Tension Piles

For checking of uplift capacity of piles, Ra and Ru shall be calculated. However, Ra and Ru are not clearly defined in the Code of Practice for Foundation 2004 (for HKSAR).

In the COP, Ru is defined as "ultimate anchoring resistance of the pile" (Clause 5.1.6 and 5.3.3) while Ra is defined as "allowable anchorage resistance of the pile" (Clause 5.3.3).

As mentioned in the CODE, in general, Ra = allowable uplift resistance of the pile shaft + effective self weight of pile.

To summarize, Ra and Ru may be taken as below:
(i) Ru = the weight of soil cone near the pile
(ii) Ra = smaller value of (Bonding capacity of pile socket, Ru/ F.O.S. of 2)

Moreover, Ra is usually taken as less than 1/2 of piles' compression capacity in order to get rid of uplift test of tension pile.

Floor Slab in ETABS

When floor plate elements are modelled in ETABS, they can be either modelled as:
(i) Plate element, which contains out-of-plane stiffness only;
(ii) Membrane element, which contains in-plane stiffness only;
(iii) Shell element, which contains both in-plane and out-of-plane stiffness;

It shall be noted that if the shell element is adopted, in normal situation, all the loading would only be transmitted to the shell's node points instead of its supporting beam/wall elements.

In order to allow gravity load transfer, the following modelling procedure shall be adopted (for normal floor slab):

(1) Use Membrane Element:

• Model the floor slab as Membrane element;
• Assign rigid diaphram for lateral load transfer;
• In "Floor Meshing Options", choose "Default";
• For the beams supporting the slab, choose "Use Line For Floor Meshing" (Especially important for using large slab element in model)

OR

(2) Use Shell Element or Plate Element:

• Model the floor slab as shell element or plate element;
• Assign rigid diaphram for lateral load transfer;
• In "Floor Meshing Options", choose "Auto Mesh Object into Structural Elements" with small element size (say 1m);

For Transfer Plate, the following procedure should be adopted:

(1) Define the transfer plate as a shell element using the actual thickness for "membrane" and "bending" properties.

(2) Draw meshing lines using "NULL" lines. Make sure all the column that is being transferred are connected.

(3) Go to Assign --> Shell/Area -->Area Object Mesh Options. Under Floor Meshing Options, choose Auto Mesh into structural elements and check mesh at beams and mesh at wall and ramp edges. Leave the other two unchecked.

Dynamic Response of Structure

For long span structure subject to periodic load (like human walking load), it is necessary to estimate the human comfort.

Computer program like ETABS, SAP2000 can be adopted to estimate the slab/ beam acceleration under such load. The procedure is outlined as follow (using ETABS as an example):

(1) Obtain the function of the periodic load from relevant code, like BS5400, PRC codes, etc;
(2) Define load case for the periodic load;
(3) Define Time History Function (say Sine Function) for the periodic load. Usually the amplitude for the function is taken as 1 for easy reference;
(4) Define Time History Case Data, with scale factor same as the amplitude stipulated in relevant codes. The load case (under item 2 above) shall be selected;
(5) Add loading on the floor elements under the load case in item 2 (Usually taken as 1 for easy reference);
(6) Carry out the modal analysis (with mass defined);
(7) After analysis, click "Display" -> "Show Time History Traces";
(8) Select the following: 
     "Define Function..." -> Select the Joint concerned -> Click "Modify/Show TH Function"->Vector Type "Accel" -> Component "UZ";

The result is displayed as follow:

The acceleration shall then check against the code requirement.