26 shows the width taken to cover the slab in the Y direction as the pile cap size is 1000mm thus the start and end end with as taken as 0.5m and strip layer type is B as we defined de fined layer B to be Y direction. Then draw the design strip at the middle of the pile cap “ Pink Line” To Show the width Set Display option
Figure 28: SAFE2016, Set Display Option
-
The same procedure is done for the other direction and for the rest for the Pile Caps. 9. Save the Model and Run.
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Modal Analysis: Piles Reaction Check: After running the Model, display the react ion from Envelope Envelope Service Combination in order to compare it with the pile’s capacity. -
Display
Show
Reaction Forces
Check
the figure below Max or Min used to check the Piles’ Piles’ capacity in Compression and Tension
Figure 29: SAFE2016, Display Reaction, Point Reactions
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-
The two figures below show two different cases for the assumption of pile’s capacity
Case 2
Case 1
Case 2
Case 1
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-
React ion from Envelope Envelope Service Ser vice Combination= 1506.566 KN Case 1: The Reaction Pile Capacity for P60= 3323 KN as per Figure 2
-
Correct
React ion from Envelope Service Combination=6918.95 Combination=6918.95 KN Case 2: The Reaction Pile Capacity for P70= 4075 KN
-
Assumption
Extra
Pile need to be added.
Follow the procedure for the rest of the piles until you e nd up with a safe foundation.
-
It is important to check the Pile’s capacity capacit y in Compression and Tension, using Max or Min display in Load Combination. Both should determine the appropriate Piles that should be used.
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Pile Caps Design:
1. Punchi Punchi ng She S hea ar C heck: heck: -
The first check of the Pile caps is “Punching”; this criterion will be the first step for the check of the cap’s thickness. However since our columns are tied with “Tie Beams”, no punching check needed and the t he program would give an “N/C” Notation. Notation. To check Punching: Display
Show
Punching Shear Design or from
Tools:
Figure 30: SAFE2016, Punching Shear Check, N/C
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2. P i le C aps Re R ei nfo nfor cem cement: To design the Pile Caps: Check Material of Rebar: Define Materials
-
Figure 31: SAFE2016, Material Property Data, Rebar
Material Type: Rebar Weight per Unit Volume: 78.5 KN/m3 Fy = 420 N/mm2
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-
Display
Show Slab Design:
Figure 32: SAFE2016, Slab Design
-
Design Basis: Strip Based as per our strip addition additio n
-
Design Type: Enveloping Flexural Reinforcement
-
Reinforcing Display Type: for example, size of bar to be added in Top and Bottom = 20 mm, it is up to the Engineer’s choice on how to display the Rebar required.
-
Show Rebar Above Specified Value: None, or it is used when the minimum rebar is defined so the addition would be shown on the plan.
-
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Apply
Figure 33: SAFE2016, Layer A, Top and Bottom Flexural Reinforcement
-
Using the cursor, the max number of Bars in Top is 15T20 and Bottom 15T32 “Long Rebar”, using “figure 37” you can check the typical detail deta il of pile cap reinforcement.
Figure 34: SAFE2016, Layer B, Top and Bottom Flexural Reinforcement
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For Shear reinforcement or face rebar check:
Figure 35: SAFE2016, Slab Design, Shear Reinforcement
Figure 36: SAFE2016, Shear Reinforcement, Layer A and B
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-
As shown in “figure 36”, the design strip A and B shows that the pile cap needs an Area of 1523.353 mm2 of shear reinforcement or face bar
Ø16
is used. -
To check Spacing: the Pile Cap width = 1600 mm, cover of rebar = 150 mm (1600 – (1600 – (2x150) – (2x150) – (15x32) (15x32) /14) = 58.57 mm bars is satisfied.
Figure 37: Schedule of Pile Cap Example
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Spacing
between
3. P i le C aps Typ T ypii cal cal deta detaii l Pla P lan: n:
Figure 38: Pile Cap Typical Detail Plan
Figure 39: Pile Cap Typical Section with Pile Lapping Reinforcement
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Conclusion: The Manual explains the concept of transferring the load from the super structure up to the soil throughout Piles, which has a capacity of (End bearing, and Skin friction). Specialized Technical laboratories provide the capacities’ value from working loads that are estimated based on the unconfined compressive strength of the rock cores extracted from various depths of the drilled Boreholes in addition to its capacity in tension. At the end of this report, you will be able to extract number of piles needed, sufficient pile cap thicknesses and an appropriate finalized foundation.
Special Thanks to my Father.. For any Information, Questions or Doubts please pl ease contact:
[email protected]
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