中震不屈服、中震弹性

在现行的《建筑抗震设计规范》（GB50011—2001）（以下简称《抗震规范》）中，对中震设计的内容涉及很少，仅在总则中提到“小震不坏、中震可修、大震不倒”的抗震设防目标，但没有给出中震设计的判断标准和设计要求。目前我国的抗震设计，是以小震为设计基础的，中震和大震则是通过地震力调整系数和施工图时的各种抗震构造措施来保证的。规范的这些规定，对于大多数工程而言，其结构的安全性是有保证的。但随着建筑工程形式多样性的飞速发展，复杂结构、超高超限结构越来越多，对中震设计的要求也越来越多。目前在超高超限施工图审查办，对于超限结构基本上都要求进行中震验算。


1.1中震弹性和中震不屈服的计算准则

1.1.1中震弹性设计

a.水平地震影响系数最大值alpha_max按小震的约2.85倍取值；

b.内力调整系数取为1(强柱弱梁,强剪弱弯等)；

c.其余分项系数/组合系数均保留；

d.抗震调整系数Gama_RE取同小震，《高规》表4.7.2；

e.材料强度用设计值。

中震弹性设计取消内力调整的经验系数,保留了荷载分项系数,也就是保留了结构的安全度和可靠度,属正常设计。

1.1.2中震不屈服设计

a.水平地震影响系数最大值alpha_max按小震的约2.85倍取值；

b. 内力调整系数取为1(强柱弱梁,强剪弱弯等)；

c. 荷载分项系数取1 ，保留组合系数；

d. 抗震调整系数Gama_RE=1 ；

e.材料强度用标准值。

中震不屈服设计已经去掉所有安全度,属于承载力极限状态设计。

1.1.3水平地震影响系数最大值的取值

见下表，7度地区水平地震影响系数最大值

小震alpha_max=0.08

中震alpha_max=0.23

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.

Subgrade Modulus for Raft Analysis

The most important analysis parameter for raft on soil is the "Subgrade Modulus", the so-called K value.

In general, higher value of K represents soil with higher stiffness. Hence the settlement induced is smaller. Local concentration of bearing (high bearing pressure) under the loading point (columns/ walls) is expected.

On the other hand, lower value of K represents weak soil and larger settlement is expected. The bearing pressure would be smaller as the loading will be "absorbed" by the deformation of raft and soil.

The procedure of determine the value of K (in unit of kN/m³) is as following (assuming soil without consolidation):

(1) Obtain the Young Modulus of the Soil by Ground Investigation Works.

(2) Build a computer model (say, by SAFE) with a first trial of K value (say, ranged from 10,000 to 50,000 for typical soil).

(3) Input all the loading on top of the raft and have analysis of the raft.

(4) Obtain the bearing stress profile for the raft.

(5) Based on the computer output, estimate the settlement based on elastic theory.

(6) Obtain a new value of K by K= stress/ settlement.

(7) Have iteration of K value by repeating steps 2-6 until the results converged.

Fire Protection and Corrosion Protection System for Structural Steel

Structural steel shall be protected against fire and corrosion.

Fire Protection System

Fire protection system shall include surface preparation, application of primer, fixing details, etc.

There are mainly two type of fire protection system, the sprayed fire protection system and intumescent coating system.

(i) Sprayed Fire Protection System

• For interior structural steelwork.


• For dry environment, the steel surface shall be blast cleaned (e.g. to Swedish Standard SIS055900 Sa2) and shall be thoroughly cleared of oil, grease, dirt or other foreign substrances which may impair the propoer adhesion of the fire protection to the substrate.


• For high humidity location, the steel surface shall be:
  (1) blast cleaned (e.g. to Swedish Standard SIS055900 Sa 2.5);
  (2) provided with corrosion protection system using two pack epoxy based zinc rich primer (say, to BS4652) with a dry film thickness of 80μm.
  (3) apply a bond coat to the primed surface
  (4) apply sprayed mineral fire coating to the surface

(ii) Intumescent Coating System

• For both interior and exterior condition.


• For Interior condition, all interior structural steelwork (including fasteners and welded connections) shall be blast cleaned to Swedish Standard SIS 055900 Sa 2 ½ and with:
  (1) Primer: 2-pack epoxy based zinc rich primer with dry film thickness = 80μm
  (2) Basement: protective fire coating with thickness depends on Hp/A and fire rating.
  (3) Finishing Coat: Compatible finishing coat with dry file thickness = 80μm
  


• For Exterior condition, all structural steelwork (including fasteners and welded connections) shall hot-dip galvanized to BSEN ISO 1461 with:
  (1) Pretreatment: Degrease and rinse, apply British Rail T-Wash.
  (2) Primer: Thickness and type to be recommended by the fire protection coating manufacturer.
  (3) Basement: protective fire coating with thickness depends on Hp/A and fire rating.
  (4) Finishing Coat: Compatible finishing coat with dry file thickness = 50μm
  

Corrosion Protection System

(i) For Interior Environment (without FRP Requirement), either

• Hot-dip galvanized to BS EN ISO 1461 and receive the protective coating below:

(1) Pre-treatment: Degrease and rinse, apply British Rail T-Wash.
(2) Finishing coat: 2-packed recoatable polyurethane finishing paint applied in 2 coats recommended as suitable by paint manufacturer for direct application to etched surface. Etched surface shall be overcoated within 24 hours or the time limit by the manufacturer. Dry film thickness = 80μm

OR

• Blast cleaned to Sa 2 ½ and shall receive the protective coating as follows:

(1) Primer: 2-pack epoxy based zinc rich primer to BS 4652, dry film thickness = 80μm
(2) Undercoat: 2-pack epoxy based micaceous iron oxide paint, dry film thickness = 100μm
(3) Finishing coat: 2-pack recoatable polyurethane coat, applied in two coats, dry film thickness = 100μm
(4) Minimum overall dry film thickness = 280μm

(ii) For External Environment (without FRP Requirement),

• Hot-dip galvanized to BS EN ISO 1461 and receive the protective coating below:

(1) Pre-treatment: Degrease and rinse, apply British Rail T-Wash.
(2) Primer: 2-pack epoxy based zinc phosphate primer to BS4652 with dry film thickness = 40μm
(3) Undercoat: 2-pack epoxy based micaceous iron oxide paint, dry film thickness = 80μm
(4) Finishing coat: 2-packed recoatable polyurethane coat, applied in two coat with dry film thickness = 100μm
(5) Minimum overall dry film thickness = 220μm

Piezometer Vs Standpipe

Piezometer and standpipe are both used to measure the ground water level.

A piezometer is a small-diameter observation well used to measure the hydraulic head of groundwater. Typical configuration of piezometer consists of filter tip surrounded by sand filter zone at the bottom. Above the tilter tip is bentonite seal and bentonite-cement grout.



As piezometer is used to measure the water pressure at the tip, long response time is required.

Contrast to piezometer, standpipe is used to measure the water level directly and hence shorter response time is expected. The configuration of standpipe is similar to that of piezometer, except that the bentonite seal/ grout are replaced by sand/ gravel. Typical standpipe arrangement is as follow: