What are the advantages and disadvantages of using grout pads under base plates?

It's interesting that you suggest grout pads / shims compensate for the variations in base plate thickness. This is just not true as base plates have a rolling tollerance of + or - 1mm depending on thickness. Its the level of the top of concrete pads that vary - commonly +or- 25mm and sometimes more. Typically 100mm square shims are placed on the conc pads and are levelled to datum before the frame is erected. These shims are placed centrally to allow the columns to be tilted during erection and also when plumbing and lining. These shims are permanent so will always transfer load through the frame into the concrete pad.

The Pros are: * Easier & Faster: Saves time and money on leveling the foundation. * Adaptable: Compensates for uneven base plate thickness. * Stronger Connection: Creates a uniform contact surface for better load transfer and stability, reducing the risk of damage to the base plate or concrete.

Load Distribution: Grout pads help distribute loads evenly across the base plate, reducing localized stress concentrations. Leveling and Alignment: Grout pads can be used to level and align base plates, ensuring that equipment or structures are installed correctly. Stability: Grout pads provide stability to the base plate and prevent rocking or tilting, which can occur when a base plate is in direct contact with an uneven or irregular surface. Vibration Damping: Grout pads can help dampen vibrations and reduce the transmission of vibrations from machinery or equipment to the surrounding structure.

Shrinkage can be avoided by using non-shrink, high strength grout with a greater compressive strength than the base concrete it is supported on. The increase in height of the column would be negligible so should have no effect on bracing or fire protection requirements.

The grout height would be taken into consideration in the original design in my experience, however, the presence of the grout, shims, or levelling nuts needs to be considered in the design of the anchor bolts ability to resist the shear loads. In some cases the reduction in capacity will push you to add shear lugs to the underside of your base plate. Shear lugs need to be grouted in.

The Cons are: * Grout can shrink and crack, reducing strength * Grout may settle unevenly, causing stress and alignment issues * May increase column height, requiring extra work

Material Selection: Choosing the right grout material is crucial, as selecting an inappropriate or low-quality material can lead to long-term problems. Inadequate grout materials may shrink, crack, or deteriorate over time. Maintenance: Grout pads require regular inspection and maintenance to ensure they remain in good condition. Potential for Errors: Poor installation or incorrect placement of grout pads can lead to uneven support or alignment issues. Limited Adjustability: Once the grout material has cured, adjustments to the base plate are challenging. This can be a disadvantage if subsequent adjustments or equipment replacements are anticipated.

Use non-shrink grout under structural steel baseplates for a thicker grout base. A thickness of 10 to 25 mm is generally too thin to allow for sufficient as-built top of slab elevation tolerance.

Agreed that 10mm would be too thin, at the lower bounds of the grout to be able to flow freely beneath the base and provide full bearing contact. Typically in the UK a nominal 25mm would be allowed for with the grout thickness being less or more depending on the construction tolerance. Generally maximum thickness of a high strength free-flowing grout suggested by the manufacturers is 110mm, though with correct planning and detailing this should never become an issue.

Design Considerations can be: * Thin grout pad (10mm-25mm) * Compatible material with concrete and base plate * Proper curing and protection * Designed for shear and uplift forces * Anchored to the foundation

Keeping the grout pad thickness as small as possible, typically within the range of 10 mm to 25 mm, can be a valid design strategy in certain situations, especially when considering the impact on anchor bolts and eccentric shear. It's essential to balance the advantages of reducing bending moments on anchor bolts with the need to provide adequate support, stability, and durability for the base plates and supported equipment or structures.

Base plates are not generally designed to bear on shims. The design is generally based on continuous contact with the supporting grout or concrete.

Alternatives: * Leveling nuts/washers: * Easier to install * Allow for adjustments * Shims/wedges: * Easier to install than grout * Can be used for small adjustments * Limitations of alternatives: * Need careful installation and inspection * May corrode or loosen over time

Shims or wedges should only be used as temporary measures during erection to enable the column to be lined and levelled. It should always then be grouted for the permanent state to ensure a full bearing contact beneath the entirety of the base plate. Similarly levelling nuts or washers would still require grouting afterwards, and the negative result could be long bolt protrusions between base material and base plate, leading to bolt bending under horizontal shear loading.

Grout Pad Designs: * Fixed Base Column: * Carries compression and bending. * Base plate bends to resist bending. * Grout pad takes shear through friction. * Pinned Base Column: * Carries compression and lateral loads. * Base plate handles compression. * Grout pad anchors the column to resist uplift.