Momentresisting frames with infills

There are several possible types of moment-resisting frames with infills, depending on the Clauses 6.10.3(2), type of infill and its connection to the frame. 6.10.3(3)

The relative stiffness of the steel moment frame and of the panels influences the distribution of forces between them. If the infill panels are stiffer than the frame, they attract the earthquake action effect. Depending on their composition, the panels can be:

8 The primary component of earthquake resistance: frames with well-designed, well-

connected reinforced concrete or masonry infill panels correspond to this situation. 8 The effective earthquake-resisting structure only in the first stage of the response, after which the infills crush and fail. Then, the moment frame takes over. However, as such panel crushing can be unevenly distributed, a soft-storey mechanism may develop and be the global failure mode.

The design approach has to consider the many possible situations by selecting some standard ones for which the behaviour can be properly assessed:

8 If the infills are well-designed reinforced concrete panels well-connected to the frame, the structure is in fact a composite wall and has to be designed as such, according to Section 7 of EN 1998-1. 8 If infills are high-quality masonry panels intended to act as part of the seismic-resistant structural system, the structure should be considered as a confined masonry one, and designed as such, according to Section 9 of EN 1998-1. 8 If infills of whatever nature are structurally disconnected from the moment-resisting frame, which means that the infills are supported on the bottom beam with a horizontal gap under the top beam and vertical gaps on both sides, then they act only as a mass and make no contribution to earthquake resistance. Then, the openings c of vertical gaps should be sufficient to prevent any contact between the infills and the structure: c > dT, where dT is the interstorey drift at the ULS. A deformable material should be used to fill side and top gaps, for tightness. Prevention of out-of-plane movement of the structurally disconnected infill (possibly leading to overturning) may prove difficult. 8 If infill panels consist of a material characterized by a low in-plane Young's modulus E

and a low yield resistance, the situation is close to total disconnection. 8 If infills are made of panels having a very low in-plane resistance and/or crush immediately at their perimeter, the result is again close to total disconnection, because the panel material behaves like a deformable gap-filling material.

In practice, the last two statements have to be assessed by comparing:

8 the order of magnitude of the shear resistance VRd of the steel columns around the infill to the yield or failure strength NRd of a compression diagonal made of the infill material 8 the interstorey drift dr of the steel moment frame under the design seismic action, to the deformation dI at maximum strength of the same compression diagonal.

One may consider the infills as structurally ineffective and ignore them, if NRd < 0.05FRd and dl > dr.

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