Design limitations of web openings to AISC 360

• Simple beams only (single span, pin ended). Although web openings can be defined on multi-span beams design is beyond scope.
  • Cantilevers are excluded.
  • Curved members are excluded.
  • Rotated members are excluded.
  • Haunched and tapered members are excluded.
• Section type: rolled symmetrical compact I sections.
  • Plated sections are excluded.
  • Non-compact sections are excluded.
• Steel yield strength is limited to 65 ksi
• Design for hogging (negative) moments is excluded.
• Axially loaded beams are excluded.
• Web opening placement and sizes: Opening depth is limited to 70 percent of beam depth. There are also two control parameters which dictate the dimensions of the opening, one being the aspect ratio, ao/ho, of the opening and the other being the opening parameter po. Both should meet the required limits.
• Multiple openings. Checks are performed to ensure that openings are spaced far enough apart so that design expressions for individual openings may be used.
• Openings cannot be placed closer than the section height to a support.
• Openings can be defined as rectangular or circular. Circular openings are then designed as equivalent rectangular openings, the dimensions of the equivalent rectangle used in all subsequent calculations being:

  • d_o' = 0.9 * opening diameter

  • l_o = 0.45 * opening diameter

• Openings can be reinforced on one side or both sides but always top or bottom, but no checks are performed on the reinforcement and its welds.
• No concentrated loads should be placed above an opening. If there is a point load above the opening then that combination gets a Beyond Scope status, (but only if the concentrated load is greater than 25% of the shear strength of the two tee sections).
• The nearest concentrated load should be placed at least d/2 from the edge of the opening.
• A check is performed to determine whether bearing stiffeners are necessary, if so these are not designed but a warning is shown.
• ASD design i.e. LRFD only.

Web opening design checks

Common design checks for both composite and non-composite beams

The following design checks are carried out at each opening for both composite and steel beams:

• Section and opening dimension limit check including the spacing of multiple openings if applicable.

• Classification check. Non-compact sections are beyond scope.

• Moment-shear interaction check. First the maximum pure flexural and shear strength is calculated following the guidelines of the Design Guide for the currently selected edition of the head code. Then the direct formulas (3-5a and 3-5b) are used to calculate design shear and bending strength.

• Deflection calculation. As deflection calculations are head code independent, for simplicity a single approach is used irrespective of the head code selected.

Additional design checks for non-composite beams or composite beams at construction stage

The following additional design checks are carried out at each opening only for non-composite beams or composite beams at construction stage.

• Lateral torsional buckling. The 'standard' lateral torsional buckling check is run but the torsional constant is multiplied by a reduction factor according to the design guide. Strength over the openings should not be the governing UR.

• Buckling of tee-shaped compression zone. The tee which is in compression is investigated as an axially loaded column following the procedures of selected head code. For unreinforced members this is not required when the aspect ratio of the tee is less than or equal to 4. For reinforced openings, this check is only required for large openings in regions of high moment.

Additional design checks for composite beams at composite stage

The following additional design checks are carried out at each opening only for composite beams at composite stage.

• Slab reinforcement check. The check of minimum transverse and longitudinal slab reinforcement ratio to prevent cracking of the slab in the vicinity of the web opening.

• Number of shear connectors above the opening. To limit the effect of bridging a minimum of two studs per foot is applied to the total number of studs. If this criterion is already satisfied by normal stud requirements, additional studs are not needed. A warning is shown when this criteria is not met.

Deflections

The simplified rules in DG2 are for limited cases and therefore have not been implemented. Instead Tekla Structural Designer uses a first principles approach as per Eurocodes.

For both non-composite and composite beams without openings the deflection analysis includes the effect of shear. For composite beams this is conservative because it uses the shear area and shear modulus of the bare beam.

The deflection of a beam with web openings should* be greater than that of the same beam without openings due to two effects,

• the reduction in the beam inertia at the positions of openings due to primary bending of the beam,

• the local deformations at the openings due to Vierendeel effects. This has two components - that due to shear deformation and that due to local bending of the upper and lower tee sections at the opening.

The primary bending deflection is established by 'discretising' the member and using a numerical integration technique based on 'Engineer's Bending Theory' - M/I = E/R = σ/y. In this way the discrete elements that incorporate all or part of an opening will contribute more to the total deflection.

The component of deflection due to the local deformations around the opening is established using a similar process to that used for cellular beams which is in turn based on the method for castellated beams given in the SCI publication, “Design of castellated beams. For use with BS 5950 and BS 449".

The method works by applying a 'unit point load' at the position where the deflection is required and using a 'virtual work technique to estimate the deflection at that position.

For each opening, the deflection due to shear deformation, δ_s, and that due to local bending, δ_bt, is calculated for the upper and lower tee sections at the opening. These are summed for all openings and added to the result at the desired position from the numerical integration of primary bending deflection.

Note that in the original source document on castellated sections, there are two additional components to the deflection. These are due to bending and shear deformation of the web post. For castellated beams and cellular beams where the openings are very close together these effects are important and can be significant. For normal beams the openings are likely to be placed a reasonable distance apart. Thus in many cases these two effects will not be significant. They are not calculated for such beams but in the event that the openings are placed close together a warning is given.

* The above technique for calculating the deflection of a beam with web openings does NOT include shear deflection due to the primary bending. Consequently, if the shear deflection component is more significant than that due to openings, it is possible that the reported deflection for a beam with web openings is less than that reported for the same beam without openings.