Rule of thumb Basic for Weld Design Based on AISC and API



As a structural engineer, we have known that connection design have 2 models.  Bolted connection and weld connection. those two are our responsible as structural engineer. for this post, we will describe rule of thumb for a practitioner like me how to deal with welding design based on AISC and API.

In construction for oil and gas, connection prefer using weld than bolt. especially in floating construction. Weld connection has many advantages than bolt connection, described below :

  1. Yield stress for welding more than base plate 
  2. Welding are watertight therefore appropriate used for Ship, FPSO, or another watertight
  3. welding strength depend on weld size 
  4. welding area relatively small and no need wide area like bolt

 Ok, how about basic concept (rule of thumb) about this welding ?
Welding has several types :

  1. Double fillet Weld
  2. Single Fillet 
  3. Full Penetration 
  4. partial penetration
in this post, only for basic concept how to understand welding strength.


Basic Concepts API/ AISC
According to API/AISC the stresses in the throat must be checked. At the throat stress in 3 direction occur. 2 “perpendicular” stress σ1 and τ1 “parallel” stress  τ11 as shown in the figure below for a fillet weld.

Forces acting on the welds should always be resolved. in 3 directions. on in the direction of τ11 (z-direction in the figure) and two in the directions at an angle of 45 degrees to the throat (X-and Y- the direction in the figure). 

The resulting stress should be less than allowable stress below :


Remarks: Fy means the yield of a base material

Basic concept API / AISC
Notes:

  • For fillet Welds τ1 = σ1, when τ1 is checked for 0.4 Fy. τ1 is automatically less than 0.6 Fy. This check can be ignored
  • when τ1 and τ11 occur, the combined shear has to be checked. this check is always governing overt the two check for the single shear directions. these checks can be left out them.
  • API dictates welding and weld procedure in accordance with AWS Strutural Welding Codes AWS. This code dictateas an allowable shear stresss of 0.3 "nominal tensile strenth, at HMC 0.4 "yield stress is used for allowable shear stress. the value used by Hereema is governing compared to the where dictate of AWS
To be continued

Reference :
- Manual Welding Design from Richard Van Bowel
- AWS A3.0 Standard welding term and definition


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