Zero Compression Element

When we want to model tension cable rods or any member with zero compression  and want to use finite element model for this purpose then follow the procedure below. This procedure is written considering CSI products (SAP or ETABS) but the concept should be similar for most of the softwares.

 

  1. Select the frame elements which should not take any compression.
    FRAME_ELVN

 

  1. Click Assign > Frame/Line > Tension/Compression Limits.

TENSIONCOMPRESSIONMENU

  1. Check the box for Compression and define a limit of zero in the text box (default=0)

TENSIONCOMPRESSIONBOX

 

In order to run the analysis for tension-only members, we have to run Non-Liner Static analysis which is very simple.

  1. Go to Define > Load Cases and select the case you want to change to non-linear static (usually lateral load cases).
    LateralLoadBox
  2. Select Non-Linear Static from the pull down menu.
    LateralLoadBoxNL

 

  1. Input other load cases in the box with appropriate load factors.
  2. Run the analysis and check the members with no compression and confirm that members are only taking tension.
    ZeroCompression
  3. Perform the design.

 

Alternate option for defining non-linear load cases:

  1. Click Define > Load Combination
  2. Select Load combinations needs to convert to Non Linear combos

LoadCombo

 

  1. Click button ” Convert Combos to Nonlinear Cases
  2. Run Analysis and check the forces in the members.
  3. Perform the design.

 

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Where is the Base?

ASCE 7-05 defined the base for seismic design as:

“The level at which the horizontal seismic ground motions are considered to be imparted to the structure”.

Most of the times this is misinterpreted or misunderstood that where the base is considered when determining the height of building for seismic calculations specially when the site is sloped or if the basement is open from one side.

Read the following article published in December 2009 issue of Structure magazine, this article will answer lots of questions.

 https://app.box.com/s/h0k4t3z8xyck45pdzyoy

Modeling Piles/Piers in FEM Software

When we use finite element software for analyzing deep foundations then it becomes important to model piles with accurate axial stiffness so that the load can distribute properly based on pile stiffnesses.

Some people suggests to calculate pile stiffness by AE/L formula. By doing this method the problem is that as you increase the length of the pile, the stiffness will reduce which is not accurate.

The best thing is to ask Geotech Engineer for the allowable loads and the allowable axial displacement on working loads. Most of the times this allowable displacement is ranged from ¼” to ½” which can be confirmed from geotech report.

Take your allowable load which will vary based on the depth of pile and divide it by the axial displacement, this will be the required pile stiffness.

k=P/∆

Where:

k = Pile axial stiffness (kips/inch )

P = Allowable load (kips)

Δ = Allowable vertical displacement (inches)

Biggest Bridge Slide Ever!!!

Watch the video and admire the engineering:

Here is the brief description of project:

On April 10, 2014, the slide of the Milton-Madison Bridge was completed. At nearly a half-mile long, this is the longest bridge slide of its type in North America, perhaps the world. The slide occurred over a two day period with the actual slide time being approximately 12 1/2 hours. The total slide distance was 55 feet, going from temporary piers to refurbished, permanent piers.

The old bridge was where the new bridge is today – after the slide. The new bridge was built on temporary supports NEXT TO the old bridge. The old bridge still carried traffic. After the new bridge was built, traffic was switched to the NEW Bridge on the temporary alignment. The old bridge was demolished. The existing piers were rehabilitated. After that work was complete, the new bridge slides back over where the old bridge used to be. By reusing the foundations and piers the owner saved many tens of millions of dollars.