Recent content by BAretired

M1 is positive and Mmax is negative by convention but they are both clockwise moments. F1 pushes the top chord to the right. Fmax pulls the top chord to the right. That is why F1 and Fmax are additive. Together, they represent the total axial force in the top chord. A free body diagram of...

In the diagram below, M1 is a positive moment and Mmax is a negative moment. If F1 and Fmax are M1/c and Fmax/c respectively where c is the moment arm, they both point right for top fibers and left for bottom fibers. The horizontal shear at the neutral axis is equal to the top or bottom chord...

We ARE talking about chord force. We are NOT talking about lateral bracing. The top chord force is equal and opposite to the bottom chord force. For a rectangular beam, the total horizontal shear force between the two chords, i.e. at the neutral axis, is equal to the chord force in each chord.

The 2% comment in red is a different matter entirely. The topic in this thread is "Shear flow for indeterminate beam". Shear flow is calculated by the formula VQ/I, or if shear stress is required it is VQ/It. That formula is correct for any beam whether cantilever, simple or continuous...

F and F' are pointing in the same direction because it is a free body diagram of a beam cut at midspan. Fixity at the support and positive moment at midspan requires F and F' to both point left for top fibers and right for bottom fibers. The horizontal shear to be transferred at the neutral...

Thanks hoshang, in that case, top struts are possible and I believe, advisable.

I agree, gte447. It seems to be a mystery.

You are too stingy with the information. Provide a sketch showing dimensions. Top struts as per your suggestion would need to be too high. Normally, the top of truss is about handrail height. You could use a Pratt truss or Warren truss with verticals moment connected to cross beams or you...

Show a cross section. If the joists are supported on hangers, I agree with Tomfh...could be joist shrinkage.

A Warren truss can have verticals if needed. Floor beams can be extended with diagonal braces to provide lateral support to the top chord. Are you talking about a pedestrian bridge, road bridge or rail bridge? Can you provide a sketch showing the approximate dimensions of the bridge. So...

If 6% relates to the length of wire, then I would say the sag to be used is S4.

6% of what? horizontal span? I would think it should be (S1+S2)/2, but the norm is not clear.

Seems like I'm talking to myself. Perhaps the OP has passed on. If so, RIP, OP.

The sketch below illustrates that the horizontal shear is the same whether the beam is fixed or simply supported.

Shear flow is calculated in exactly the same manner for a simple and fixed beam. In both beams, the shear flow is zero at midspan and maximum at supports. In the simple beam, top fibers are compressed throughout the span, whereas in the fixed beam, top fibers near midspan are compressed while...