The Structural Engineer's Corner

Eng. Onorio Francesco Salvatore

Ultimate Limit States and Serviceability Limit States in Australian Standard AS 1170

Written By: Lexatus - Sep• 30•13

The Limit States used in Australia (AS 1170.0 – Structural design actions) are similar to those of other codes, like the ISO ones.
There are:
Ultimate Limit States, ULS;
Serviceability Limit States, SLS.

There are two different types of Serviceability Limit States:

SLS1 => the structure and the non-structural components do not require repair after the SLS1 earthquake, snow or wind event;
SLS2 => the structure mantains operational continuity after the SLS2 earthquake.

The Table C1 (more…)

Methods of Structural Analysis according to AS 4100 – Part 3: Braced frame vs Sway frame in the moment amplification method for elastic analysis

Written By: Lexatus - Sep• 15•13

1599R-15351

In the previous post, we discussed about the Elastic analysis at first-order with moment amplification. We have seen how this method can be a good alternative to a second-order analysis. Basically, we need to increase the moment by a certain quantity including in this way the second-order effects.

We have also introduced the moment amplification factors. The factors are two:

– δb => moment amplification factor for a braced member (Clause 4.4.2.2);

– δs => moment amplification factor for a sway member (Clause 4.4.2.3).

This method can always be used?

No, there is a limitation. When the factor is (more…)

Excel spreadsheet for the evaluation of the Dynamic Response Factor as per AS 1170.2

Written By: Lexatus - Sep• 14•13

Dynamic Response Factor evaluation

As announced in a previous post (The Dynamic Response Factor as per Australian Standard 1170.2), I’m putting on free download an Excel tool I just created for the evaluation of the Dynamic Response Factor as per Section 6 of the Australian Standard 1170.2.

The inputs needed are the following:

  1. Design wind speed for chosen direction;
  2. First-mode natural frequency of vibration in the along-wind direction (how to calculate it will be discussed in a future post);
  3. Height of the level at which action effects are calculated;
  4. Height to the top of the structure;
  5. Turbulence intensity;
  6. Peak factor for the upwind velocity fluctuactions;
  7. Average breadth of the structure between heights s and h;
  8. Average breadth of the structure between heights 0 and h’
  9. Ratio of structural damping to critical damping of a structure.

The outputs are the following: (more…)

The Dynamic Response Factor as per Australian Standard 1170.2

Written By: Lexatus - Sep• 14•13

First-mode Onorio

In order to evaluate the design wind pressure on a structure, the basic pressure has to be multiplied by an Aerodynamic Shape Factor (as per Section 5 of Australian Standard AS 1170.2) and by a Dynamic Response Factor (as per Section 6 of Australian Standard AS 1170.2).

The Standard specifies that the value of the Dynamic Response Factor Cdyn is 1.0 except where the structure is dynamically wind sensitive. This coefficient is variable with the natural first-mode fundamental frequency. The cases are:

Greater than 1 Hz => Cdyn = 1.0;

Less than 1 Hz => Cdyn to be calculated.

When the natural first-mode fundamental frequency is less than (more…)

Orientation factor in wind action evaluation according to Australian Standard 1170.2

Written By: Lexatus - Sep• 14•13

Once obtained the Site wind speed, the Structural Engineer needs a Design wind speed. As said in previous article, the Site wind speed – as the name says – is only related to a specific site, no matter of what structure we are analysing. In order to evaluate the specific wind speed on structure, the Structural Engineer has to consider if the orientation is known or unknown. In the first case, for the specific structural element he wants to analyse, an orientation modified wind speed is needed.

In order to determine the design wind speed, one of the following approaches has to be used: (more…)

Factors affecting the Site wind speed in Australian Standards

Written By: Lexatus - Sep• 14•13

In the previous article we have discussed about the Site wind speed. It is independent of the structure shape and has to be seen only as a wind speed characteristic for a specific site.

In fact, to determine the Site wind speed, the Structural Engineer takes the Regional wind speed and multiplies it by some factors. We have: (more…)

Corrective factors for uncertainties in wind action calculation

Written By: Lexatus - Sep• 14•13

In the previous article we have talked about the Regional wind speed and we have seen as this value is function of an additional corrective factor for tropical cyclones regions C and D. This factor is included to allow for uncertainties in the prediction of ultimate design wind speeds for those regions.

Structural Engineering often deals with  (more…)

Site wind speed according to AS/NZS 1170.2

Written By: Lexatus - Sep• 14•13

In order to obtain the Design wind velocity, the Structural Engineer needs to define the Site wind velocity. This value is substantially defined as the Regional wind velocity modified taking into account the direction and the exposure of the site.

The formula used is:

Md is a wind directional multiplier for the 8 cardinal directions (b). The values in the brackets are used to take into account the exposure of the site, as said before. These last three coefficients are named as Terrain/height multiplier, Shielding multiplier and Topographic multiplier.

The design height, z, at which the site and design wind speeds are calculated is (more…)

Regional wind speed according to AS/NZS 1170:2

Written By: Lexatus - Sep• 14•13

In order to obtain the design wind speed to be used in the calculation of design wind pressure, a regional wind velocity and a site wind velocity must be calculated.

VR, the Regional wind speed, is defined as a 3-second gust wind speed. It is an all-directional basic action because no considerations on direction from which wind gust is originated are made. Its unit of measure is metres/second. In order to obtain this value, a return period R, defined as the reciprocal of the annual probability of exceedance must be evaluated. (more…)

Methods of Structural Analysis according to AS 4100 – Part 2: Elastic Analysis

Written By: Lexatus - Sep• 14•13

Second-order Onorio

In the Australian Standard 4100, the second-order effects for elastic analysis are defined in the Clause 4.4.1.2.
It is stated that the analysis shall allow for the effects of the design loads acting on the structure and its members in their displaced and deformed configuration.

Second-order bending moments caused by the member axial forces and the displacements are neglected in the first-order (linear) methods of elastic analysis. The problem is that when we have compressive forces, the second-order bending moments may increase the first-order bending moment. In this case we need to allow for this effect.

As stated in any Structural Engineering books, (more…)